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SPS10c
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Generators


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Transformers


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Generators (Mechanical Energy is converted to Electrical Energy)


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Practice - Parallel Circuit Problems


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Practice - Series Circuit Problems


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Generators (Mechanical Energy is converted to Electrical Energy)


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Electric Motors


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Electric Motor (Electrical Energy is converted to Mechanical Energy)


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Direction of Current and Electrons; Right hand Rule; Electromagnets


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Permanent vs Temporary Magnets


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Magnetism


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Properties of Magnets


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Schematic Diagrams, Series and Parallel Circuits


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Sample Problems  V=IR


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V=IR (Ohm's Law)


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Alternating and Direct Current - More complete explanation and some interesting related facts.


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Alternating and Direct Current - Simplest Explanation  (Video will not run on iPads!  Check this one out on a desktop.) 


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Charging by Friction, Conduction and Induction

"Under Construction"


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Static Electricity


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Static Electricity - Examples of charging because of friction; Static Discharge


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Electricity - The Big Picture!


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Doppler Effect


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Speed of Sound


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Sound


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Wave Interference; Standing Wave


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Lenses (and refraction)


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Reflection (Regular and Diffuse), Refraction and Diffraction

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Sound Waves


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Visible Light

Select Source!


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Light


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Wave-Particle Duality of Light   Go to Source!


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Electromagnetic Spectrum


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Electromagnetic Waves


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Properties of Waves; Transverse and Longitudinal


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Properties of Waves; Electromagnetic and Sound Waves


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Waves - Tsunamis


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Mechanical Advantage & Pulleys


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Practice - Mechanical Advantage Problems (Check with me for answers!)


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Simple Machines (lots of links!)


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Mass and Weight


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Practice - Graphic Organizer: Free Fall


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Free Falling in Outer Space


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Terminal Velocity & Free Fall


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Falling Objects & Gravitational Force


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Gravity (Go to Source)


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Newton's Second Law Problem


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Newton's 3 Laws ( Go to Source)


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Newton's Three Laws


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Practice - Motion Graphs


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Distance - Time and Velocity - Time Graphs


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Velocity & Acceleration Calculations


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Heating & Cooling Curve (Not the Same Thing as a Phase Diagram!)


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Practice Phase Diagram #2


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Practice Phase Diagram #1


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Phase Diagram


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Calculate Heat Capacity - Q (Don't  memorize specific heats!)


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Conduction, Convection, Radiation


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Energy Transformation


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Testing for Acids and Bases


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Neutralization (Acids & Bases)


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Acids & Bases


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Properties of Acids and Bases


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Practice - Solubility Curves (Check with me for answers!)


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Solubility Curves


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Observe Factors Affecting the Rate of Solution Formation (Start at 9:04)


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Rate of Solution Formation (Not the Same as Solubility)


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Solutions - Why don't oil and water mix?


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Conductivity of Solutions


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Solutions


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Gases - Pressure, Temperature, Volume & # of Molecules (Scientist's names are not the important part!).  Select Source to link to the web page and video.


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Heating & Cooling Cuves


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Plasma


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Valence Electrons & Periodic Table (Bohr Models)


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Solids, Liquids & Gases 

In a solid, the particles do move, they vibrate,  back and forth around a fixed point.  The particles in a liquid move more freely.  The particles in a gas move the fastest and even farther apart.


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How many elements are there?  Nuclear Forces


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Mendeleev's Importance


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Metals, Metalloids & Nonmetals in the Periodic Table  (Zig-Zag line is not drawn quite right, look at your EOCT periodic table;  Po and At are also metalloids)


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Groups in Periodic Table


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Nuclear Energy


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Nuclear Energy (Excellent Summary - Go to the very end!)


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Half-life Problems - Examples

1st Problem:  44 days divided by 11 days per half-life = 4 half-lives


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Carbon Dating (Nuclear Decay and HalfLife)


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Half-Life Problems


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Nuclear Energy


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Half-life & Radioactive Decay


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Fission or Fusion for Energy


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Fission & Fusion


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Fission & Fusion


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Alpha Decay

Georgia: Use He (symbol for helium) behind the 4 and 2 to represent the alpha particle in the equation.


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Beta Decay

Georgia:  Use the e instead of the Greek letter.


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3 Types of Nuclear Radiation


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Intro to Radioactivity

Go to the Source for video and web page.


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Types of Chemical Reactions


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Some Tips for Balancing Equations


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Practice Balancing Equations (answers included)


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Balancing Chemical Equations

Reminder - The diatomic elements (HOF BrINCl) are always written with the subscript 2.  For example, nitrogen as a reactant or product is N2. I can't get the 2 to show as a subscript, but you know what I mean.


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"Counting Atoms"

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Law of Conservation of Matter Problem (Check with me for answer!)


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Law of Conservation of Matter


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Practice Covalent Names & Formulas (Check with me for correct answers!)


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Covalent (or Molecular) Compounds, Formulas and names - Stop at 7:43!


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Practice Ionic Names & Formulas (Check with me for correct answers!)


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Ionic Compounds, Formulas  (LCM & Criss Cross Methods) and Names - Stop at 9:26!


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Formulas for Binary Ionic Compounds (Watch to the very end!)


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Practice with Isotopes & Ions (Check with me for answers!)


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Chemical Bonding


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Atoms (identifying), Isotopes, Ions 

Intro to the Periodic Table


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Chemical Bonding (Mrs. Carter - Linked specifically to the GA standard)


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Practice with Isotopes & Ions (Check with me for answers!)


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SPS1a (4a)
SPS1a (4a)
Atoms (identifying), Isotopes, Ions 

Intro to the Periodic Table


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Protons, electrons, neutrons (Follow link to You-Tube) Only in a neutral atom is the #p+ = #e- . You must know the number of protons to identify the element!

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Structure of the Atom

(comprehensive)


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More about those electrons

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Atoms - Isotopes, Ions etc.

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Electrons (first 20 elements) and Shells (Relate to Energy Levels and Bohr Models)

(Listen carefully, very well done and worth the effort.) 


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Isotopes (Great explanation and examples!)

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Practice with Isotopes & Ions (Check with me for answers!)

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Mass # and Atomic Mass (The actual calculation of atomic mass is part of the Chemistry course in GA, don't worry about being asked to do that! - Another, great explanation!)


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Atomic # and Mass # [You are correct our EOCT periodic table does not give mass numbers. Also, we write the mass # and atomic # to the left of the symbol]


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Ga Standards High School Physical Science
Ga Standards High School Physical Science

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EOCT Reference Sheet
EOCT Reference Sheet

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Periodic Table
Periodic Table

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Generators

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by Chris Woodford. Last updated: April 13, 2018.

Oil may be the world's favorite fuel, but not for much longer. Modern homes are powered mostly by electricity and it won't be long before most of us are driving electric cars as well. Electricity is superbly convenient. You can produce it in all kinds of different ways using everything from coal and oil to wind and waves. You can make it in one place and use it on the other side of the world if you want to. And, once you've produced it, you can store it in batteries and use it days, weeks, months, or even years later. What makes electric power possible-and indeed practical-is a superb electromagnetic device called an electricity generator: a kind of electric motor working in reverse that converts ordinary energy into electricity. Let's take a closer look at generators and find out how they work!

Photo: A diesel electricity generator from the mid-20th century, pictured at the REA Power Plant Museum near Hampton, Iowa. Courtesy of Photographs in the Carol M. Highsmith Archive, Library of Congress, Prints and Photographs Division.

Where does electricity come from?

The best way to understand electricity is to start by giving it its proper name: electrical energy. If you want to run anything electrical, from a toaster or a toothbrush to an MP3 player or a television, you need to feed it a steady supply of electrical energy. Where are you going to get that from? There's a basic law of physics called the conservation of energy that explains how you can get energy-and how you can't. According to this law, there's a fixed amount of energy in the universe and some good news and some bad news about what we can do with it. The bad news is that we can't create more energy than we have already; the good news is that we can't destroy any energy either. All we can ever do with energy is convert it from one form into another.

Photo: A large electricity generator driven by steam at CalEnergy's Leathers geothermal power plant in Imperial County, California. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

If you want to find some electricity to power your television, you won't be making energy out of thin air: the conservation of energy tells us that's impossible. What you'll be doing is using energy converted from some other form into the electrical energy you need. Generally, that happens in a power plant some distance from your home. Plug in your TV and electrical energy flows into it through a cable. The cable is much longer than you might think: it actually runs all the way from your TV-underground or through the air-to the power plant where electrical energy is being prepared for you from an energy-rich fuel such as coal, oil, gas, or atomic fuel. In these eco-friendly times, some of your electricity will also be coming from wind turbines, hydroelectric power plants (which make power using the energy in dammed rivers), or geothermal energy (Earth's internal heat). Wherever your energy comes from, it'll almost certainly be turned into electricity with the help of a generator. Only solar cells make electricity without using generators.

How can we generate electricity?

Photo: A typical electricity generator. This one can make up to 225kW of electric power and is used for testing prototype wind turbines. Photo by Lee Fingersh courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

If you've read our detailed article about electric motors, you'll already know pretty much how generators work: a generator is just an electric motor working in reverse. If you've not read that article, you might like to take a quick look before reading on- but here's a quick summary either way.

An electric motor is essentially just a tight coil of copper wire wrapped around an iron core that's free to rotate at high speed inside a powerful, permanent magnet. When you feed electricity into the copper coil, it becomes a temporary, electrically powered magnet-in other words, an electromagnet-and generates a magnetic field all around it. This temporary magnetic field pushes against the magnetic field that the permanent magnet creates and forces the coil to rotate. By a bit of clever design, the coil can be made to rotate continuously in the same direction, spinning round and round and powering anything from an electric toothbrush to an electric train.

Photo: The rotating part (rotor) of a typical, small electric motor. An electricity generator has exactly the same components but works in the opposite way, turning motion into electrical energy.

So how is a generator different? Suppose you have an electric toothbrush with a rechargeable battery inside. Instead of letting the battery power the motor that pushes the brush, what if you did the opposite? What if you turned the brush back and forth repeatedly? What you'd be doing would be manually turning the electric motor's axle around. That would make the copper coil inside the motor turn around repeatedly inside its permanent magnet. If you move an electric wire inside a magnetic field, you make electricity flow through the wire-in effect, you generate electricity. So keep turning the toothbrush long enough and, in theory, you would generate enough electricity to recharge its battery. That, in effect, is how a generator works. (Actually, it's a little bit more tricky than this and you can't actually recharge your toothbrush this way, though you're welcome to try!)

How does a generator work?

Artwork: A simple generator like this produces alternating current (an electric current that periodically reverses). Each side of the generator (green or orange) is either moving up or down. When it moves up, it will generate a current flowing one way; when it moves down, the current flows the other way. If you're a meter connected to the wire, you don't know which way the wire is moving: all you see is that the direction of the current periodically reverses: you see an alternating current.

Take a length of wire, hook it up to an ammeter (something that measures current), and place it between the poles of a magnet. Now move the wire sharply through the invisible magnetic field the magnet produces and a current will briefly flow through the wire (registering on the meter). This is the basic science behind the electricity generator, demonstrated in 1831 by British scientist Michael Faraday (read a short biography or a long biography). If you move the wire in the opposite direction, you generate a current that flows the other way. (If you're interested, you can figure out the direction in which the current flows using something called the right-hand rule or generator rule, which is the mirror image of the left-hand rule used to figure out how motors work.)

The important thing to notice is that you generate a current only when you move the wire through the magnetic field (or when you move a magnet past a wire, which amounts to the same thing). It's not enough just to place a wire near a magnet: to generate electricity, either the wire has to move past the magnet or vice-versa. Suppose you want to generate lots of electricity. Lifting a wire up and down all day isn't going to be much fun-so you need to devise some way of moving a wire past a magnet by mounting one or the other of them on a wheel. Then, when you turn the wheel, the wire and magnet will move with respect to one another and an electric current will be produced.

Now here's the interesting part. Suppose you bend a wire into a loop, sit it between the poles of a magnet, and arrange it so it will constantly rotate-as in the diagram here. You can probably see that as you turn the loop, each side of the wire (either the orange side or the green side) will sometimes be moving up and sometimes moving down. When it's moving up, electricity will flow one way; when it's moving down, the current will flow the other way. So a basic generator like this will produce an electric current that reverses direction every time the loop of wire flips over (in other words, an alternating current or AC). However, most simple generators actually produce direct current-so how do they manage it?

DC generators

Just as a simple DC electric motor uses direct current (DC) electricity to produce continual, rotary motion, so a simple DC generator produces a steady supply of direct current electricity when it spins around. Like a DC motor, a DC generator uses a commutator. It sounds technical, but it's just a metal ring with splits in it that periodically reverses the electrical contacts from the generator coil, reversing the current at the same time. As we saw up above, a simple loop of wire automatically reverses the current it produces every half-turn, simply because it's rotating, and the commutator's job is to cancel out the effect of the coil's rotation, ensuring that a direct current is produced.

Artwork: Comparing the simplest DC generator with the simplest AC alternator. In this design, the coil (gray) spins between the poles of a permanent magnet. Each time it rotates through a half-turn, the current it generates reverses. In the DC generator (top), a commutator reverses the current every time the coil moves a half-turn, cancelling out the reversal of the current. In the AC alternator (bottom), there is no commutator so the output simply rises, falls, and reverses as the coil rotates. You can see the output current from each type of generator in the chart on the right.

AC Alternators

What if you want to generate alternating current (AC) instead of direct current? Then you need an alternator, which is simply an AC generator. The simplest kind of alternator is like a DC generator without a commutator. As the coil or magnets spin past one another, the current naturally rises, falls, and reverses, giving an AC output. Just as there are AC induction motors, which use electromagnets to produce a rotating magnetic field instead of permanent magnets, so there are alternators that work by induction in a similar way.

Alternators are mostly used for generating electricity from vehicle engines. Cars use alternators, driven by their gasoline engines, which charge up their batteries as they drive along (the AC current is converted to DC by diodes or rectifier circuits).

Generators in the real world

Photo: An alternator is a generator that produces AC (alternating current) instead of DC (direct current). Here we can see a mechanic removing the alternator from an outboard motor boat engine. Photo by Yesenia Rosas courtesy of US Navy.

Generating electricity sounds simple-and it is. The hard thing is that you need to put in a huge amount of physical effort to generate even small amounts of power. You'll know this if you have a bicycle with dynamo lights powered from the wheels: you have to pedal somewhat harder to make the lights glow-and that's just to produce the tiny amount of electricity you need to power a couple of torch bulbs. A dynamo is simply a very small electricity generator. At the opposite extreme, in real power plants, gigantic electricity generators are powered by steam turbines. These are a bit like spinning propellers or windmills driven using steam. The steam is made by boiling water using energy released from burning coal, oil, or some other fuel. (Note how the conservation of energy applies here too. The energy that powers the generator comes from the turbine. The energy that powers the turbine comes from the fuel. And the fuel-if it's coal or oil-originally came from plants powered by the Sun's energy. The point is simple: energy always has to come from somewhere.)

Portable generators

Photo: A portable electricity generator powered by a diesel engine. Photo by JoAnn S. Makinano courtesy of US Air Force and Defense Imagery.

Most of the time we take electricity for granted. We switch on lights, TVs, or clothes washers without stopping to think that the electrical energy we're using has to come from somewhere. But what if you're working outdoors, in the middle of nowhere, and there's no electricity supply you can use to power your chainsaw or your electric drill?

One possibility is to use cordless tools with rechargeable batteries. Another option is go for pneumatic tools, such as jackhammers. These are entirely mechanical and powered by compressed air instead of electricity. A third option is to use a portable electricity generator. It's simply a small gasoline engine (petrol engine), similar to the compact engine you get on a motorcycle, with an electricity generator attached. As the engine chugs away, burning up gasoline, it pushes a piston back and forth, turning a generator and producing a steady electric current as its output. With the help of a transformer, you can use a generator like this to produce pretty much any voltage you need, anywhere you need it. As long as you have enough gasoline, you can make your own electricity supply indefinitely. But remember the conservation of energy: run out of gas and you run out of electricity as well!

Artwork: Generator technology advanced rapidly during the 19th century. English chemist and physicist Michael Faraday built the first primitive generator in 1831. Within a few decades, numerous inventors were building practical electric generators. This one (a "dynamo-electric machine") was designed by Edward Weston in the 1870s as a way to "transmute mechanical energy into electrical energy with greater efficiency than heretofore." It has a static outer ring of magnets (blue) and a rotating armature (coils) in the center (red). A commutator (green) converts the generated current to DC. From US Patent 180,082 reissue 8,141 by Edward Weston, courtesy of US Patent and Trademark Office.

Find out more

On this website

You might like these other articles on our site covering related topics:

Videos

How does a generator work?: An excellent short video by Dr Jonathan Hare and the Vega Science Trust demonstrates very clearly how moving a coil through a magnetic field can produce electricity.

Simple generator: electric generator for science fair: William Beaty gives a step-by-step guide to building a simple generator using easy-to-find components (enamel wire, magnets, cardboard, and so on).

Bicycle generator: How to power a food processor with a bicycle driving an alternator (a type of electricity generator). Quite a neat experiment, though the commentary could be a little clearer.

Books

For older readers

For younger readers

Articles

More to explore on our website...

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How electricity generators and dynamos work

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by Chris Woodford. Last updated: April 13, 2018. Oil may be the world's favorite fuel, but not for much longer. Modern homes are powered mostly by electricity and it won't be long before most of us are driving electric cars as well. Electricity is superbly convenient.

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Transformers

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by Chris Woodford. Last updated: March 10, 2018.

The mighty power lines that criss-cross our countryside or wiggle unseen beneath city streets carry electricity at enormously high voltages from power plants to our homes. It's not unusual for a power line to be rated at 400,000 to 750,000 volts! But the appliances in our homes use voltages thousands of times smaller-typically just 110 to 250 volts. If you tried to power a toaster or a TV set from an electricity pylon, it would instantly explode! (Don't even think about trying, because the electricity in overhead lines will almost certainly kill you.) So there has to some way of reducing the high voltage electricity from power plants to the lower voltage electricity used by factories, offices, and homes. The piece of equipment that does this, humming with electromagnetic energy as it goes, is called a transformer. Let's take a closer look at how it works!

Photo: Blast from the past: A strangely shaped transformer at the Chickamauga Dam near Chattanooga, Tenn. Photographed in 1942 by Alfred T. Palmer, Office of War Administration, courtesy of US Library of Congress.

Why do we use high voltages?

Photo: Coming down: This old substation (step-down electricity transformer) supplies power in the small English village where I live. It's about 1.5m (5ft) high and its job is to convert several thousand volts of incoming electricity to the hundreds of volts we use in our homes.

Your first question is probably this: if our homes and offices are using photocopiers, computers, washing machines, and electric shavers rated at 110-250 volts, why don't power stations simply transmit electricity at that voltage? Why do they use such high voltages? To explain that, we need to know a little about how electricity travels.

As electricity flows down a metal wire, the electrons that carry its energy jiggle through the metal structure, bashing and crashing about and generally wasting energy like unruly schoolchildren running down a corridor. That's why wires get hot when electricity flows through them (something that's very useful in electric toasters and other appliances that use heating elements). It turns out that the higher the voltage electricity you use, and the lower the current, the less energy is wasted in this way. So the electricity that comes from power plants is sent down the wires at extremely high voltages to save energy.

But there's another reason too. Industrial plants have huge factory machines that are much bigger and more energy-hungry than anything you have at home. The energy an appliance uses is directly related (proportional) to the voltage it uses. So, instead of running on 110-250 volts, power-hungry machines might use 10,000-30,000 volts. Smaller factories and machine shops may need supplies of 400 volts or so. In other words, different electricity users need different voltages. It makes sense to ship high-voltage electricity from the power station and then transform it to lower voltages when it reaches its various destinations. (Even so, centralized power stations are still very inefficient. About two thirds of the energy that arrives at a power plant, in the form of raw fuel, is wasted in the plant itself and on the journey to your home.)

Photo: Making large electricity transformers at a Westinghouse factory during World War II. Photo by Alfred T. Palmer, Office of War Administration, courtesy of US Library of Congress.

How does a transformer work?

A transformer is based on a very simple fact about electricity: when a fluctuating electric current flows through a wire, it generates a magnetic field (an invisible pattern of magnetism) or "magnetic flux" all around it. The strength of the magnetism (which has the rather technical name of magnetic flux density) is directly related to the size of the electric current. So the bigger the current, the stronger the magnetic field. Now there's another interesting fact about electricity too. When a magnetic field fluctuates around a piece of wire, it generates an electric current in the wire. So if we put a second coil of wire next to the first one, and send a fluctuating electric current into the first coil, we will create an electric current in the second wire. The current in the first coil is usually called the primary current and the current in the second wire is (surprise, surprise) the secondary current. What we've done here is pass an electric current through empty space from one coil of wire to another. This is called electromagnetic induction because the current in the first coil causes (or "induces") a current in the second coil. We can make electrical energy pass more efficiently from one coil to the other by wrapping them around a soft iron bar (sometimes called a core):

To make a coil of wire, we simply curl the wire round into loops or ("turns" as physicists like to call them). If the second coil has the same number of turns as the first coil, the electric current in the second coil will be virtually the same size as the one in the first coil. But (and here's the clever part) if we have more or fewer turns in the second coil, we can make the secondary current and voltage bigger or smaller than the primary current and voltage.

One important thing to note is that this trick works only if the electric current is fluctuating in some way. In other words, you have to use a type of constantly reversing electricity called alternating current (AC) with a transformer. Transformers do not work with direct current (DC), where a steady current constantly flows in the same direction.

Step-down transformers

If the first coil has more turns that the second coil, the secondary voltage is smaller than the primary voltage:

This is called a step-down transformer. If the second coil has half as many turns as the first coil, the secondary voltage will be half the size of the primary voltage; if the second coil has one tenth as many turns, it has one tenth the voltage. In general:

Secondary voltage ÷ Primary voltage = Number of turns in secondary ÷ Number of turns in primary

The current is transformed the opposite way-increased in size-in a step-down transformer:

Secondary current ÷ Primary current = Number of turns in primary ÷ Number of turns in secondary

So a step-down transformer with 100 coils in the primary and 10 coils in the secondary will reduce the voltage by a factor of 10 but multiply the current by a factor of 10 at the same time. The power in an electric current is equal to the current times the voltage (watts = volts x amps is one way to remember this), so you can see the power in the secondary coil is theoretically the same as the power in the primary coil. (In reality, there is some loss of power between the primary and the secondary because some of the "magnetic flux" leaks out of the core, some energy is lost because the core heats up, and so on.)

Step-up transformers

Reversing the situation, we can make a step-up transformer that boosts a low voltage into a high one:

This time, we have more turns on the secondary coil than the primary. It's still true that:

Secondary voltage ÷ Primary voltage = Number of turns in secondary ÷ Number of turns in primary

and

Secondary current ÷ Primary current = Number of turns in primary ÷ Number of turns in secondary

In a step-up transformer, we use more turns in the secondary than in the primary to get a bigger secondary voltage and a smaller secondary current.

Considering both step-down and step-up transformers, you can see it's a general rule that the coil with the most turns has the highest voltage, while the coil with the fewest turns has the highest current.

Transformers in your home

Photo: Typical home transformers. Anticlockwise from top left: A modem transformer, the white transformer in an iPod charger, and a cellphone charger.

As we've already seen, there are lots of huge transformers in towns and cities where the high-voltage electricity from incoming power lines is converted into lower-voltages. But there are lots of transformers in your home also. Big electric appliances such as washing machines and dishwashers use relatively high voltages of 110-240 volts, but electronic devices such as laptop computers and chargers for MP3 players and mobile cellphones use relatively tiny voltages: a laptop needs about 15 volts, an iPod charger needs 12 volts, and a cellphone typically needs less than 6 volts when you charge up its battery. So electronic appliances like these have small transformers built into them (often mounted at the end of the power lead) to convert the 110-240 volt domestic supply into a smaller voltage they can use. If you've ever wondered why things like cellphones have those big fat chunky power cords, it's because they contain transformers!

Photos: An electric toothbrush standing on its charger. The battery in the brush charges by induction: there is no direct electrical contact between the plastic brush and the plastic charger unit in the base. An induction charger is a special kind of transformer split into two pieces, one in the base and one in the brush. An invisible magnetic field links the two parts of the transformer together.

Induction chargers

Many home transformers (like the ones used by iPods and cellphones) are designed to charge up rechargeable batteries. You can see exactly how they work: electricity flows into the transformer from the electricity outlet on your wall, gets transformed down to a lower voltage, and flows into the battery in your iPod or phone. But what happens with something like an electric toothbrush, which has no power lead? It charges up with a slightly different type of transformer, which has one of its coils in the base of the brush and the other in the charger that the brush stands on. You can find out how transformers like this work in our article about induction chargers.

Transformers in practice

If you've got some of these transformer chargers at home (normal ones or induction chargers), you'll have noticed that they get warm after they've been on for a while. Because all transformers produce some waste heat, none of them are perfectly efficient: less electrical energy is produced by the secondary coil than we feed into the primary, and the waste heat accounts for most of the difference. On a small home cellphone charger, the heat loss is fairly minimal (less than that from an old-fashioned, incandescent light bulb) and not usually something to worry about. But the bigger the transformer, the bigger the current it carries and the more heat it produces. For a substation transformer like the one in our top photo, which is about as wide as a small car, the waste heat can be really significant: it can damage the transformer's insulation, seriously shorten its life, and make it much less reliable (let's not forget that hundreds or even thousands of people can depend on the power from a single transformer, which needs to operate reliably not just from day to day, but from year to year). That's why the likely temperature rise of a transformer during operation is a very important factor in its design. The typical "load" (how heavily it's used), the seasonal range of outdoor (ambient) temperatures, and even the altitude (which reduces the density of the air and therefore how effectively it cools something) all need to be taken into account to figure out how effectively an outdoor transformer will operate.

In practice, most large transformers have built-in cooling systems that use air, liquid (oil or water), or both to remove any waste heat. Typically, the main part of the transformer (the core, and the primary and secondary windings) is immersed in an oil tank with a heat exchanger, pump, and cooling fins attached. Hot oil is pumped from the top of the transformer through the heat exchanger (which cools it down) and back into the bottom, ready to repeat the cycle. Sometimes the oil moves around a cooling circuit by convection alone without the use of a separate pump. Some transformers have electric fans that blow air past the heat exchanger's cooling fins to dissipate heat more effectively.

Artwork: Large transformers have built-in cooling systems. In this case, the transformer core and coil (red) sit inside a large oil tank (gray). Hot oil taken from the top of the tank circulates through one or more heat exchangers, which dissipate the waste heat using cooling fins (green), before returning the oil to the same tank at the bottom. Artwork from US Patent 4,413,674: Transformer Cooling Structure by Randall N. Avery et al, Westinghouse Electric Corp., courtesy of US Patent and Trademark Office.

Find out more

On this website

On other websites

Books

For older readers

Transformers Design and Applications by Robert M. Del Vecchio et al. CRC Press, 2018. A detailed guide to power supply transformers.

Transformer and Inductor Design Handbook by Colonel William T. McLyman. CRC Press, 2011. A detailed, practical guide to designing electrical machines using inductance.

Electrical Transformers and Power Equipment by Anthony J. Pansini. Fairmont Press, 1999. Explains the theory, construction, installation, and maintenance of transformers and the different types of transformers before going on to cover related power devices such as circuit breakers, fuses, and protective relays.

Transformers and Motors by George Patrick Schultz. Newnes, 1997. This book has a much more "hands-on," practical feel than some of the other books listed here; it's intended more for electricians and people who have to work with transformers than those who want to design them.

Transformers & Induction Machines by M.V.Bakshi and U.A.Bakshi. Technical Publications, 2009. Explains different kinds of transformers and related electrical equipment that works by induction.

More general books for younger readers

DK Eyewitness: Electricity by Steve Parker. Dorling Kindersley, 2005. A historic look at electricity and how people have put it to practical use.

Power and Energy by Chris Woodford. Facts on File, 2004. One of my own books, this describes how humans have harnessed energy (including electricity) throughout history.

Patents

There are hundreds of patents covering electricity transformers of different kinds. Here are a few particularly interesting (early) ones from the US Patent and Trademark Office database:

US Patent 351,589: System of electric distribution by Lucien Gaulard and John Gibbs, October 26, 1886. Gaulard and Gibbs outline how transformers can be used to step up and step down voltages for efficient power distribution-the basis of the modern electricity supply system throughout the world.

US Patent 433,702: Electrical transformer or induction device by Nikola Tesla, August 5, 1890. Tesla outlines a phase-shift transformer (one that can produce a phase difference between the primary and secondary currents).

US Patent 497,113: Transformer motor by Otto Titus Bláthy, May 9, 1893. A combined transformer and motor produced by one of the inventors of the transformer.

US Patent 1,422,653: Electrical transformer for regulating or varying the voltage of the current supplied therefrom by Edmund Berry, July 11, 1922. A transformer with a dial that allows you to adjust the output voltage.

News articles

More to explore on our website...

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by Chris Woodford. Last updated: March 10, 2018. The mighty power lines that criss-cross our countryside or wiggle unseen beneath city streets carry electricity at enormously high voltages from power plants to our homes. It's not unusual for a power line to be rated at 400,000 to 750,000 volts!

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Generators (Mechanical Energy is converted to Electrical Energy)

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Series Resistor Circuit Analysis Worked Example

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Physical Science 6.8d - The Generator Effect

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From the Physical Science course by Derek Owens. The fundamental ideas behind the generator effect. Distance learning classes are available at http://www.derekowens.com

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Practice - Parallel Circuit Problems

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Practice - Series Circuit Problems

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Generators (Mechanical Energy is converted to Electrical Energy)

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Series Resistor Circuit Analysis Worked Example

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Physical Science 6.8d - The Generator Effect

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From the Physical Science course by Derek Owens. The fundamental ideas behind the generator effect. Distance learning classes are available at http://www.derekowens.com

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Electric Motors

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by Chris Woodford. Last updated: April 4, 2018.

Flick a switch and get instant power-how our ancestors would have loved electric motors! You can find them in everything from electric trains to remote-controlled cars-and you might be surprised how common they are. How many electric motors are there in the room with you right now? There are probably two in your computer for starters, one spinning your hard drive around and another one powering the cooling fan. If you're sitting in a bedroom, you'll find motors in hair dryers and many toys; in the bathroom, they're in extractor fans, and electric shavers; in the kitchen, motors are in just about every appliance from clothes washing machines and dishwashers to coffee grinders, microwaves, and electric can openers. Electric motors have proved themselves to be among the greatest inventions of all time. Let's pull some apart and find out how they work!

Photo: Even small electric motors are surprisingly heavy. That's because they're packed with tightly wound copper and heavy magnets. This is the motor from an old electric lawn mower. The copper-colored thing toward the front of the axle, with slits cut into it, is the commutator that keeps the motor spinning in the same direction (as explained below).

Electricity, magnetism, and movement

The basic idea of an electric motor is really simple: you put electricity into it at one end and an axle (metal rod) rotates at the other end giving you the power to drive a machine of some kind. How does this work in practice? Exactly how do your convert electricity into movement? To find the answer to that, we have to go back in time almost 200 years.

Suppose you take a length of ordinary wire, make it into a big loop, and lay it between the poles of a powerful, permanent horseshoe magnet. Now if you connect the two ends of the wire to a battery, the wire will jump up briefly. It's amazing when you see this for the first time. It's just like magic! But there's a perfectly scientific explanation. When an electric current starts to creep along a wire, it creates a magnetic field all around it. If you place the wire near a permanent magnet, this temporary magnetic field interacts with the permanent magnet's field. You'll know that two magnets placed near one another either attract or repel. In the same way, the temporary magnetism around the wire attracts or repels the permanent magnetism from the magnet, and that's what causes the wire to jump.

Fleming's Left-Hand Rule

You can figure out the direction in which the wire will jump using a handy mnemonic (memory aid) called Fleming's Left-Hand Rule (sometimes called the Motor Rule).

Hold out the thumb, first finger, and second finger of your left hand so all three are at right angles. If you point the seCond finger in the direction of the Current (which flows from the positive to the negative terminal of the battery), and the First finger in the direction of the Field (which flows from the North to the South pole of the magnet), your thuMb will show the direction in which the wire Moves.

That's...

A quick word about current

If you're confused by me saying that the current flows from positive to negative, that just happens to be a historical convention. People like Benjamin Franklin, who helped figure out the mystery of electricity back in the 18th century, believed it was a flow of positive charges, so it flowed from positive to negative. We call this idea conventional current and still use it to this day in things like Fleming's Left-Hand Rule. Now we have better ideas about how electricity works, we tend to talk about current as a flow of electrons, from negative to positive, in the opposite direction to the conventional current. When you're trying to figure out the rotation of a motor or a generator, be sure to remember that current means conventional current and not electron flow.

How an electric motor works-in theory

Photo: An electrician repairs an electric motor onboard an aircraft carrier. The shiny metal he's using may look like gold, but it's actually copper, a good conductor that is much less expensive. Photo by Jason Jacobowitz courtesy of US Navy.

The link between electricity, magnetism, and movement was originally discovered in 1820 by French physicist André-Marie Ampère (1775-1867) and it's the basic science behind an electric motor. But if we want to turn this amazing scientific discovery into a more practical bit of technology to power our electric mowers and toothbrushes, we've got to take it a little bit further. The inventors who did that were Englishmen Michael Faraday (1791-1867) and William Sturgeon (1783-1850) and American Joseph Henry (1797-1878). Here's how they arrived at their brilliant invention.

Suppose we bend our wire into a squarish, U-shaped loop so there are effectively two parallel wires running through the magnetic field. One of them takes the electric current away from us through the wire and the other one brings the current back again. Because the current flows in opposite directions in the wires, Fleming's Left-Hand Rule tells us the two wires will move in opposite directions. In other words, when we switch on the electricity, one of the wires will move upward and the other will move downward.

If the coil of wire could carry on moving like this, it would rotate continuously-and we'd be well on the way to making an electric motor. But that can't happen with our present setup: the wires will quickly tangle up. Not only that, but if the coil could rotate far enough, something else would happen. Once the coil reached the vertical position, it would flip over, so the electric current would be flowing through it the opposite way. Now the forces on each side of the coil would reverse. Instead of rotating continuously in the same direction, it would move back in the direction it had just come! Imagine an electric train with a motor like this: it would keep shuffling back and forward on the spot without ever actually going anywhere.

How an electric motor works-in practice

There are two ways to overcome this problem. One is to use a kind of electric current that periodically reverses direction, which is known as an alternating current (AC). In the kind of small, battery-powered motors we use around the home, a better solution is to add a component called a commutator to the ends of the coil. (Don't worry about the meaningless technical name: this slightly old-fashioned word "commutation" is a bit like the word "commute". It simply means to change back and forth in the same way that commute means to travel back and forth.) In its simplest form, the commutator is a metal ring divided into two separate halves and its job is to reverse the electric current in the coil each time the coil rotates through half a turn. One end of the coil is attached to each half of the commutator. The electric current from the battery connects to the motor's electric terminals. These feed electric power into the commutator through a pair of loose connectors called brushes, made either from pieces of graphite (soft carbon similar to pencil "lead") or thin lengths of springy metal, which (as the name suggests) "brush" against the commutator. With the commutator in place, when electricity flows through the circuit, the coil will rotate continually in the same direction.

Artwork: A simplified diagram of the parts in an electric motor. Animation: How it works in practice. Note how the commutator reverses the current each time the coil turns halfway. This means the force on each side of the coil is always pushing in the same direction, which keeps the coil rotating clockwise.

A simple, experimental motor such as this isn't capable of making much power. We can increase the turning force (or torque) that the motor can create in three ways: either we can have a more powerful permanent magnet, or we can increase the electric current flowing through the wire, or we can make the coil so it has many "turns" (loops) of very thin wire instead of one "turn" of thick wire. In practice, a motor also has the permanent magnet curved in a circular shape so it almost touches the coil of wire that rotates inside it. The closer together the magnet and the coil, the greater the force the motor can produce.

Although we've described a number of different parts, you can think of a motor as having just two essential components:

There's a permanent magnet (or magnets) around the edge of the motor case that remains static, so it's called the stator of a motor.

Inside the stator, there's the coil, mounted on an axle that spins around at high speed-and this is called the rotor. The rotor also includes the commutator.

Universal motors

DC motors like this are great for battery-powered toys (things like model trains, radio-controlled cars, or electric shavers), but you don't find them in many household appliances. Small appliances (things like coffee grinders or electric food blenders) tend to use what are called universal motors, which can be powered by either AC or DC. Unlike a simple DC motor, a universal motor has an electromagnet, instead of a permanent magnet, and it takes its power from the DC or AC power you feed in:

When you feed in DC, the electromagnet works like a conventional permanent magnet and produces a magnetic field that's always pointing in the same direction. The commutator reverses the coil current every time the coil flips over, just like in a simple DC motor, so the coil always spins in the same direction.

When you feed in AC, however, the current flowing through the electromagnet and the current flowing through the coil both reverse, exactly in step, so the force on the coil is always in the same direction and the motor always spins either clockwise or counter-clockwise. What about the commutator? The frequency of the current changes much faster than the motor rotates and, because the field and the current are always in step, it doesn't actually matter what position the commutator is in at any given moment.

Animation: How a universal motor works: The electricity supply powers both the magnetic field and the rotating coil. With a DC supply, a universal motor works just like a conventional DC one, as above. With an AC supply, both the magnetic field and coil current change direction every time the supply current reverses. That means the force on the coil is always pointing the same way.

Photo: Inside a typical universal motor: The main parts inside a medium-sized motor from a coffee grinder, which can run on either DC or AC. The gray electromagnet round the edge is the stator (static part) and its powered by the orange-colored coils. Note also the slits in the commutator and the carbon brushes pushing against it, which provide power to the rotor (rotating part). Induction motors in such things as electric railroad trains are many times bigger and more powerful than this, and always work using high-voltage alternating current (AC), instead of low-voltage direct current (DC), or the moderately low voltage household AC that powers universal motors.

Other kinds of electric motors

In simple DC and universal motors, the rotor spins inside the stator. The rotor is a coil connected to the electric power supply and the stator is a permanent magnet or electromagnet. Large AC motors (used in things like factory machines) work in a slightly different way: they pass alternating current through opposing pairs of magnets to create a rotating magnetic field, which "induces" (creates) a magnetic field in the motor's rotor, causing it to spin around. You can read more about this in our article on AC induction motors. If you take one of these induction motors and "unwrap" it, so the stator is effectively laid out into a long continuous track, the rotor can roll along it in a straight line. This ingenious design is known as a linear motor, and you'll find it in such things as factory machines and floating "maglev" (magnetic levitation) railroads.

Another interesting design is the brushless DC (BLDC) motor. The stator and rotor effectively swap over, with multiple iron coils static at the center and the permanent magnet rotating around them, and the commutator and brushes are replaced by an electronic circuit. You can read more in our main article on hub motors. Stepper motors, which turn around through precisely controlled angles, are a variation of brushless DC motors.

Find out more

Related articles on our site

Books

For younger readers

For older readers

Articles

Power and Electric Motors by Rhett Allain. Wired, November, 2011. Why do electric motors draw much more power when they're just starting up?

Activities

Here are some safe and simple motor-making activities you can try for yourself. In order of difficulty, the first is a simple screw motor; the last is a fully-fledged DC commutator motor.

Patents

Patents are a great source of detailed technical information and drawings. Here are a few I've dug out from the USPTO database:

More to explore on our website...

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by Chris Woodford. Last updated: April 4, 2018. Flick a switch and get instant power-how our ancestors would have loved electric motors! You can find them in everything from electric trains to remote-controlled cars-and you might be surprised how common they are. How many electric motors are there in the room with you right now?

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Electric Motor (Electrical Energy is converted to Mechanical Energy)

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Physical Science 6.8c - The Motor Effect

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From the Physical Science course by Derek Owens. The motor effect and how it it is fundamentally caused by forces on charges moving through a magnetic field. Distance learning classes are available at http://www.derekowens.com

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Direction of Current and Electrons; Right hand Rule; Electromagnets

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Physical Science 6.8a - Electric Current and Magnetic Fields

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Permanent vs Temporary Magnets

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A brief lesson about permanent and temporary magnets.

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Magnetism

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by Chris Woodford. Last updated: September 26, 2018.

Science is our understanding of how the world works-and generally the world works fine whether we understand it or not. Take magnetism, for example. People have known about magnets for thousands of years and they've been using them practically, as compasses, for almost as long. The ancient Greeks and Romans knew as well as we do that lodestone (an iron-rich mineral) can attract other pieces of iron, while the ancient Chinese were making magnetic compasses set in intricate wooden inlays for their practice of Feng Shui (the art of carefully arranging a room) thousands of years before interior designers came on board. Science can sometimes be slow to catch up: we've only really learned how magnetism works in the last century, since the world inside atoms was first discovered and explored.

Photo: A typical horseshoe magnet. See the trace of brown rust on the top of the magnet's upper "leg"? That happens because the magnet is made of iron, which corrodes (rusts) in damp air.

What is magnetism?

Photo: The magnetic field between the opposite poles of two bar magnets that strongly attract one another. We can't normally see magnetic fields, but if you sprinkle iron filings (tiny bits shaved off an iron bar with a file) onto a piece of paper and hold it above the magnets you can see the field underneath. Photo by courtesy of Wikimedia Commons (where you'll find a bigger version of this image).

Playing with magnets is one of the first bits of science most children discover. That's because magnets are easy to use, safe, and fun. They're also quite surprising. Remember when you first discovered that two magnets could snap together and stick like glue? Remember the force when you held two magnets close and felt them either attract (pull toward one another) or repel (push away)? One of the most amazing things about magnets is the way they can attract other magnets (or other magnetic materials) "at a distance," invisibly, through what we call a magnetic field.

To ancient people, magnetism must have seemed like magic. Thousands of years down the line, we understand what happens inside magnetic materials, how their atomic structure causes their magnetic properties, and how electricity and magnetism are really just two sides of the same coin: electromagnetism. Once scientists would have said magnetism was the strange, invisible force of attraction between certain materials; today, we're more likely to define it as a force created by electric currents (themselves caused by moving electrons).

SIX things to know about magnets

Almost everyone knows these six basic facts about how magnets behave:

1. A magnet has two ends called poles, one of which is called a north pole or north-seeking pole, while the other is called a south pole or south-seeking pole.

2. The north pole of one magnet attracts the south pole of a second magnet, while the north pole of one magnet repels the other magnet's north pole. So we have the common saying: like poles repel, unlike poles attract.

3. A magnet creates an invisible area of magnetism all around it called a magnetic field.

4. The north pole of a magnet points roughly toward Earth's north pole and vice-versa. That's because Earth itself contains magnetic materials and behaves like a gigantic magnet.

5. If you cut a bar magnet in half, it's a bit like cutting an earthworm in half! You get two brand new, smaller magnets, each with its own north and south pole. (This is, of course, a joke. You don't get two worms if you cut a worm in half. But you do get two magnets.)

6. If you run a magnet a few times over an unmagnetized piece of a magnetic material (such as an iron nail), you can convert it into a magnet as well. This is called magnetization.

What is a magnetic field?

Photo: A colorful way to visualize invisible magnetic fields using a computer graphics program developed at Los Alamos National Laboratory. In this three-dimensional chart, the height and color of the peaks shows the strength of the magnetic field at each point. Photo by courtesy of US Department of Energy.

Suppose you put a bar magnet (shaped like a rectangle, sometimes with the north and south poles painted different colors) or a horseshoe magnet (bent round into a tight U-shape) onto a table and place an iron nail nearby. If you push the magnet slowly toward the nail, there will come a point when the nail jumps across and sticks to the magnet. That's what we mean by magnets having an invisible magnetic field that extends all around them. Another way of describing this is to say that a magnet can "act at a distance": it can cause a pushing or pulling force on other objects it isn't actually touching).

Magnetic fields can penetrate through all kinds of materials, not just air. You probably have little notes stuck to the door of your refrigerator with brightly colored magnets-so you can see that magnetic fields cut through paper. You may have done the trick where you use a magnet to pick up a long chain of paperclips, with each clip magnetizing the next one along. That little experiment tells us that a magnetic field can penetrate through magnetic materials such as iron.

Earth the Magnet

Why do magnets point north or south? A great English scientist named William Gilbert answered that question in 1600 when he suggested that Earth is a giant magnet.

Photo: We can use magnetic compasses like this to navigate because Earth is itself a giant magnet. Photo by Staff Sgt. Jacob N. Bailey courtesy of US Air Force.

Gilbert's theory was published in De Magnete (Of Magnets, Magnetic Bodies, and the Great Magnet of the Earth), the first big scientific book published in the English language (previously, scientific books had always been written in Latin). Like all true scientists, Gilbert tested many of his ideas with careful experiments.

We now know that Earth is magnetic because it's packed with molten rocks rich in magnetic materials such as iron. Just like a bar magnet, Earth's magnetic field stretches out into space, in a region called the magnetosphere, and can affect things around it. When energetic particles zooming in from the Sun (the so-called solar wind) interact with Earth's magnetic field, we get amazing auroras in the sky (the northern lights or aurora borealis and the southern lights or aurora australis).

Photo: The Northern Lights above Bear Lake, Alaska. Photo by Senior Airman Joshua Strang courtesy of US Air Force.

What about other stars and planets-do they have magnetism too? We know the Sun has a magnetic field several times stronger than Earth's, but the Moon has little or no magnetism. The other planets have magnetic fields too. Saturn, Jupiter, Neptune, and Uranus have fields stronger than Earth's, while Mars, Mercury, and Venus have weaker fields. It's not yet known whether Pluto has a magnetic field (but then astronomers are still arguing over whether it's even a planet!).

How can we measure magnetism?

The strength of the field around a magnet depends on how close you get: it's strongest very near the magnet and falls off quickly as you move away. (That's why a small magnet on your table has to be quite close to things to attract them.) We measure the strength of magnetic fields in units called gauss and tesla (the modern SI unit, named for Croatian-born US electricity pioneer Nikola Tesla, 1856-1943). It's interesting to note that the strength of Earth's magnetic field is very weak-about 100-1000 times weaker than that of a typical bar or fridge magnet. On Earth, gravity, not magnetism, is the force that sticks you to the floor. We'd notice Earth's magnetism much more if its gravity weren't so very strong.

Chart: Comparing the strength of some "everyday" sources of magnetism. Please note that the vertical scale is logarithmic: each step up the scale means the strength of the magnetic field has increased ten times. The main thing to note here is how very weak Earth's magnetism is (the green block on the extreme left) compared to everything else we routinely encounter (never mind the giant magnets used in hospitals and labs). The record-breaking lab magnetic field shown on the extreme right, created in Japan in April 2018, is about 24 million times stronger than Earth's magnetic field. My data for this chart comes from the following sources: Earth (goo.gl/TkxfO3), Sun (goo.gl/8uigAU), appliances (goo.gl/P3l487), fridge (goo.gl/OhrDKt), small neodymium (goo.gl/avODib), junkyard (goo.gl/owWZer), MRI (goo.gl/jQ8cTD), loudspeaker (goo.gl/oIwNlS), biggest MRI (goo.gl/8zkACY), biggest lab (bit.ly/2zvH7On). Almost everything produces magnetism-even our own bodies, which make something like a puny 0.000000001 tesla.

What is an electromagnet?

The Homer Simpson or Mickey Mouse magnet that holds things to your refrigerator is a permanent magnet: it keeps hold of its magnetism all the time. Not all magnets work this way. You can make a temporary magnet by passing electricity through a coil of wire wrapped around an iron nail (a device you'll sometimes see referred to as a solenoid). Switch on the current and the nail becomes a magnet; switch it off again and the magnetism disappears. (This is the basic idea behind an electric chime doorbell: you make an electromagnet when you press the button, which pulls a hammer onto the chime bar-ding-dong!) Temporary magnets like this are called electromagnets -magnets worked by electricity-and they hint at a deeper connection between electricity and magnetism that we'll come on to in a moment.

Like permanent magnets, temporary electromagnets come in different sizes and strengths. You can make an electromagnet powerful enough to pick up paperclips with a single 1.5-volt battery. Use a much bigger voltage to make a bigger electric current and you can build an electromagnet powerful enough to pick up a car. That's how scrapyard electromagnets work. The strength of an electromagnet depends on two main things: the size of the electric current you use and the number of times you coil the wire. Increase either or both of these and you get a more powerful electromagnet.

Magnetism and electricity: the theory of electromagnetism

Electromagnets show that you can make magnetism using electricity. In fact, as European scientists discovered in the 19th century, electricity always makes magnetism when it moves about or changes. Every time an electric current flows in a wire, it generates a magnetic field all around it. Changing electricity, in short, produces magnetism.

Photo: The father of electromagnetism, James Clerk Maxwell. Public domain photo by courtesy of Wikimedia Commons.

The reverse is true as well: you can make electricity using a changing pattern of magnetism. That's just as well or you wouldn't be reading these words now. Virtually all the electricity we use (with the exception of electricity produced from solar cells) is made by devices called generators. These use powerful magnets and coils of wire to produce electricity with the help of turbines, devices that capture kinetic energy from fluids that move past them (typically wind, water, or steam made from coal, oil, or nuclear power).

How do you get electricity from magnetism? Simply speaking, you put a metal wire near a magnet (so the wire is inside the magnetic field). Move the wire or move the magnet so the magnetic field inside the wire fluctuates and electricity will flow through the wire. Keep moving the wire or magnet and you'll make electricity continually.

You can see from this that electricity and magnetism are partners. Wherever you find one, the other is never far away. The first person to explain this properly, in the mid-19th century, was a brilliant Scottish physicist named James Clerk Maxwell. His theory summed up everything then known about electricity and magnetism in four relatively simple mathematical formulas. Maxwell's equations, as we now call them, combined electricity and magnetism into a single, powerful theory we call electromagnetism. We now know that electromagnetism is one of the four fundamental forces that control everything that happens in our universe-and that's a powerful idea indeed!

What use are magnets?

Maybe you think magnets are interesting; maybe you think they're boring! What use are they, you might ask, apart from in childish magic tricks and scrapyards?

You might be surprised just how many things around you work by magnetism or electromagnetism. Every electric appliance with an electric motor in it (everything from your electric toothbrush to your lawn mower) uses magnets to turn electricity into motion. Motors use electricity to generate temporary magnetism in wire coils. The magnetic field thus produced pushes against the fixed field of a permanent magnet, spinning the inside part of the motor around at high speed. You can harness this spinning motion to drive all kinds of machines.

There are magnets in your refrigerator holding the door closed. Magnets read and write data (digital information) on your computer's hard drive and on cassette tapes in old-fashioned personal stereos. More magnets in your hi-fi loudspeakers or headphones help to turn stored music back into sounds you can hear. If you're sick with a serious internal illness, you might have a type of body scan called NMR (nuclear magnetic resonance), which draws the world beneath your skin using patterns of magnetic fields. Magnets are used to recycle your metal trash ( steel food cans are strongly magnetic but aluminum drinks cans are not, so a magnet is an easy way to separate the two different metals).

Photo: An NMR scan like this builds up a detailed image of a patient's body (or, in this case, their head) on a computer screen using the magnetic activity of atoms in their body tissue. You can see the patient going into the scanner at the top and the image of their head on the screen below. Photo by courtesy of Warren Grant Magnuson Clinical Center (CC) and US National Institutes of Health (NIH).

Which materials are magnetic?

Iron is the king of magnetic materials-the metal we all think of when we think of magnets. Most other common metals (such as copper, gold, silver, and aluminum) are, at first sight, nonmagnetic and most nonmetals (including paper, wood, plastic, concrete, glass, and textiles such as cotton and wool) are nonmagnetic too. But iron's not the only magnetic metal. Nickel, cobalt, and elements that belong to a part of the Periodic Table (the orderly arrangement chemists use to describe all the known chemical elements) known as the rare-Earth metals (notably samarium and neodymium) also make good magnets. Some of the best magnets are alloys (mixtures) of these elements with one another and with other elements. Ferrites (compounds made of iron, oxygen, and other elements) also make superb magnets. Lodestone (which is also called magnetite) is an example of a ferrite that's commonly found inside Earth (it has the chemical formula FeO·Fe2O3).

Materials like iron turn into good temporary magnets when you put a magnet near them, but tend to lose some or all of their magnetism when you take the magnet away again. We say these materials are magnetically soft. By contrast, alloys of iron and the rare-Earth metals retain most of their magnetism even when you remove them from a magnetic field, so they make good permanent magnets. We call those materials magnetically hard.

Is it true to say that all materials are either magnetic or nonmagnetic? People used to think that but scientists now know that the materials we consider to be nonmagnetic are also affected by magnetism, though extremely weakly. The extent to which a material can be magnetized is called its susceptibility.

How different materials react to magnetism

Scientists have a number of different words to describe how materials behave when you put them near a magnet (which is another way of saying when you put them inside a magnetic field). Broadly speaking, we can divide all materials into two kinds called paramagnetic and diamagnetic, while some of the paramagnetic materials are also ferromagnetic. It's important to be clear what these confusing words actually mean...

Paramagnetic

Make a sample of a magnetic material and hang it from a thread so it dangles in a magnetic field, and it will magnetize and line itself up so its magnetism is parallel to the field. As people have known for thousands of years, this is how exactly a compass needle behaves in Earth's magnetic field. Materials that behave this way are called paramagnetic. Metals such as aluminum and most nonmetals (which you might think aren't magnetic at all) are actually paramagnetic, but so weakly that we don't notice. Paramagnetism depends on temperature: the hotter a material is, the less it's likely to be affected by nearby magnets.

Photo: We think of aluminum (used in drinks cans like these) as nonmagnetic. That helps us separate for recycling our aluminum cans (which don't stick to magnets) from our steel ones (which do). In fact, both materials are magnetic. The difference is that aluminum is very weakly paramagnetic, while steel is strongly ferromagnetic. Photo courtesy of US Air Force.

Ferromagnetic

Some paramagnetic materials, notably iron and the rare-Earth metals, become strongly magnetized in a field and usually stay magnetized even when the field is removed. We say materials like this are ferromagnetic, which really just means they're "magnetic like iron." However, a ferromagnetic material will still lose its magnetism if you heat it above a certain point, known as its Curie temperature. Iron has a Curie temperature of 770°C (1300°F), while for nickel the Curie temperature is ~355°C (~670°F). If you heat an iron magnet to 800°C (~1500°F), it stops being a magnet. You can also destroy or weaken ferromagnetism if you hit a magnet repeatedly.

Diamagnetic

We can think of paramagnetic and ferromagnetic materials as being "fans" of magnetism: in a sense, they "like" magnetism and respond positively to it by allowing themselves to be magnetized. Not all materials respond so enthusiastically. If you hang some materials in magnetic fields, they get quite worked up inside and resist: they turn themselves into temporary magnets to resist magnetization and weakly repel magnetic fields outside themselves. We call these materials diamagnetic. Water and lots of organic (carbon-based) substances, such as benzene, behave this way. Tie a diamagnetic material to a thread and hang it in a magnetic field and it will turn so it makes an angle of 180° to the field.

What causes magnetism?

In the early 20th century, before scientists properly understood the structure of atoms and how they work, they came up with an easy-to-understand idea called the domain theory to explain magnetism. A few years later, when they understood atoms better, they found the domain theory still worked but could itself be explained, at a deeper level, by the theory of atoms. All the different aspects of magnetism we observe can be explained, ultimately, by talking about either domains, electrons in atoms, or both. Let's look at the two theories in turn.

Explaining magnetism with the domain theory

Imagine a factory somewhere that makes little bar magnets and ships them out to schools for their science lessons. Picture a guy called Dave who has to drive their truck, transporting lots of cardboard boxes, each one with a magnet inside it, to a different school. Dave doesn't have time to worry which way the boxes are stacked, so he piles them inside his truck any old how. The magnet inside one box could be pointing north while the one next to it is pointing south, east, or west. Overall, the magnets are all jumbled up so, even though magnetic fields leak out of each box, they all cancel one another out.

The same factory employs another truck driver called Bill who couldn't be more different. He likes everything tidy, so he loads his truck a different way, stacking all the boxes neatly so they line up exactly the same way. Can you see what will happen? The magnetic field from one box will align with the field from all the other boxes... effectively turning the truck into one giant magnet. The cab will be like a giant north pole and the back of the truck a huge south pole!

What happens inside these two trucks is what happens on a tiny scale inside magnetic materials. According to the domain theory, something like an iron bar contains lots of tiny pockets called domains. Each domain is a bit like a box with a magnet inside. See where we're heading? The iron bar is just like the truck. Normally, all its onboard "boxes" are arranged randomly and there's no overall magnetism: the iron is not magnetized. But arrange all the boxes in order, make them all face the same way, and you get an overall magnetic field: hey presto, the bar is magnetized. When you bring a magnet up to an unmagnetized iron bar and stroke it systematically and repeatedly up and down, what you're doing is rearranging all the magnetic "boxes" (domains) inside so they point the same way.

Domain theory explains what happens inside materials when they are magnetized. In an unmagnetized material (left), the domains are randomly arranged so there is no overall magnetic field. When you magnetize a material (right), by stroking a bar magnet over it repeatedly in the same direction, the domains rearrange so their magnetic fields align, producing a combined magnetic field in the same direction.

This theory explains how magnetism can arise, but can it explain some of the other things we know about magnets? If you chop a magnet in half, we know you get two magnets, each with a north and south pole. That makes sense according to the domain theory. If you cut a magnet in half, you get a smaller magnet that's still packed with domains, and these can be arranged north-south just like in the original magnet. What about the way magnetism disappears when you hit a magnet or heat it? That can be explained too. Imagine the van full of orderly boxes again. Drive it erratically, at really high speed, and it's a bit like shaking or hammering it. All the boxes will jumble up so they face different ways and the overall magnetism will disappear. Heating a magnet agitates it internally and jumbles up the boxes in much the same way.

Explaining magnetism with the atomic theory

The domain theory is easy enough to understand, but it's not a complete explanation. We know that iron bars aren't full of boxes packed with little magnets-and, if you think about, trying to explain a magnet by saying it's full of smaller magnets isn't really an explanation at all, because it immediately prompts the question: what are the smaller magnets made of? Fortunately, there's another theory we can turn to.

Back in the 19th century, scientists discovered they could use electricity to make magnetism and magnetism to make electricity. James Clerk Maxwell said that the two phenomena were really different aspects of the same thing-electromagnetism-like two sides of the same piece of paper. Electromagnetism was a brilliant idea, but it was more of a description than an explanation: it showed how things were rather than explaining why they were that way. It wasn't until the 20th century, when later scientists came to understand the world inside atoms, that the explanation for electromagnetism finally appeared.

We know everything is made of atoms and that atoms are made up of a central lump of matter called the nucleus. Minute particles called electrons move around the nucleus in orbit, a bit like satellites in the sky above us, but they also spin on their axis at the same time (just like spinning tops). We know electrons carry electric currents (flows of electricity) when they move through materials such as metals. Electrons are, in a sense, tiny particles of electricity. Now back in the 19th century, scientists knew that moving electricity made magnetism. In the 20th century, it became clear that magnetism was caused by electrons moving inside atoms and creating magnetic fields all around them. Domains are actually groups of atoms in which spinning electrons produce an overall magnetic field pointing one way or another.

Artwork: Magnetism is caused by electrons orbiting and spinning inside atoms. Note that this picture is not drawn to scale: most of an atom is empty space and the electrons are actually much further from the nucleus than I've drawn here.

Like the domain theory, atomic theory can explain many of the things we know about magnets, including paramagnetism (the way magnetic materials line up with magnetic fields). Most of the electrons in an atom exist in pairs that spin in opposite directions, so the magnetic effect of one electron in a pair cancels out the effect of its partner. But if an atom has some unpaired electrons (iron atoms have four), these produce net magnetic fields that line up with one another and turn the whole atom into a mini magnet. When you put a paramagnetic material such as iron in a magnetic field, the electrons change their motion to produce a magnetic field that lines up with the field outside.

What about diamagnetism? In diamagnetic materials, there are no unpaired electrons, so this doesn't happen. The atoms have little or no overall magnetism and are less affected by outside magnetic fields. However, the electrons orbiting inside them are electrically charged particles and, when they move in a magnetic field, they behave like any other electrically charged particles in a magnetic field and experience a force. That changes their orbits very slightly, producing some net magnetism that opposes the very thing that causes it (according to the classic bit of electromagnetic theory know as Lenz's law, which is related to the law of conservation of energy). As a result, the weak magnetic field they produce opposes the magnetic field that causes it-which is exactly what we see when diamagnetic materials try to "fight" the magnetic field they're placed in.

A brief history of magnetism

Ancient world: Magnetism is known to the ancient Greeks, Romans, and Chinese. The Chinese use geomantic compasses (ones with wooden inscriptions arranged in rings around a central magnetic needle) in Feng Shui. Magnets gain their name from Manisa in Turkey, a place once named Magnesia, where magnetic lodestone was found in the ground.

13th century: Magnetic compasses are first used for navigation in western nations. Frenchman Petrus Perigrinus (also called Peter of Maricourt) makes the first proper studies of magnetism.

17th century: English physician and scientist William Gilbert (1544-1603) publishes On Magnets, his monumental scientific study of magnetism, and proposes that Earth is a giant magnet.

18th century: Englishman John Michell (1724-93) and FrenchmanCharles Augustin de Coulomb (1736-1806) study the forces magnets can exert. Coulomb also makes important studies of electricity, but fails to connect electricity and magnetism as parts of the same underlying phenomenon.

19th century: Danishman Hans Christian Oersted (1777-1851), FrenchmenAndré-Marie Ampère (1775-1836) andDominique Arago (1786-1853), and EnglishmanMichael Faraday (1791-1867) explore the close connections between electricity and magnetism.James Clerk Maxwell (1831-1879) publishes a relatively complete explanation of electricity and magnetism (the theory of electromagnetism) and suggests electromagnetic energy travels in waves (paving the way for the invention ofradio).Pierre Curie (1859-1906) demonstrates that materials lose their magnetism above a certain temperature (now known as the Curie temperature).Wilhelm Weber (1804-1891) develops practical methods for detecting and measuring the strength of a magnetic field.

20th century: Paul Langevin (1872-1946) elaborates on Curie's work with a theory explaining how magnetism is affected by heat. French physicistPierre Weiss (1865-1940) proposes there are particles called magnetrons, equivalent to electrons, that cause the magnetic properties of materials and outlines the theory of magnetic domains. Two American scientists,Samuel Abraham Goudsmit (1902-78) andGeorge Eugene Uhlenbeck (1900-88), show how magnetic properties of materials result from the spinning motion of electrons inside them.

Find out more

On this website

We have lots of other articles related to magnetism-and the things that use it, including:

Books

For younger readers

For older readers

Hidden Attraction: The History and Mystery of Magnetism by Gerrit L. Verschuur. Oxford University Press, 2013. More history than mystery, thank goodness, and well worth a read.

The Man Who Changed Everything by Basil Mahon, John Wiley, 2003. A readable and easy-to-understand introduction to James Clerk Maxwell's life and work.

Electricity and Magnetism by W.J. Duffin. McGraw Hill, 2001. A brilliantly clear undergraduate text (and the one I used as a physics student).

James Clerk Maxwell by James R. Newman, Scientific American, 1955. Quite an old article, but still worth reading. It's an excellent summary of Maxwell's life as a mathematical physicist and explains very vividly how he visualized electromagnetism before figuring out the math behind it. [Subscription/payment]

De Magnete by William Gilbert, Chiswick Press, 1600. Thanks to Project Gutenberg, you can read the whole text of Gilbert's book online.

Videos

What is the magnetic field? by David Colarusso. A good, clear introduction to magnetic fields and how to draw them with magnetic field lines (2.5 minutes).

MIT: 8.02: Electricity and Magnetism by Walter Lewin. A much more detailed undergraduate course from the inspiring Dutch-born MIT physics professor (48 minutes), including some fun demonstrations.

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Static Electricity

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by Chris Woodford. Last updated: January 9, 2018.

Zap! When a bolt of lightning leaps to the ground, we get a sudden, very vivid demonstration of the power of static electricity (electrical energy that has gathered in one place). Most of us know that static builds up when we rub things together, although that's not really a satisfying explanation. What is it about rubbing things that produces an electrical phenomenon? Although lightning is a spectacular example of static electricity, it's not something we can harness. But there are many other places where static electricity is incredibly useful; from laser printers and photocopiers to pollution-busting power plants, static can be really fantastic. So let's take a closer look at what it is and how it works!

Photo: A lightning bolt is a huge release of static electricity, in which built-up electrical potential energy shoots from the sky to the ground in a sudden, improvised, electric current. Picture by David Parsons courtesy of US DOE/NREL (Department of Energy/National Renewable Energy Laboratory).

What is static electricity?

Photo: Classic static: When you rub a balloon on your pullover, you create static electricity that makes it stick. The rubbing shifts electrons from your pullover (which becomes positively charged) to the latex rubber in the balloon (which becomes negatively charged). The opposite charges make the two things stick.

We take electricity for granted: it's easy to forget that homes, offices, and factories have been powered in this clean and convenient way only since the end of the 19th century-which, in the broader run of human history, is no time at all. It was during the 19th century that pioneers such as Alessandro Volta, Michael Faraday, Joseph Henry, and Thomas Edison figured out the secrets of electricity, how to produce it, and how to make it do useful things. Before that, electricity was largely a curiosity: it was very interesting for scientists to study and play with, but there wasn't much else they could do with it. In those days, people cooked and heated their homes using wood or coal stoves and lit their rooms with candles or oil lamps; there were no such things as radios or TVs, much less cellphones or computers.

The "modern electricity" that powers everything from the phone in your pocket to the subway you ride to school or work is what we call current electricity (or electric current). It's energy that travels down a metal wire from the place where it's produced (anything from a gigantic power plant to a tiny battery) to the thing it powers (often an electric motor, heating element, or lamp). Current electricity is always on the move, carrying energy from one place to another.

Photo: Current electricity: When electricity flows around a closed loop, called a circuit, we can use it to transport energy from a source (in this case, an electricity outlet on the wall) so it does something useful (in this case, warming the heating elements on an electric fire).

Before the 19th century, the only kind of electricity people really knew about or tried to use was static electricity. The ancient Greeks understood that things could be given a static electric "charge" (a buildup of static) simply by rubbing them, but they had no idea that the same energy could be used to generate light or power machines. One of the people who helped to make the connection between static and current electricity was American statesman, publisher, and scientist Benjamin Franklin. In 1752, when Franklin tried to figure out the mysteries of electricity, he famously did so by flying a kite in a thunderstorm to catch himself some electrical energy (an extremely dangerous thing to do). A lightning bolt zapped down the kite to the ground and, had Franklin not been insulated, he might well have been killed. Franklin realized that static electricity, accumulating in the sky, became current electricity when a lightning bolt carried it down to the surface of the Earth. It was through research such as this that he developed one of his most famous inventions, the lightning rod (lightning conductor). Franklin's work paved the way for the electrical revolution of the 19th century-and the world really changed when people such as Volta and Faraday, building on Franklin's discoveries, learned how to produce electricity at will and make it do useful things.

Potential and kinetic energy

Just quickly, in passing, it's worth noting that there's another way to think of static and current electricity and to relate them to things we already know about energy. We can think of static electricity as a kind of potential energy: it's stored energy ready and waiting to do something useful for us. In a similar way, current electricity is (loosely speaking) analogous to kinetic energy: energy in movement-albeit of an electrical kind. Just as you can turn potential energy into kinetic energy (for example, by letting a bolder roll down a hill), so you can turn static electricity into current electricity (that's what a lightning bolt does) and back again (that's how a Van de Graaff generator works).

What makes static electricity?

Until a few years ago, scientists were confident that they understood static electricity and exactly how it worked. The explanation went like this...

Just like the ancient Greeks, we tend to think static electricity comes from rubbing things. So if you live in a home with nylon carpets and metal doorknobs, you'll soon learn that your body builds up a static charge when you walk across the floor, which can discharge when you touch a doorknob, giving you a tiny electric shock. In most school experiments, we also learn about static by rubbing things. You've probably tried that trick where you rub a balloon on your clothes to make it stick? You might conclude from this that static electricity is somehow connected to friction-that it's the very act of rubbing something vigorously that produces a buildup of electrical energy (in the same way that friction can produce heat and even fire).

The triboelectric effect

It's not the rubbing that's important but the fact that we're bringing two different materials into contact. Rubbing two things together vigorously simply brings them into contact again and again-and it's this that produces the static electricity through a phenomenon known as triboelectricity (or the triboelectric effect). All materials are built from atoms, which have a positive central core (the nucleus) surrounded by a kind of fuzzy "cloud" of electrons, which are the really exciting bits. Now some atoms have a more powerful pull on electrons than others; a great deal of chemistry stems from that fact. If we put two different materials in contact, and one attracts electrons more than the other, it's possible for electrons to be pulled from one of the materials to the other. When we separate the materials, the electrons effectively jump ship to the material that attracts them most strongly. As a result, one of the materials has gained some extra electrons (and becomes negatively charged) while the other material has lost some electrons (and becomes positively charged). Hey presto, we have static electricity! When we rub things together again and again, we increase the chances that more atoms will take part in this electron-swap, and that's why a static charge builds up.

Photo: How the triboelectric effect explains static electricity: 1. Ebonite (hard vulcanized rubber-shown here as a black rod) and wool (shown as gray) normally have no electric charge. 2) Put them in contact and the ebonite attracts electrons from the wool. 3) Separate them and the electrons remain on the ebonite, making it negatively charged and leaving the wool with a lack of electrons (or a positive charge). Rubbing the two substances together increases the contact between them and makes it more likely that electrons will migrate from the wool to the ebonite. The negative charge on the ebonite is exactly the same size as the positive charge on the wool; in other words, no net charge is created.

The triboelectric series

If you experiment with different materials, you find some gain positive charges when they're rubbed and some gain negative charges; some materials also gain more charge than others. It turns out that we can rank materials in order according to the charge they gain, giving us a kind of league table of materials running from positive to negative. Different books and web pages show slightly different lists, but they all broadly run from minerals (positive) through such things as wood and paper (neutral) to plastics (negative). Don't worry too much about the exact order of the list; it's going to vary for all kinds of reasons (the kind of glass or the additives in the latex, for example).

This list is called the triboelectric series. The further apart two materials are in the series, the more static electricity will build up when you rub them together. If two materials are very close in the series, it's hard to get them to build up any charge at all no matter how hard you rub them. That would seem to confirm that static electricity isn't about rubbing, per se, but about the nature of the materials we bring into contact.

Rethinking static electricity

What you've just read is the traditional, widely accepted explanation of static electricity-and you'll still find it described that way in most school books.

But in 2011, scientists reported some important new discoveries that seemed to suggest much more was going on. Instead of being purely a matter of physics, and a simple transfer of charged electrons from one material to another, it seemed static electricity could also be caused by chemistry (movement of ions and other essentially chemical processes). And it could also happen through a swapping of small amounts of actual material (a bit of balloon shifting to your pullover or vice versa). Where we used to think of static as a simple "pile" of negative or positive charge (electrons or a lack of them), on closer inspection, it now appears to be a "mosaic" of both positive and negative charges that add up to an overall charge (positive or negative). This research is very new, and still evolving, but it seems clear that our traditional explanation of static electricity is an oversimplified version of what's really happening, even if we've faithfully believed it for over 2000 years!

Artwork: Top: The traditional theory sees a static charge on a balloon as an even spread of charged particles across its surface. Bottom: According to the latest thinking, a static charge is actually a random "mosaic" of much bigger charges, which can be both positive and negative, and which add up to an overall charge. In this case, there's much more negative charge than positive (yellow than red), so our balloon has an overall negative charge.

Further reading

Simple introductions

More complex articles

The Mosaic of Surface Charge in Contact Electrification by H. T. Baytekin, A. Z. Patashinski, M. Branicki, B. Baytekin, S. Soh, and B. A. Grzybowski. Science, July 15, 2011, Vol. 333, Issue 6040, pp.308-312.

Antioxidants dispel static electricity by Richard Van Noorden. Nature, September 19, 2013.

What Creates Static Electricity? by Meurig W. Williams. American Scientist, Volume 100, July/August 2012, pp.316-323.

What use is static electricity?

Now static electricity is all very interesting, but what possible use is it? You can't make toast from a lightning bolt and you can't charge your cellphone simply by rubbing its case on your pullover. You might think static is one of those fascinating but ultimately quite useless bits of science that has no practical applications-but you'd be wrong: static electricity is used in all kinds of everyday technology!

Laser printers and photocopiers use static electricity to build up ink on a drum and transfer it to paper. Crop spraying also relies on static electricity to help herbicides stick to the foliage of plants and distribute themselves evenly over the leaves. Factory paint-spraying robots use a similar trick to ensure that paint droplets are attracted to metal car bodies and not the machinery around them. In many power plants and chemical factories, static electricity is used in smokestacks to scrub away pollution (read more in our article on electrostatic smoke precipitators).

Photo: How can you stop air pollution spewing out from smokestacks? One way is to give a static electric charge to smoke, then funnel it through a grid of metal plates with an opposite charge, so the dirty soot particles are removed. That's how "scrubbers" (electrostatic smoke precipitators) work, such as the ones installed in these smokestacks at the McNeil biomass power plant in Burlington, VT. Photo by Warren Gretz courtesy of US DOE National Renewable Energy Laboratory (NREL).

Of course, static electricity has its drawbacks too. It can cause sparks and explosions in fuel depots and stray static is a real nuisance if you're working with electronic components. That's why engineers and chemists have developed all kinds of anti-static technologies (from simple wires to ingenious, slightly conducting paints and coatings) that prevent static buildup in sensitive places. While you're reading these words, you can be sure that someone, somewhere is trying to find a new way to harness static electricity or a better way to stop it causing problems. Static electricity may be stationary, but it's never standing still!

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On this site

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Books

For younger readers

Eyewitness: Electricity by Steve Parker. New York: Dorling Kindersley, 2005. A good solid introduction to electricity from a dependable children's science writer.

Routes of Science: Electricity by Chris Woodford. New York: Facts on File, 2004: One of my own books, this volume takes us through the complete history of electricity, from the ancient Greeks to modern times.

Cool Science: Experiments with Electricity and Magnetism by Chris Woodford. New York: Gareth Stevens, 2010: Another of my books, this one is a short and simple hands-on guide to electricity and magnetism.

For older readers

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What is static electricity and what causes it?

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by Chris Woodford. Last updated: January 9, 2018. Zap! When a bolt of lightning leaps to the ground, we get a sudden, very vivid demonstration of the power of static electricity (electrical energy that has gathered in one place). Most of us know that static builds up when we rub things together, although that's not really a satisfying explanation.

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Static Electricity - Examples of charging because of friction; Static Discharge

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Electricity - The Big Picture!

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by Chris Woodford. Last updated: October 21, 2021.

If you've ever sat watching a thunderstorm, with mighty lightning bolts darting down from the sky, you'll have some idea of the power of electricity. A bolt of lightning is a sudden, massive surge of electricity between the sky and the ground beneath. The energy in a single lightning bolt is enough to light 100 powerful lamps for a whole day or to make about twenty thousand slices of toast! [1]

Electricity is the most versatile energy source that we have; it is also one of the newest: homes and businesses have been using it for not much more than a hundred years. Electricity has played a vital part of our past. But it could play a different role in our future, with many more buildings generating their own renewable electric power using solar cells and wind turbines. Let's take a closer look at electricity and find out how it works!

What is electricity?

Electricity is a type of energy that can build up in one place or flow from one place to another. When electricity gathers in one place it is known as static electricity (the word static means something that does not move); electricity that moves from one place to another is called current electricity.

Static electricity

Photo: Lightning happens when static electricity (built up in one place) turns to current electricity (flowing from one place to another).

Static electricity often happens when you rub things together. If you rub a balloon against your pullover 20 or 30 times, you'll find the balloon sticks to you. This happens because rubbing the balloon gives it an electric charge (a small amount of electricity). The charge makes it stick to your pullover like a magnet, because your pullover gains an opposite electric charge. So your pullover and the balloon attract one another like the opposite ends of two magnets.

Have you ever walked across a nylon rug or carpet and felt a slight tingling sensation? Then touched something metal, like a door knob or a faucet (tap), and felt a sharp pain in your hand? That is an example of an electric shock. When you walk across the rug, your feet are rubbing against it. Your body gradually builds up an electric charge, which is the tingling you can sense. When you touch metal, the charge runs instantly to Earth-and that's the shock you feel.

Lightning is also caused by static electricity. As rain clouds move through the sky, ice crystals inside them sink to the bottom, while water droplets rise to the top. The crystals have one kind of charge (negative) while the water droplets have the other kind (positive). It's the separation of these charges that allows a cloud to build up its power. Eventually, when the charge is big enough, it leaps to Earth as a bolt of lightning. You can often feel the tingling in the air when a storm is brewing nearby. This is the electricity in the air around you. Read more about this in our article on capacitors.

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How static electricity works

Electricity is caused by electrons, the tiny particles that "orbit" around the edges of atoms, from which everything is made. Each electron has a small negative charge. An atom normally has an equal number of electrons and protons (positively charged particles in its nucleus or center), so atoms have no overall electrical charge. A piece of rubber is made from large collections of atoms called molecules. Since the atoms have no electrical charge, the molecules have no charge either-and nor does the rubber.

Suppose you rub a balloon on your pullover over and over again. As you move the balloon back and forward, you give it energy. The energy from your hand makes the balloon move. As it rubs against the wool in your pullover, some of the electrons in the rubber molecules are pulled free and gather on your body. This leaves the balloon with slightly too few electrons. Since electrons are negatively charged, having too few electrons makes the balloon slightly positively charged. Your pullover meanwhile gains these extra electrons and becomes negatively charged. Your pullover is negatively charged, and the balloon is positively charged. Opposite charges attract, so your pullover sticks to the balloon.

That's a very brief introduction to static electricity. You'll find much more about it (and why it's caused by something called triboelectricity) in our main article on static electricity.

Photo: A classic demonstration of static electricity you may have seen in your school. When you touch the metal ball of a Van de Graaff static electricity generator, you receive a huge electric charge and your hair literally stands on end! Each strand of hair gets the same static charge and like charges repel, so the hairs push away from one another. In a bit more detail: the Van de Graaff ball builds up a huge positive charge. This "sucks" electrons (e) out of your body, and from the hairs in your head, leaving each clump of hair with a positive charge that repels the other hairs. Find out how a Van de Graaff Generator works.

Current electricity

When electrons move, they carry electrical energy from one place to another. This is called current electricity or an electric current. A lightning bolt is one example of an electric current, although it does not last very long. Electric currents are also involved in powering all the electrical appliances that you use, from washing machines to flashlights and from telephones to MP3 players. These electric currents last much longer.

Photo: Appliances use wires (cables) like this to carry electric current around inside them. The electricity travels through the brown-colored copper metal on the inside. The blue plastic coating on the outside protects you from the current if you touch the wire. It also stops two wires making electrical "contact" if they happen to touch.

Have you heard of the terms potential energy and kinetic energy? Potential energy means energy that is stored somehow for use in the future. A car at the top of a hill has potential energy, because it has the potential (or ability) to roll down the hill in future. When it's rolling down the hill, its potential energy is gradually converted into kinetic energy (the energy something has because it's moving). You can read more about this in our article on energy.

Static electricity and current electricity are like potential energy and kinetic energy. When electricity gathers in one place, it has the potential to do something in the future. Electricity stored in a battery is an example of electrical potential energy. You can use the energy in the battery to power a flashlight, for example. When you switch on a flashlight, the battery inside begins to supply electrical energy to the lamp, making it give off light. All the time the light is switched on, energy is flowing from the battery to the lamp. Over time, the energy stored in the battery is gradually turned into light (and heat) in the lamp. This is why the battery runs flat.

Picture: A battery like this stores electrical potential energy in a chemical form. When the battery is flat, it means you've used up all the stored energy inside by converting it into other forms.

Electric circuits

For an electric current to happen, there must be a circuit. A circuit is a closed path or loop around which an electric current flows. A circuit is usually made by linking electrical components together with pieces of wire cable. Thus, in a flashlight, there is a simple circuit with a switch, a lamp, and a battery linked together by a few short pieces of copper wire. When you turn the switch on, electricity flows around the circuit. If there is a break anywhere in the circuit, electricity cannot flow. If one of the wires is broken, for example, the lamp will not light. Similarly, if the switch is turned off, no electricity can flow. This is why a switch is sometimes called a circuit breaker.

You don't always need wires to make a circuit, however. There is a circuit formed between a storm cloud and the Earth by the air in between. Normally air does not conduct electricity. However, if there is a big enough electrical charge in the cloud, it can create charged particles in the air called ions ( atoms that have lost or gained some electrons). The ions work like an invisible cable linking the cloud above and the air below. Lightning flows through the air between the ions.

How electricity moves in a circuit

Materials such as copper metal that conduct electricity (allow it to flow freely) are called conductors. Materials that don't allow electricity to pass through them so readily, such as rubber and plastic, are called insulators. What makes copper a conductor and rubber an insulator?

A current of electricity is a steady flow of electrons. When electrons move from one place to another, round a circuit, they carry electrical energy from place to place like marching ants carrying leaves. Instead of carrying leaves, electrons carry a tiny amount of electric charge.

Electricity can travel through something when its structure allows electrons to move through it easily. Metals like copper have "free" electrons that are not bound tightly to their parent atoms. These electrons flow freely throughout the structure of copper and this is what enables an electric current to flow. In rubber, the electrons are more tightly bound. There are no "free" electrons and, as a result, electricity does not really flow through rubber at all. Conductors that let electricity flow freely are said to have a high conductance and a low resistance; insulators that do not allow electricity to flow are the opposite: they have a low conductance and a high resistance.

For electricity to flow, there has to be something to push the electrons along. This is called an electromotive force (EMF). A battery or power outlet creates the electromotive force that makes a current of electrons flow. An electromotive force is better known as a voltage.

Direct current and alternating current

Electricity can move around a circuit in two different ways. In the big picture up above, you can see electrons racing around a loop like race cars on a track, always going in the same direction. This type of electricity is called direct current (DC) and most toys and small gadgets have circuits that work this way.

Artwork: Top: In a direct current (DC) circuit, electrons always flow in the same direction. Bottom: In an alternating current (AC) circuit, the electrons reverse direction many times each second.

The bigger appliances in your home use a different kind of electricity called alternating current (AC). Instead of always flowing the same way, the electrons constantly reverse direction-about 50-60 times every second. Although you might think that makes it impossible for energy to be carried round a circuit, it doesn't! Take the flashlight bulb in the circuit above. With direct current, new electrons keep streaming through the filament (a thin piece of wire inside the bulb), making it heat up and give off light. With alternating current, the same old electrons whiz back and forth in the filament. You can think of them running on the spot, heating up the filament so it still makes bright light we can see. So both types of current can make the lamp work even though they flow in different ways. Most other electric appliances can also work using either direct or alternating current, though some circuits do need AC to be changed to DC (or vice versa) to work correctly.

Electromagnetism

Electricity and magnetism are closely related. You might have seen giant steel electromagnets working in a scrapyard. An electromagnet is a magnet that can be switched on and off with electricity. When the current flows, it works like a magnet; when the current stops, it goes back to being an ordinary, unmagnetized piece of steel. Scrapyard cranes pick up bits of metal junk by switching the magnet on. To release the junk, they switch the magnet off again.

Electromagnets show that electricity can make magnetism, but how do they work? When electricity flows through a wire, it creates an invisible pattern of magnetism all around it. If you put a compass needle near an electric cable, and switch the electricity on or off, you can see the needle move because of the magnetism the cable generates. The magnetism is caused by the changing electricity when you switch the current on or off.

This is how an electric motor works. An electric motor is a machine that turns electricity into mechanical energy. In other words, electric power makes the motor spin around-and the motor can drive machinery. In a clothes washing machine, an electric motor spins the drum; in an electric drill, an electric motor makes the drill bit spin at high speed and bite into the material you're drilling. An electric motor is a cylinder packed with magnets around its edge. In the middle, there's a core made of iron wire wrapped around many times. When electricity flows into the iron core, it creates magnetism. The magnetism created in the core pushes against the magnetism in the outer cylinder and makes the core of the motor spin around. Read more in our main article on electric motors.

Make an electromagnet

Picture: Why not make an electromagnet? All you need is a few common household items.

You can make a small electromagnet using a battery, some insulated (plastic-covered) copper wire, and a nail. Here are a couple of websites that tell you what to do step-by-step:

Making electricity

Just as electricity can make magnetism, so magnetism can make electricity. A dynamo is a bit like an electric motor inside. When you pedal your bicycle, the dynamo clipped to the wheel spins around. Inside the dynamo, there is a heavy core made from iron wire wrapped tightly around-much like the inside of a motor. The core spins freely inside some large fixed magnets. As you pedal, the core rotates inside these outer magnets and generates electricity. The electricity flows out from the dynamo and powers your bicycle lamp.

The electric generators used in power plants work in exactly the same way, only on a much bigger scale. Instead of being powered by someone's legs, pedaling furiously, these large generators are driven by steam. The steam is made by burning fuels or by nuclear reactions. Power plants can make enormous amounts of electricity, but they waste quite a lot of the energy they produce. The energy has to flow from the plant, where it is made, to the homes, offices, and factories where it is used down many miles of electric power cable. Making electricity in a power plant and delivering it to a distant building can waste up to two thirds of the energy that was originally present in the fuel!

Another problem with power plants is that they make electricity by burning "fossil fuels" such as coal, gas, or oil. This creates pollution and adds to the problem known as global warming (the way Earth is steadily heating up because of the energy people are using). Another problem with fossil fuels is that supplies are limited and they are steadily running out.

Photo: Making clean, renewable energy from the wind. Each of these turbines contains an electricity generator in the top section, just behind the spinning rotors.

There are other ways to make energy that are more efficient, less polluting, and do not contribute to global warming. These types of energy are called renewable, because they can last indefinitely. Examples of renewable energy include wind turbines and solar power. Unlike huge electric power plants, they are often much more efficient ways of making electricity. Because they can be sited closer to where the electricity is used, less energy is wasted transmitting power down the wires.

Wind turbines are effectively just electric generators with a "propeller" on the front. The wind turns the propeller, which spins the generator inside, and makes a study current of electricity.

Unlike virtually every other way of making electricity, solar cells (like the ones on calculators and digital watches) do not work using electricity generators and magnetism. When light falls on a solar cell, the material it is made from (silicon) captures the light's energy and turns it directly into electricity. Potentially, this means solar cells are an extremely efficient way to make electricity. A home with solar electric panels on the roof might be able to make most of its own electricity, for example.

Electricity and electronics

Photo: A transistor (a typical electronic component) on a circuit board. Components like this run on electricity, just like clothes washing machines, but they use much smaller currents and voltages.

Electricity is about using relatively large currents of electrical energy to do useful jobs, like driving a washing machine or powering an electric drill. Electronics is a very different kind of electricity. It's a way of controlling things using incredibly tiny currents of electricity-sometimes even individual electrons! Suppose you have an electronic clothes washing machine. Large currents of electricity come from the power outlet (mains supply) to make the drum rotate and heat the water. Smaller currents of electricity operate the electronic components in the washing machine's programmer unit. These tiny currents control the bigger currents, making the drum rotate back and forth, starting and stopping the water supply, and so on. Read more in our main article on electronics.

The power of electricity

Before the invention of electricity, people had to make energy wherever and whenever they needed it. Thus, they had to make wood or coal fires to heat their homes or cook food. The invention of electricity changed all that. It meant energy could be made in one place then supplied over long distances to wherever it was needed. People no longer had to worry about making energy for heating or cooking: all they had to do was plug in and switch on-and the energy was there as soon as they wanted it.

Another good thing about electricity is that it's like a common "language" that all modern appliances can "speak." You can run a car using the energy in gasoline, or you can cook food on a barbecue in your garden using charcoal, though you can't run your car on charcoal or cook food with gasoline. But electricity is quite different. You can cook with it, run cars on it, heat your home with it, and charge your cellphone with it. This is the great beauty and the power of electricity: it's energy for everyone, everywhere, and always.

Measuring electricity

We can measure electricity in a number of different ways, but a few measurements are particularly important.

Photo: You can use a digital multimeter like this to measure voltage, current, and resistance.

Voltage

The voltage is a kind of electrical force that makes electricity move through a wire and we measure it in volts. The bigger the voltage, the more current will tend to flow. So a 12-volt car battery will generally produce more current than a 1.5-volt flashlight battery.

Current

Voltage does not, itself, go anywhere: it's quite wrong to talk about voltage "flowing through" things. What moves through the wire in a circuit is electrical current: a steady flow of electrons, measured in amperes (or amps).

Power

Together, voltage and current give you electrical power. The bigger the voltage and the bigger the current, the more electrical power you have. We measure electric power in units called watts. Something that uses 1 watt uses 1 joule of energy each second.

The electric power in a circuit is equal to the voltage × the current (in other words: watts = volts × amps). So if you have a 100-watt (100 W) light and you know your electricity supply is rated as 120 volts (typical household voltage in the United States), the current flowing must be 100/120 = 0.8 amps. If you're in Europe, your household voltage is more likely 230 volts. So if you use the same 100-watt light, the current flowing is 100/230 = 0.4 amps. The light burns just as brightly in both countries and uses the same amount of power in each case; in Europe it uses a higher voltage and lower current; in the States, there's a lower voltage and higher current. (One quick note: 120 volts and 230 volts are the "nominal" or standard household voltages-the voltages you're supposed to have, in theory. In practice, your home might have more or less voltage than this, for all sorts of reasons, but mainly because of how far you are from your local power plant or power supply.)

Energy

Power is a measurement of how much energy you're using each second. To find out the total amount of energy an electric appliance uses, you have to multiply the power it uses per second by the total number of seconds you use it for. The result you get is measured in units of power × time, often converted into a standard unit called the kilowatt hour (kWh). If you used an electric toaster rated at 1000 watts (1 kilowatt) for a whole hour, you'd use 1 kilowatt hour of energy; you'd use the same amount of energy burning a 2000 watt toaster for 0.5 hours or a 100-watt lamp for 10 hours. See how it works?

Electricity meters (like the one shown in the photo above, from my house) show the total number of kilowatt hours of electricity you've used. 1 kilowatt hour is equal to 3.6 million joules (J) of energy (or 3.6 megajoules if you prefer).

You can measure your energy consumption automatically with an energy monitor.

A brief history of electricity

Picture: Nikola Tesla (1856-1943) pioneered the alternating current power system most of us use today. Even so, his rival, Thomas Edison (1846-1931), is still popularly remembered as the inventor who gave the world electric power. Photograph by Sarony; engraving by T. Johnson, c.1906, courtesy of US Library of Congress.

600 BCE: Greek philosopher Thales of Miletus (c.624-546 BCE) discovered static electricity.

1600 CE: English scientist William Gilbert (1544-1603) was the first person to use the word "electricity." He believed electricity was caused by a moving fluid called humor.

1733: French scientist Charles du Fay (1698-1739) found that there were two different kinds of static electric charge.

1752: American printer, journalist, scientist, and statesman Benjamin Franklin (1706-1790) carried out further experiments and named the two kinds of electric charge "positive" and "negative."

1780: Italian biologist Luigi Galvani (1737-1798) touched two pieces of metal to a dead frog's leg and made it jump. This led him to believe electricity is made inside animals' bodies.

1785: French scientist Charles Augustin de Coulomb (1736-1806) explored the mysteries of electric fields: the electrically active areas around electric charges.

1800: One of Galvani's friends, an Italian physics professor named Alessandro Volta (1745-1827), realized "animal electricity" was made by the metals Galvani had used. After further research, he found out how to make electricity by joining different metals together and invented batteries.

1827: German physicist Georg Ohm (1789-1854) found some materials carry electricity better than others and developed the idea of resistance.

1820: Danish physicist Hans Christian Oersted (1777-1851) put a compass near an electric cable and discovered that electricity can make magnetism.

1821: A French physicist called Andre-Marie Ampère (1775-1836) put two electric cables near to one another, wired them up to a power source, and watched them push one another apart. This showed electricity and magnetism can work together to make a force.

1821: Michael Faraday (1791-1867), an English chemist and physicist, developed the first, primitive electric motor.

1830s: American physicist Joseph Henry (1797-1879) and British inventor William Sturgeon (1783-1850) independently made the first practical electromagnets and electric motors.

1831: Building on his earlier discoveries, Michael Faraday invented the electric generator.

1840: Scottish physicist James Prescott Joule (1818-1889) proved that electricity is a kind of energy.

1870s: Belgian engineer Zénobe Gramme (1826-1901) made the first large-scale electric generators.

1873: James Clerk Maxwell (1831-1879), another British physicist, set out a detailed theory of electromagnetism (how electricity and magnetism work together).

1881: The world's first experimental electric power plant opened in Godalming, England.

1882: Thomas Edison (1846-1931) built the first large-scale electric power plants in the USA.

1890s: Edison's former employee Nikola Tesla (1856-1943) promoted alternating current (AC) electricity, a rival to the direct current (DC) system promoted by Edison. Edison and Tesla battled for supremacy and, although Edison is remembered as the pioneer of electric power, it was Tesla's AC system that ultimately triumphed.

DON'T ever play with electricity!

Electricity is amazingly useful-but it can be really dangerous as well. When electricity zaps from power plants to your home, it's at thousands of times higher voltages and massively more dangerous than the electricity in your home. If you are silly enough to touch or play near power equipment, you could die an extremely nasty and unpleasant death-electricity doesn't just shock you, it burns you alive. Heed warnings like this one and stay well away.

Electricity can also be dangerous in your home. Household electric power can kill you, so be sure to treat it with respect too. Don't play with household power sockets or push things into them. Don't take apart electrical appliances, because dangerous voltages can linger inside for a long time after they are switched off. If you want to know what something electrical looks like inside, search on the web-you'll find a safe answer that way.

It's generally okay to use small (1.5 volt) flashlight batteries for your experiments if you want to learn about electricity; they make small and safe voltages and electric currents that will do you no harm. Ask an adult for advice if you're not sure what's safe.

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Books

For younger children (ages 9-12)

The Shocking World of Electricity with Max Axiom Super Scientist by Liam O'Donnell. Capstone, 2019. A graphic-novel (comic) style introduction likely to appeal to reluctant readers. Ages 7-10.

Electricity (Science in a Flash) by Georgia Amson-Bradshaw. Hachette/Franklin Watts, 2017/2018. A clearly written 32-page guide for ages 7-9, with some basic hands-on activities and a helpful glossary.

Electricity for Young Makers: Fun and Easy Do-It-Yourself Projects by Marc de Vinck. Maker Media, 2017. A safe and friendly hands-on introduction in which you get to build a flashlight, a loudspeaker, and a couple of electric motors!

A Beginner's Guide to Electricity and Magnetism by Gill Arbuthnot. A&C Black/Bloomsbury, 2016. Another 64-page overview for ages 7-10.

Eyewitness: Electricity by Steve Parker. Dorling Kindersley, 2005. A classic glossy Eyewitness book that blends facts and history. Also worth investing in the same series:Eyewitness: Electronics by Roger Bridgman. Dorling Kindersley, 2007. This one takes a similar approach but covers electronics and electronic components.

Charged Up: The Story of Electricity by Jackie Bailey and Matthew Lilly. Picture Window Books/A & C Black, 2004. A humorous, cartoon-style tour through the history of electricity. (For some reason, it's also published under the title "Charging About.")

For older children (ages 10+)

MAKE Electronics by Charles Platt. O'Reilly, 2015. A great hands-on guide to learning about electronic components and circuits.

Electronic Gadgets for the Evil Genius by Roger Iannini. McGraw-Hill Education, 2013. There are quite a few "Evil Genius" books in this series that will appeal to budding young hackers keen to experiment with more advanced circuits.

Children's books by me

Scientific Pathways: Electricity by Chris Woodford. Rosen, 2013: A simple introduction to the history of electricity, from the ancient Greeks to modern times. This book aims to show how science and technology progresses from one discovery to the next, a bit like a relay race, through the work of many different people. (This is an updated version of a book originally published by Blackbirch in 2004 under the series title Routes of Science.)

Cool Science: Experiments with Electricity and Magnetism by Chris Woodford. Gareth Stevens, 2010: A 32-page, hands-on, practical approach to understanding electricity and magnetism.

History of electricity

Websites

Notes and references

↑ Calculations: A typical lightning bolt might release about 500-1000 megajoules (500-1000 million joules) of energy. A powerful, old-fashioned incandescent lamp uses 100 watts (100 joules per second). One lamp lit for 24 hours uses 24 hours × 3600 seconds × 100 joules/second = 8,640,000 joules or 8.6 megajoules. So 100 of these lamps would use 860 megajoules. If you're using energy-saving lamps, you could light up 5-10 more. What about toasters? Suppose your toaster uses 1000 watts (1000 joules per second) and takes 2 minutes to make two pieces of toast. There are 120 seconds in two minutes, so your toaster needs 1000 × 120 = 120,000 joules of energy for two pieces or 60,000 joules per piece. Dividing our 1000 million joule lightning bolt by 60,000, we get about 17,000.

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Advertisement by Chris Woodford. Last updated: October 21, 2021. If you've ever sat watching a thunderstorm, with mighty lightning bolts darting down from the sky, you'll have some idea of the power of electricity. A bolt of lightning is a sudden, massive surge of electricity between the sky and the ground beneath.

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Sound

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by Chris Woodford. Last updated: February 20, 2018.

Sound-it's almost impossible to imagine a world without it. It's probably the first thing you experience when you wake up in the morning-when you hear birds chirping or your alarm clock bleeping away. Sound fills our days with excitement and meaning, when people talk to us, when we listen to music, or when we hear interesting programs on the radio and TV. Sound may be the last thing you hear at night as well when you listen to your heartbeat and drift gradually into the soundless world of sleep. Sound is fascinating-let's take a closer look at how it works!

Photo: Sound is energy we hear made by things that vibrate. Photo by William R. Goodwin courtesy of US Navy.

What is sound?

Sound is the energy things produce when they vibrate (move back and forth quickly). If you bang a drum, you make the tight skin vibrate at very high speed (it's so fast that you can't usually see it), forcing the air all around it to vibrate as well. As the air moves, it carries energy out from the drum in all directions. Eventually, even the air inside your ears starts vibrating-and that's when you begin to perceive the vibrating drum as a sound. In short, there are two different aspects to sound: there's a physical process that produces sound energy to start with and sends it shooting through the air, and there's a separate psychological process that happens inside our ears and brains, which convert the incoming sound energy into sensations we interpret as noises, speech, and music. We're just going to concentrate on the physical aspects of sound in this article.

Sound is like light in some ways: it travels out from a definite source (such as an instrument or a noisy machine), just as light travels out from the Sun or a light bulb. But there are some very important differences between light and sound as well. We know light can travel through a vacuum because sunlight has to race through the vacuum of space to reach us on Earth. Sound, however, cannot travel through a vacuum: it always has to have something to travel through (known as a medium), such as air, water, glass, or metal.

Robert Boyle's classic experiment

The first person to discover that sound needs a medium was a brilliant English scientist known as Robert Boyle (1627-1691). He carried out a classic experiment that you've probably done yourself in school: he set an alarm clock ringing, placed it inside a large glass jar, and while the clock was still ringing, sucked all the air out with a pump. As the air gradually disappeared, the sound died out because there was nothing left in the jar for it to travel through.

Artwork: Robert Boyle's famous experiment with an alarm clock.

1. Put a ringing alarm clock inside a large glass case with a valve on top. Close the valve so no air can get in.

2. You can easily hear the clock ringing because the sound travels through the air in the case and the glass, before continuing to your ears.

3. Switch on the vacuum pump and remove the air from the case. As the case empties, the ringing clock sounds fainter and fainter until you can barely hear it at all. With little or no air in the case, there's nothing to carry the sound to your ears.

4. Switch off the pump. With the clock still ringing, open the valve on top of the case. As air rushes back in, you'll hear the clock ringing once again. Why? Because with air once again inside the case, there's a medium to carry the sound waves from the ringing clock to your ears.

How sound travels

When you hear an alarm clock ringing, you're listening to energy making a journey. It sets off from somewhere inside the clock, travels through the air, and arrives some time later in your ears. It's a little bit like waves traveling over the sea: they start out from a place where the wind is blowing on the water (the original source of the energy, like the bell or buzzer inside your alarm clock), travel over the ocean surface (that's the medium that allows the waves to travel), and eventually wash up on the beach (similar to sounds entering your ears). If you want to learn more about how sea waves travel, read our article on surfing science.

Artwork: Sound waves and ocean waves compared. Top: Sound waves are longitudinal waves: the air moves back and forth along the same line as the wave travels, making alternate patterns of compressions and rarefactions. Bottom: Ocean waves are transverse waves: the water moves back and forth at right angles to the line in which the wave travels.

There is one crucially important difference between waves bumping over the sea and the sound waves that reach our ears. Sea waves travel as up-and-down vibrations: the water moves up and down (without really moving anywhere) as the energy in the wave travels forward. Waves like this are called transverse waves. That just means the water vibrates at right angles to the direction in which the wave travels. Sound waves work in a completely different way. As a sound wave moves forward, it makes the air bunch together in some places and spread out in others. This creates an alternating pattern of squashed-together areas (known as compressions) and stretched-out areas (known as a rarefactions). In other words, sound pushes and pulls the air back and forth where water shakes it up and down. Water waves shake energy over the surface of the sea, while sound waves thump energy through the body of the air. Sound waves are compression waves. They're also called longitudinal waves because the air vibrates along the same direction as the wave travels.

To get the difference between transverse and longitudinal waves clear in your mind, take a look at these two little animations on Wikimedia Commons:

The science of sound waves

If you've ever got time on your hands while you're lazing on the beach, try watching the different ways in which waves can behave. You'll notice that waves traveling on water can do all kinds of clever things, like smashing into a wall and reflecting straight back with more or less the same intensity. They can also spread out in ripples, creep their way up the beach, and do other clever stuff. What's happening here with water waves doesn't actually have anything to do with the water: it's simply the way energy behaves when it's carried along by waves. Similar things happen with other kinds of waves-with light and with sound too.

You can reflect a sound wave off something the same way light will reflect off a mirror or water waves will bounce off a sea wall and go back out to sea. Stand some distance from a large flat wall and clap your hands repeatedly. Almost immediately you'll hear a ghostly repeat of your clapping, slightly out of step with it. What you hear is, of course, sound reflection, better known as an echo: it's the sound energy in your clap traveling out to the wall, bouncing back, and eventually entering your ears. There's a delay between the sound and the echo because it takes time for the sound to race to the wall and back (the bigger the distance, the longer the delay).

Picture: Reflected sound is extremely useful for "seeing" underwater where light doesn't really travel-that's the basic idea behind sonar. Here's a side-scan sonar (reflected sound) image of a World War II boat wrecked on the seabed. Photo courtesy of U.S. National Oceanographic and Atmospheric Administration and US Navy.

Sound waves lose energy as they travel. That's why we can only hear things so far and why sounds travel less well on blustery days (when the wind dissipates their energy) than on calm ones. Much the same thing happens on the oceans. Crisp water waves can sometimes travel vast distances across the ocean, but they can also be messed up when squally weather dissipates their energy over shorter distances.

Sound waves are like light and water waves in other ways too. When water waves traveling long distances across the ocean flow around a headland or into a bay, they spread out in circles like ripples. Sound waves do exactly the same thing, which is why we can hear around corners. Imagine you're sitting in a room off a corridor and, much further up the corridor, there's an identical room where someone is practicing a trumpet inside. Sound waves travel out from the trumpet, spreading out as they go. They ripple out down the corridor, race along it, ripple through the doorway into your room and eventually reach your ears. The tendency waves have to spread out as they travel and bend around corners is called diffraction.

Whispering galleries and amphitheaters

You might not think you could hear someone whispering if they sat a long way away, but if they can make the sound of their voice bounce off something into your ears, their voice will travel much further than usual.

If you're inside a building with a giant dome, the sounds you make will reflect off the curved roof like light rays bouncing off a mirror. Buildings that work this way are sometimes called whispering galleries. The dome of the US Capitol and the famous reading room in the British Museum in London are two well known examples. You can hear the same effect at work outside when you sit in a naturally curved area called an amphitheater. You can talk in a normal voice and still be heard very clearly a considerable distance away.

Photos: 1) The Capitol in Washington, DC has a whispering gallery inside its dome. Photo by courtesy of The Architect of the Capitol. 2) It's easy to hear people talking in this amphitheater in Iraq. Photo by Jason L. Andrade courtesy of US Marine Corps.

Measuring waves

All sound waves are the same: they travel through a medium by making atoms or molecules shake back and forth. But all sound waves are different too. There are loud sounds and quiet sounds, high-pitched squeaks and low-pitched rumbles, and even two instruments playing exactly the same musical note will produce sound waves that are quite different. So what's going on?

The energy something makes when it vibrates produces sound waves that have a definite pattern. Each wave can be big or small: big sound waves have what's called a high amplitude or intensity and we hear them as louder sounds. Loud sounds are equivalent to larger waves moving over the sea (except that, as you'll remember from up above, the air is moving back and forth, not up and down as the water does).

Apart from amplitude, another thing worth noting about sound waves is their pitch, also called their frequency. Soprano singers make sound waves with a high pitch, while bass singers make waves with a much lower pitch. The frequency is simply the number of waves something produces in one second. So a soprano singer produces more energy waves in one second than a bass singer and a violin makes more than a double bass.

Understanding amplitude and frequency

This artwork shows four waves that would sound different. If you're using a fairly up-to-date web browser, you can compare how they sound by playing the samples below. (If you don't see the sound clips, please try a different browser.)

1. The top wave represents a typical sound wave vibrating at a certain amplitude (its height) and frequency (how many peaks and troughs there are in a certain amount of time).
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2. This wave has the same frequency as the first wave (the same number of peaks and troughs) but twice the amplitude (it's twice as high). A sound wave like this would sound louder than the first wave but the same pitch.
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3. This wave has half the frequency of the second wave (half the number of peaks and troughs) but the same amplitude (it's exactly the same height). A sound wave like this would sound deeper (lower pitched) than the second wave, about as loud as the second wave, and louder than the first wave.
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4. This wave has twice the frequency of waves 1 and 2 and four times the frequency of wave 3, so it would sound much higher in pitch than the other waves. It has the same amplitude as waves 2 and 3, so it would sound just about as loud.
Your browser does not support the audio element.

Remember, however, that sound waves don't look like this as they travel. These up-and-down patterns are what you'll see if you study sound wave signals with an oscilloscope (a kind of electronic graph-drawing machine). Sound waves travel through the air as squashed-up compressions and stretched-out rarefactions. They only look like this on an oscilloscope trace.
Why instruments sound different

But here's a conundrum. If a violin and a piano make sound waves with the same amplitude and frequency, how come they sound so different? If the waves are identical, why don't the two instruments sound exactly the same? The answer is that the waves aren't identical! An instrument (or a human voice, for that matter) produces a whole mixture of different waves at the same time. There's a basic wave with a certain amplitude and pitch, called the fundamental, and on top of that there are lots of higher-pitched sounds called harmonics or overtones. Each harmonic has a frequency that's exactly two, three, four, or however many times higher than the fundamental. Every instrument produces a unique pattern of a fundamental frequency and harmonics, called timbre (or sound quality). All these waves add together to give a unique shape to the sound wave produced by different instruments, and that's one reason why they sound different. The other reason is that the amplitude of the waves made by a particular instrument changes in a unique way as the seconds tick by. Flute sounds are immediate and die quickly, while piano sounds take longer to build up and die out more slowly as well. You'll find a much longer explanation of this in our article about electronic music synthesizers.

The speed of sound

When we talk about the speed of sound, what exactly do we mean? Now you know that sound carries energy in a pattern of waves, you can see that the speed of sound means the speed at which the waves move-the speed at which the energy travels between two places. When we say that a jet airplane "breaks through the sound barrier," we mean that it accelerates so fast that it overtakes the incredibly high-intensity (that is, noisy) sound waves its engines are making, producing a horrible noise called a sonic boom in the process. That's why you'll see a fighter plane whizz overhead a second or two before you hear the vicious scream of its jet engines.

Photo: Breaking through the sound barrier creates a sonic boom. The mist you can see, which is called a condensation cloud, isn't necessarily caused by an aircraft flying supersonic: it can occur at lower speeds too. It happens because moist air condenses due to the shock waves created by the plane. You might expect the plane to compress the air as it slices through. But the shock waves it generates alternately expand and contract the air, producing both compressions and rarefactions. The rarefactions cause very low pressure and it's these that make moisture in the air condense, producing the cloud you see here. Photo by John Gay courtesy of US Navy.

The speed of sound in air (at sea level) is about 1220 km/h (760 mph or 340 meters per second). Compared to light waves, sound waves creep along at a snail's pace-about a million times slower. You see lightning much sooner than you hear it because the light waves reach you pretty much instantly, while the sound waves take about 5 seconds to cover each 1.6 km (1 mile).

One thing to note about the speed of sound is that there's really no such thing! Sound travels at different speeds in solids, liquids, and gases and even its speed in one material can change. Very roughly speaking, how fast it goes varies according to the density of the medium (how much stuff there is there). It's faster in solids than in liquids and faster in liquids than in gases: for example, sound travels about 15 times faster in steel than in air, and about four times faster in water than in air.That's why whales use sound to communicate over such long distances and why submarines use SONAR (sound navigation and ranging; a sound-based navigation system similar to radar only using sound waves instead of radio waves). It's also one of the reasons why it's very hard to figure out where the noise of a boat engine is coming from if you're swimming in the sea. (Note: The way sound travels through different materials is much more complex than this. A good explanation of why sound goes so fast in solids would mean I'd have to introduce a concept called phonons-and I'm going to skip that in this simple, introductory article, gloss over the complexities, and "pretend" that density is what makes the difference.)

Sound travels at different speeds in different gases-and can go at different speeds even in the same gas. It travels much faster in warm air near the ground than in colder air higher up, for example. And it travels roughly three times faster in helium gas than in ordinary air, because helium is much less dense. That's why people who breathe in helium talk in funny voices: the sound waves their voices make travel faster-with higher frequency.

Sound in practice

Photo: Musical sound is arguably humankind's greatest invention. This superb Steinway grand piano dates from 1876 and sits in the amazing gallery at the National Trust's Lanhydrock country house in Cornwall, England.

Sound is a hugely important part of life on Earth. Most animals listen out for noises-things that signal the possibility of eating or being eaten. Many creatures also exchange meaningful sounds, either to communicate with members of the same species or warn off predators and rivals. Humans have evolved this ability into spoken language (as a way of exchanging information) and music (essentially, a sound-based system for communicating emotion).

We've also developed a variety of different sound technologies. We've invented musical instruments that can make a huge range of different musical sounds, from simple drums and percussion instruments to sophisticated electronic synthesizers that can generate any sound you care to imagine. We can record sounds on such things as compact discs or with newer technologies like MP3 (sound files stored in highly compressed forms on computers). We can also use very high frequency sounds, known as ultrasound, for everything from cleaning false teeth to studying the development of a baby inside a mother's womb. We've even taught computers to listen to our spoken words and turn them into written language using voice recognition software-appropriately enough, that's how I wrote this article for you today!

Find out more

On this website

On other sites

Explore Sound: A comprehensive educational site from the Acoustical Society of America, with activities for students of all ages.

Sounds Amazing: A great collection of interactive science lessons from the University of Salford, which explains what sound waves are and the different ways in which they behave.

Books

Educational books for younger readers

Sound by Angela Royston. Raintree, 2017. A basic introduction to sound and musical sounds, including simple activities. Ages 7-9.

Sound (Science in a Flash) by Georgia Amson-Bradshaw. Franklin Watts/Hachette, 2017. Simple facts, experiments, and quizzes fill this book; the visually exciting design will appeal to reluctant readers. Also for ages 7-9.

Experimenting with Sound Science Projects by Robert Gardner. Enslow Publishers, 2013. A comprehensive 120-page introduction, running through the science of sound in some detail, with plenty of hands-on projects and activities (including welcome coverage of how to run controlled experiments using the scientific method). Ages 9-12.

Cool Science: Experiments with Sound and Hearing by Chris Woodford. Gareth Stevens Inc, 2010. One of my own books, this is a short introduction to sound through practical activities, for ages 9-12.

Popular science

Academic books

Master Handbook of Acoustics by F. Alton Everest and Ken Pohlmann. McGraw-Hill Education, 2015. A comprehensive reference for undergraduates and sound-design professionals.

The Science of Sound by Thomas D. Rossing, Paul A. Wheeler, and F. Richard Moore. Pearson, 2013. One of the most popular general undergraduate texts.

More to explore on our website...

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Sound - The science of waves, how they travel, how we use them

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by Chris Woodford. Last updated: February 20, 2018. Sound-it's almost impossible to imagine a world without it. It's probably the first thing you experience when you wake up in the morning-when you hear birds chirping or your alarm clock bleeping away.

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Wave Interference; Standing Wave

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Lenses (and refraction)

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by Chris Woodford. Last updated: May 7, 2017.

Microscopes let us peer inside invisible worlds our eyes could never see, telescopes take us far beyond the Earth to the stars and planets of the night sky, movie projectors throw enormous images onto screens, and lighthouses cast reassuring beams of light far across the ocean. Amazing curves of glass or plastic called lenses make all these things possible. Let's take a closer look at what they are and how they work!

Photo: Lenses in the headlamps of this car focus beams of light down onto the road so you can see where you're going. Some car headlights use Fresnel lenses to make powerful beams, just like lighthouses!

What are lenses?

A lens is a transparent piece of glass or plastic with at least one curved surface. It gets its name from the Latin word for "lentil" (a type of pulse used in cooking), but don't let that confuse you. There's no real reason for this other than that the most common kind of lens (called a convex lens) looks very much like a lentil!

Photo: Lentils gave lenses their name. Convex lenses bulge out in the middle like lentils, while concave lenses "cave in" in the middle and bulge out at the edges.

How do lenses work?

A lens works by refraction: it bends light rays as they pass through it so they change direction. (You can read a full explanation of why this happens in our article on light.) That means the rays seem to come from a point that's closer or further away from where they actually originate-and that's what makes objects seen through a lens seem either bigger or smaller than they really are.

Types of lenses

There are two main types of lenses, known as convex (or converging) and concave (or diverging).

Convex lenses

In a convex lens (sometimes called a positive lens), the glass (or plastic) surfaces bulge outwards in the center giving the classic lentil-like shape. A convex lens is also called a converging lens because it makes parallel light rays passing through it bend inward and meet (converge) at a spot just beyond the lens known as the focal point.

Photo: A convex lens makes light rays converge (come together) at the focal point or focus. The distance from the center of the lens to the focal point is the focal length of the lens.

Convex lenses are used in things like telescopes and binoculars to bring distant light rays to a focus in your eyes.

Concave lenses

A concave lens is exactly the opposite with the outer surfaces curving inward, so it makes parallel light rays curve outward or diverge. That's why concave lenses are sometimes called diverging lenses. (One easy way to remember the difference between concave and convex lenses is to think of concave lenses caving inwards.)

Photo: A concave lens makes light rays diverge (spread out).

Concave lenses are used in things like TV projectors to make light rays spread out into the distance. In a flashlight, it's easier to do this job with a mirror, which usually weighs much less than a lens and is cheaper to manufacture as well.

Compound lenses

It's possible to make lenses that behave in more complex ways by combining convex and concave lenses. A lens that uses two or more simpler lenses in this way is called a compound lens.

How do you measure the power of a lens?

If you've ever looked through binoculars, a telescope, or a magnifying glass, you'll know that some lenses magnify (or reduce) the apparent size of an object much more than others. There's a simple measurement that tells you how powerful a lens is and it's known as the focal length. The focal length of a lens is the distance from the center of the lens to the point at which it focuses light rays. The shorter the focal length, the more powerful the lens. (It's easy to see why: an ordinary piece of glass would be like a lens of infinite focal length and wouldn't bring light rays to a focus at all. On the other hand, an infinitely powerful lens would bring lays rays to a focus in an infinitely short distance, with zero focal length. A real lens is somewhere between these two extremes.)

You'll find focal lengths written either in ordinary units of length (such as centimeters, millimeters, or inches) or in special optical units called diopters. The diopter measurement of a lens is the reciprocal of the focal length in meters (one divided by the focal length), so 1 diopter = 1 m, 2 diopters = 0.5 m, 3 diopters = 0.33 meters, and so on. Eyeglass prescriptions from opticians typically show the strength of the corrective lenses you need in diopters.

The focal length isn't the only important feature of a lens. Bigger lenses gather more light than smaller ones, so they make a brighter image. This is particularly important if you're choosing a lens for a camera, because the amount of light the lens gathers will determine what the image looks like. Camera lenses are usually rated with a measurement called the f-number, which is the focal length divided by the diameter. Generally speaking, lenses with a small f-number make brighter images. Lenses with a higher f-number have a bigger depth of focus: essentially, more of the object you're photographing and its surroundings are in focus at the same time. (If you want to know more, take a look at Louis Bloomfield's explanation of lens size.)

How are lenses made?

Photo: This magnifying glass uses a single convex lens made from plastic.

Until plastics became common in the 20th century, virtually all lenses were made by grinding solid pieces of glass into different shapes. Convex lenses were made by using a concave-shaped grinding tool (and vice-versa), and then the roughly shaped lens was polished to make its final shape. The ordinary glass we use in windows and crockery isn't good enough to use for lenses, because it contains air bubbles and other imperfections. These cause light rays to divert from their correct path, making a fuzzy image or one that makes different colors of light behave in different ways (problems that optical scientists refer to as aberrations). Instead, lenses are made using a more refined material known as optical glass. For eyeglasses, many people now prefer plastic lenses because they're much lighter and safer than optical glass. Plastic lenses can be molded to shape, instead of being ground, so they can be made in huge quantities far more cheaply than glass lenses. Although ordinary plastic scratches easily, it can be coated with a thin layer of a protective material such as diamond-like carbon (DLC) to reduce the risk of damage. Some optical lenses are also coated with thin plastic to reduce annoying reflections; you can read how these anti-reflective coatings work in our article on thin-film interference.

Make a water lens!

Photo: I made this water lens by cutting a small piece of plastic from a grocery bag and laying it over a newspaper. I dropped the water on, very slowly and carefully, using a teaspoon.

Do this in your kitchen or bathroom to avoid making a mess.

1. Take an old newspaper or magazine no-one wants anymore.

2. Lay a small piece of cellophane, cling film, or clear plastic on top of the newspaper. You don't need much-maybe a piece half the size of a paperback book cover.

3. Using an eye-dropper, pipette, syringe, teaspoon, or even the tip of your pinkie, place a single, small drop of water on top of the cling film.

4. Look at the newsprint and you should be able to see that the water drop (which has a curved upper edge and a flat lower edge) magnifies the words.

5. Well done, you just made a lens!

6. What happens if you make the water drop bigger or smaller? What if you lift the plastic away from the paper and move your lens nearer or further from the print? What other cunning things can you do to change the way your lens works? Like all great scientists, take the chance to play around and experiment.

What are lenses used for?

Photo: You might not have noticed, but the tiny LEDs (light-emitting diodes) used in instrument panels have tiny plastic lenses built into them to magnify the light they produce. The lens is the curved plastic on the left (the top of the LED that shines toward you.)

Lenses are everywhere in the world around us-in everything from car headlamps and flashlights to the LED lights used in electronic instrument panels.

Our eyes contain probably the most amazing lenses of all. Think what happens when you look at the world around you. One minute you're staring at the ground in front of your feet. Seconds later, you hear an airplane screaming past, turn your head, and watch it fly by. Do this trick with a pair of binoculars and you'll find it takes you quite a while to adjust the focus from near-sight (looking at the ground) to far (watching the plane). Try it with the naked eye and you won't even notice what you're doing. That's because your eyes have flexible lenses, controlled by tiny muscles, that can bulge in and out, changing shape instantly to focus on anything from the prints on your finger to the surface of the Moon. How amazing is that?

Photos: Lighthouses don't use huge and heavy lenses: instead, they rely on Fresnel lenses (ones with a stepped surface pattern of concentric rings) and prisms, like the one in this exhibit at Think Tank, the science museum in Birmingham, England. Read how they work in our article on Fresnel lenses.

We all have lenses in our eyes, but many of us balance extra ones on the end of our noses to correct long and short sight: more glass and plastic lenses are used for eyeglasses and contact lenses than for any other purpose. There are all kinds of eyeglass lenses, including light-sensitive photochromic ones that darken in sunlight and double-up as sunglasses.

You'll also find lenses in binoculars (which use two or three lenses in each of the cylinders serving your eyes) and telescopes, though not all microscopes use them. Ordinary (optical) microscopes use a series of glass lenses to magnify tiny objects, while super-powerful electron microscopes use electromagnets to bend electron beams that help us see in even more detail. Movie projectors and projection televisions use lenses to convert small movie pictures into giant images that lots of people can view at once. Cameras work the opposite way, catching light rays from a distance and bringing them to focus on chemically treated plastic film or light-sensitive electronic chips called CCDs. You can even find lenses built into magazine and book covers to make images change as you shift your head from side to side; this cunning trick is called lenticular printing -but it really just means "printing with built-in lenses."

Find out more

On this website

We've got lots of other articles about optics, including:

For older readers

For younger readers

Light by David Burnie. Dorling Kindersley, 1998. An introduction to the science, technology, and history of light from the popular DK Eyewitness series. For ages 9-12.

Routes of Science: Light by Chris Woodford. Blackbirch, 2004. This is one of my own books, and it describes how scientists have understood light and put it to use throughout history. Suitable for ages 9-12.

Articles

The problems with lenses, and the 19th-century solution: This article from Cambridge University's Whipple Museum explains why lenses distort images and how inventors got around the problem to make the first microscopes.

Who Made That Contact Lens? by Daniel Engber. The New York Times. April 13, 2014. The idea of using artificial (contact) lenses instead of eyeglasses dates back to at least the 19th century.

Clearer Vision After Cataracts by Peter Jaret. The New York Times, May 15, 2009. The lenses in our eyes can degrade as we get older, turning cloudy as cataracts form. Fortunately, corrective lenses can solve the problem.

Other useful websites

Optics for Kids: Lots of good educational material from the Optical Society of America.

The MusEYEum: A museum in London, England run by the College of Optometrists. The website includes quite a few online exhibits that are worth a browse.

More to explore on our website...

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How do lenses work? | What are the different types of lens?

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by Chris Woodford. Last updated: May 7, 2017. Microscopes let us peer inside invisible worlds our eyes could never see, telescopes take us far beyond the Earth to the stars and planets of the night sky, movie projectors throw enormous images onto screens, and lighthouses cast reassuring beams of light far across the ocean.

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Reflection (Regular and Diffuse), Refraction and Diffraction

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AQA Core Physics Lesson 12 Reflection, Refraction and Diffraction

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Sound Waves

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Mr. Andersen explains how sound waves are created and perceived. A brief discussion of pitch and loudness are included. A generated sound of varying pitches is also included. Intro Music Atribution Title: I4dsong_loop_main.wav Artist: CosmicD Link to sound: http://www.freesound.org/people/CosmicD/sounds/72556/ Creative Commons Atribution License

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Visible Light

Select Source!

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Have you ever wondered what color is? In this first installment of a series on light, Colm Kelleher describes the physics behind colors-- why the colors we see are related to the period of motion and the frequency of waves.

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Light

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Light waves, visible and invisible - Lucianne Walkowicz

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Each kind of light has a unique wavelength, but human eyes can only perceive a tiny slice of the full spectrum -- the very narrow range from red to violet. Microwaves, radio waves, x-rays and more are hiding, invisible, just beyond our perception. Lucianne Walkowicz shows us the waves we can't see.

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Wave-Particle Duality of Light Go to Source!

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Is light a particle or a wave? - Colm Kelleher

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Can we accurately describe light as exclusively a wave or just a particle? Are the two mutually exclusive? In this third part of his series on light and color, Colm Kelleher discusses wave-particle duality and its relationship to how we see light and, therefore, color.

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Electromagnetic Spectrum

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Electromagnetic Waves

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Maxwell 's Equations and Electromagnetic Waves

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Accelerating Charges Emit Electromagnetic Waves

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Hertz Experiment on Electromagnetic Waves

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Properties of Waves; Transverse and Longitudinal

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Transverse and Longitudinal Waves | GCSE Physics | Doodle Science

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Properties of Waves; Electromagnetic and Sound Waves

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Transverse and Longitudinal Waves in Physics

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Elements of Physics: Waves; Sound and Electromagnetism

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Waves - Tsunamis

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The immense swell of a tsunami can grow up to 100 feet, hitting speeds over 500 mph -- a treacherous combination for anyone or anything in its path. Alex Gendler details the causes of these towering terrors and explains how scientists are seeking to reduce their destruction in the future.

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Mechanical Advantage & Pulleys

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Download/DVD: http://hilaroad.com/video/ This video uses working simple machines to demonstrate how to calculate mechanical advantage. Suitable for any school program involving simple machines. Demonstrates pulley systems including a chain block.

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Practice - Mechanical Advantage Problems (Check with me for answers!)

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Simple Machines (lots of links!)

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by Chris Woodford. Last updated: May 16, 2018.

It's built to last you a lifetime-quite literally. But though the human body is the most amazing tool at your disposal, it often needs a helping hand. Tools made from metal, wood, and plastic work like extensions of your body, making you feel stronger and helping you work faster and more efficiently. In science, tools like this are called simple machines. And although you might think there's a big difference between a tiny little wrench and a huge great earthmover, exactly the same physics is at work in both. Let's take a closer look at tools and machines and how they work!

Photo: This hydraulic digger uses a collection of simple machines (wheels, axles, and levers) to magnify the force its driver can exert.

What is a machine?

To do anything at all-to lift a box, to push a car, to get out of bed, to jump in the air, to brush your teeth-you need to use a pushing or pulling action called a force. If you go around telling people you're strong, what you really mean is that your body can apply a lot of force. You may have watched incredibly strong people on TV pulling trucks or trains with their bare hands, but there's a limit to what even the most muscle-bound human body can do. Simple machines let us go beyond that limit. Simple machines can make us all strong!

Photo: Thumbtacks (sometimes called drawing pins) are a bit like nails with built-in hammers. When you push on the large, flattened head, the force you apply (to the large flattened end) is effectively magnified because it's concentrated into a much smaller area at the tiny pin head. According to science, even thumbtacks are simple machines.

When you hear the word "machine", you probably think of something like a bulldozer or a steam locomotive. But in science, a machine is anything that makes a force bigger. So a hammer is a machine. A knife and fork are a pair of machines. And even an egg whisk is a machine. All these machines have one thing in common: when you apply a force to them, they increase its size and apply a greater force somewhere else. You can't cut meat with your hand alone, but if you push down on a knife, the long handle and the sharpened blade magnify the force you apply with your hand-and the meat slices effortlessly. When you pound a nail with a hammer, the handle increases the force you apply. And because the head of the hammer is bigger than the head of the nail, the force you apply is exerted over a smaller area with much greater pressure-and the nail easily enters the wood. Try pushing in a nail with your finger and you'll appreciate the advantage a hammer gives you.

There are five main types of simple machine: levers, wheels and axles (which count as one), pulleys, ramps and wedges (which also count as one), and screws. Let's look at them more closely.

Levers

A lever is the simplest machine of all: it's just a long bar that helps you exert a bigger force when you turn it. When you sit on a see-saw, you've probably figured out that you need to sit further from the balance point (known as the pivot point or fulcrum) if the person at the opposite end is heavier than you. The further away from the fulcrum you sit, the more you can multiply the force of your weight. If you sit a long way from the fulcrum, you can even lift a much heavier person sitting at the other end-providing they sit very close to the fulcrum on their side. The force you apply with your weight is called the effort. Thanks to the fulcrum, it produces a bigger force to lift the load (the weight of the other person). The words "effort" and "load" can be very confusing, so we've avoided using them in this article. The important thing to remember about levers is that the force you produce is bigger than the force you apply:

With a long lever, you can exert a lot of leverage. When you use an axe or a wrench, the long handle helps to magnify the force you can apply. The longer the handle, the more leverage you get. So a long-handled wrench is always easier to use than a short-handled one. And if you can't budge a nut or bolt with a short wrench, try one with a longer handle.

Photo: Two tools that are levers. Left: A garden fulcrum weeder (green, top) and a pipe wrench (red, bottom). The weeder is a class-1 lever, while the wrench is a class-2 lever (these terms are explained immediately below). Right: Here's the weeder in action. The built-in fulcrum makes it easy to lift weeds with a long, strong tap root.

Types of lever

Levers are all around us. Hammers, axes, tongs, knives, screwdrivers, wrenches, scissors-all these things contain levers. All of them give leverage, but not all of them work the same way. There are actually three different kinds of levers (sometimes known as classes).

Class-1 levers

In a class-1 lever, the force you apply is on the opposite side of the fulcrum to the force you produce. A see-saw is an example of a class-1 lever. So is a pair of scissors:

Class-2 levers

A class-2 lever is arranged a slightly different way, with the fulcrum at one end. You apply force at the other end and the force you produce is in the middle. Nutcrackers, garlic presses, and wheelbarrows are all examples of class-2 levers:

Class-3 levers

A class-3 lever is different again. Like a class-2 lever, it has the fulcrum at one end. But the two forces switch around. Now you apply the force in the middle and the force you produce is at the opposite end. Class-3 levers are unlike other machines in that they reduce the force you apply, giving you much greater control. Tweezers and tongs are an example of class-3 levers:

Pens are class-3 levers too: by pivoting them on our hands and holding them in the middle, we get much more control over the nib or ballpoint.

Wheels and axles

The invention of the wheel and axle (the rod around which a wheel turns), around 5500 years ago in the Middle East, revolutionized transportation and gradually brought huge changes to society, but what made it so special? It's easier to push a cart loaded with a heavy box than to push the same box along the ground because the cart's wheels and axles reduce friction and provide leverage. You can find out how in our main article on how wheels work.

Artwork: A wheel can work as either a force multiplier or a speed multiplier (but not both at the same time). If you turn the outside (rim) of a wheel, the axle at the center turns with less speed but more force, so the wheel works as a force multiplier. If you turn the axle instead (as a car does), the wheel becomes a speed multiplier. The axle turns only a short distance (blue arrow), but the leverage of the wheel means the outer rim turns much further (red arrow) in the same time. That's how a wheel helps you go faster.

Big wheels are used to multiply force in other ways too. Pipes, for example, have wheels called stopcocks (or stop valves) fitted to them. When you turn the outer rim of a stopcock, the inner axle turns with much greater force-so the pipe is easier to close. Steering wheels work this way too. A truck or a bus often has a bigger steering wheel than a car, because it takes more force to turn its wheels. The bigger wheel gives the driver more leverage.

Wheels can multiply distance and speed as well as force. Bicycles have large wheels so they go faster. When you pedal, you power the inside of the wheel. But the wheel's outer rim turns around faster and covers more ground, so your pedaling has a much greater effect. Car wheels work the same way.

Wheelbarrows combine wheels and levers to brilliant effect. A wheelbarrow makes it really easy to transport a load from one place to another-for two reasons. First, its long frame acts like a lever, so the load is much easier to lift. Second, it's easier to push the load using a wheelbarrow because the only friction is between the wheel and the axle. If you pushed the load across the rough surface of the ground without using a wheelbarrow, the friction would be much greater.

Gears

Photo: A gear is made from two or more wheels of different sizes with teeth cut into their edges to ensure they "mesh" (turn together without slipping).

Gears are wheels with teeth that can either increase the speed of a machine or its force, but not both at the same time. Bicycles use gears in both ways. If you want to pedal up a hill, you use gears to increase your force so you don't have to work quite so hard, although the catch is that they reduce your speed at the same time. If you're racing along a straight road, you can use gears to increase your speed, but this time the catch is that they'll reduce your force. Although it's not obvious just by looking at them, gears work in exactly the same way as levers (just as wheels do). That takes quite a bit of explaining so we won't go into more details here. Instead, you can read all about it in our gears article.

Pulleys

Put two or more wheels together and loop a rope around them several times and you create a powerful lifting machine called a pulley. Each time the rope wraps around the wheels, you create more lifting power or mechanical advantage. If there are four wheels and the rope wraps around them, the pulley works as though four ropes are supporting the load. So you can lift four times as much, although the catch is that you have to pull the rope four times further. Read more in our pulleys article.

Ramps and wedges

Artwork: The head of an axe works like a ramp. When it powers into wood, the wood splits apart along the diagonal. That means you can cut the wood by applying a smaller force over a larger distance. If you wanted to pull a log apart with your bare hands, you'd need to apply a much bigger force (though over a much shorter distance).

If you've ever helped pull a boat out of the sea, you'll know it's easier to do it if there's a ramp on the shore. Instead of lifting the boat vertically, straight up, you can get it out of the sea with much less force if you go up the ramp. You use less force, but you need to pull the boat a longer distance-so you use the same amount of energy in each case. Hillwalkers sometimes use the idea of a ramp to get to the top of a steep climb. By zig-zagging from side to side across their climb, they effectively create their own ramp. The hill becomes less steep, but they have to walk quite a bit further to get to the top.

Ramps are sometimes known as inclined planes or wedges. The head of an axe is a wedge working in a different way. An axe forces wood apart in two ways. The handle works like a lever, magnifying the force you apply. The wedge-shaped blade concentrates the force over a smaller area, increasing the pressure on the wood and splitting it apart. The blade of a knife works the same way.

Screws

Photo: The spiral thread on a screw means it takes longer to drive it into wood but-theoretically at least-you need less effort. The grooves also help the screw to remain in place.

A screw bites into wood when you turn it around. You often read science books that say a screw is "like a ramp wrapped around in a circle", which is pretty confusing and hard to understand. But imagine you're an ant and you want to climb from the bottom of a screw to the top. If you climb vertically up the outside, you go a relatively short distance but it takes an awful lot of climbing force. If you walk up the screw thread, winding around and around, you're really walking up a kind of spiral staircase-a ramp wrapped around in a circle. Yes, you walk much further, but it's a whole lot easier. There's another good thing about a screw too: because the head is bigger than the shaft beneath it, a screw works like a wheel (or lever): each time you turn the head, the sharpened point beneath bites into the wood with greater force. The tapering (cone-shaped) design makes it easier to drive in the screw.

Machines are all around us!

That's pretty much all there is to the science of simple machines. Once you understand how machines work, you start seeing them everywhere. Even your body is packed with machines. Your skeleton, for example, is a collection of levers! Take a look around your home and see how many more "simple machines" you can spot. You'll be amazed how many there are!

Find out more

On this website

Books

These are broadly particularly suitable for ages 7-12:

Simple machines

Forces

Can You Feel the Force by Richard Hammond. DK, 2007/2015. A sparky, humorous look at the physics of forces. (I was one of the consultants and contributors to this book.)

Force and Motion by Peter Lafferty. DK, 2000. One of the series of excellent Eyewitness volumes from Dorling Kindersley covering the history, science, and technology of forces in our world.

More to explore on our website...

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Nuclear Energy

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by Chris Woodford. Last updated: April 9, 2017.

Atomic energy has had a mixed history in the half-century or so since the world's first commercial nuclear power plant opened at Calder Hall (now Sellafield) in Cumbria, England in 1956. Huge amounts of world energy have been produced from atoms ever since, but amid enormous controversy. Some people believe nuclear power is a vital way to tackle climate change; others insist it is dirty, dangerous, uneconomic, and unnecessary. Either way, it helps if you understand what nuclear energy is and how it works-so let's forget the politics for a moment and take a closer look at the science.

Photo: Nuclear energy-the past or the future? Sleek modern solar panels in the foreground with the now-decommissioned Rancho Seco nuclear plant, Sacramento, California, right behind them. Will nuclear energy tide us over until we can convert the world to renewable energy? Or is it an expensive distraction? Photo by Warren Gretz courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

What is atomic energy?

Photo: Atoms have energy. Picture courtesy of Lawrence Livermore National Laboratory and US Department of Energy.

It's not immediately obvious but tall buildings store energy-potential energy. You have to work hard to lift bricks and other building materials up off the ground into the right position and, as long as they remain where you put them, they can store that energy indefinitely. But a tall, unstable building is bound to collapse sooner or later and, when it does so, the materials from which it was built come crashing back down to the ground, releasing their stored potential energy as heat, sound, and kinetic energy (the bricks could fall on your head!).

Atoms (the building blocks of matter) are much the same. Some large atoms are very stable and quite happy to stay as they are pretty much forever. But other atoms exist in unstable forms called radioactive isotopes. They're the atomic equivalents of wobbly old buildings: sooner or later, they're bound to fall apart, splitting into bits like a large building tumbling to the ground and releasing energy on the way. When large atoms split into one or more smaller atoms, giving off other particles and energy in the process, we call it nuclear fission. That's because the central part of the atom (the nucleus) is what breaks up and fission is another word for splitting apart. Nuclear fission can happen spontaneously, in which we case we call it radioactive decay (the conversion of unstable, radioactive isotopes into stable atoms that aren't radioactive). It can also be made to happen on demand-which is how we get energy out of atoms in nuclear power plants. That type of fission is called a nuclear reaction.

Photo: Carefully controlled: Before it was closed in the 1970s, NASA's scientific nuclear reactor at Plum Brook Station in Sandusky Ohio was used for developing materials for the space program. The site now does other kinds of cutting-edge space research. Picture courtesy of NASA Glenn Research Center (NASA-GRC).

How much energy can one atom make?

Photo: Albert Einstein-godfather of nuclear energy. Photo courtesy of US Library of Congress.

A surprisingly large amount! That was what physicist Albert Einstein meant when he wrote out this simple and now famous equation:

If E is energy, m is mass (the scientific word for the ordinary stuff around us), and c is the speed of light, Einstein's equation says that you can turn a tiny amount of mass into a huge amount of energy. How come? Looking at the math, c is a really huge number (300,000,000) so c2 is even bigger: 90,000,000,000,000,000. That's how many joules (the standard measurement of energy) you'd get from a kilogram of mass. In theory, if you could turn about seven billion hydrogen atoms completely to energy, you'd get about one joule (that's about as much energy as a 10-watt lightbulb consumes in a tenth of a second). Remember, though, these are just ballpark, guesstimate numbers. The only point we really need to note is this: since there are billions and billions of atoms in even a tiny spec of matter, it should be possible to make lots of energy from not very much at all. That's the basic idea behind nuclear power.

In practice, nuclear power plants don't work by obliterating atoms completely; instead, they split very large atoms into smaller, more tightly bound, more stable atoms. That releases energy in the process-energy we can harness. According to a basic rule of physics called the law of conservation of energy, the energy released in a nuclear fission reaction is equal to the total mass of the original atom (and all the energy holding it together) minus the total mass of the atoms it splits into (and all the energy holding them together). For a more detailed explanation of why nuclear reactions release energy, and how much they can release, see the article binding energy on Hyperphysics.

What is a chain reaction?

What if you could make lots of atoms split up one after another? In theory, you could get them to release a huge amount of energy. If breaking up billions of atoms sounds like a real bore (like breaking billions of eggs to make an omelet), there's one more handy thing that helps: some radioactive isotopes will go on splitting themselves automatically in what's called a chain reaction, producing power for pretty much as long as you want.

Suppose you take a really heavy atom-a stable kind of uranium called uranium-235. Each of its atoms has a nucleus with 92 protons and 143 neutrons. Fire a neutron at uranium-235 and you turn it into uranium-236: an unstable version of the same atom (a radioactive isotope of uranium) with 92 protons and 144 neutrons (remember that you fired an extra one in). Uranium-236 is too unstable to hang around for long so it splits apart into two much smaller atoms, barium and krypton, releasing quite a lot of energy and firing off three spare neutrons at the same time.

Now the brilliant thing is that the spare neutrons can crash into other uranium-235 atoms, making them split apart too. And when each of those atoms splits, it too will produce spare neutrons. So a single fission of a single uranium-235 atom rapidly becomes a chain reaction-a runaway, nuclear avalanche that releases a huge amount of energy in the form of heat.

Photo: Chain reaction! Fire a neutron (1) at a large uranium-235 atom (2). You make an even larger, unstable radioactive isotope of uranium, uranium-236, that promptly splits into two smaller and more stable atoms krypton and barium (3). In the process, heat energy is released and there are three spare neutrons left over (4). The neutrons can go on to react with more uranium-235 atoms (5) in a hugely energetic chain reaction. Other fission reactors are possible when a neutron hits uranium-235, producing either two or four spare neutrons. That's why (confusingly) you'll sometimes read in books that uranium-235 fission produces "two or three" spare neutrons (and an average of 2.47) per reaction.

What's the difference between a nuclear power plant and a nuclear bomb?

In a nuclear bomb, the chain reaction isn't controlled, and that's what makes nuclear weapons so terrifyingly destructive. The entire chain reaction happens in a fraction of a second, with one splitting atom producing two, four, eight, sixteen, and so on, releasing a massive amount of energy in the blink of an eye. In nuclear power plants, the chain reactions are very carefully controlled so they proceed at a relatively slow rate, just enough to sustain themselves, releasing energy very steadily over a period of many years or decades. There is no runaway, uncontrolled chain reaction in a nuclear power plant.

How does a nuclear power plant work?

Okay, we've figured how to get energy from an atom, but the energy we've got isn't that helpful: it's just a huge amount of heat! How do we turn that into something much more useful, namely electricity? A nuclear power plant works pretty much like a conventional power plant, but it produces heat energy from atoms rather than by burning coal, oil, gas, or another fuel. The heat it produces is used to boil water to make steam, which drives one or more giant steam turbines connected to generators -and those produce the electricity we're after. Here's how:

1. First, uranium fuel is loaded up into the reactor-a giant concrete dome that's reinforced in case it explodes. In the heart of the reactor (the core), atoms split apart and release heat energy, producing neutrons and splitting other atoms in a carefully controlled nuclear reaction.

2. Control rods made of materials such as cadmium and boron can be raised or lowered into the reactor to soak up neutrons and slow down or speed up the chain reaction.

3. Water is pumped through the reactor to collect the heat energy that the chain reaction produces. It constantly flows around a closed loop linking the reactor with a heat exchanger.

4. Inside the heat exchanger, the water from the reactor gives up its energy to cooler water flowing in another closed loop, turning it into steam. Using two unconnected loops of water and the heat exchanger helps to keep water contaminated with radioactivity safely contained in one place and well away from most of the equipment in the plant.

5. The steam from the heat exchanger is piped to a turbine. As the steam blows past the turbine's vanes, they spin around at high speed.

6. The spinning turbine is connected to an electricity generator and makes that spin too.

7. The generator produces electricity that flows out to the power grid-and to our homes, shops, offices, and factories.

Can a nuclear power plant explode like a nuclear bomb?

Artwork: Nuclear explosion: Oil painting of a Pacific nuclear test at Bikini Atoll in the 1950s by war artist Charles Bittinger courtesy of the US Naval History and Heritage Command, (classified as public domain).

One reason many people oppose nuclear power is because they think nuclear plants are like enormous nuclear bombs, just waiting to explode and wipe out civilization. It's true that nuclear plants and nuclear bombs are both based on nuclear reactions in which atoms split apart, but that's generally where the similarity begins and ends.

To start with, very different grades of uranium are used in power plants and nuclear bombs (some bombs use plutonium, but that's another story). Bombs need extremely pure (enriched) uranium-235, which is made by removing contaminants (notably another isotope of uranium, uranium-238) from naturally occurring uranium. Unless the contaminants are removed, they stop a nuclear chain reaction from occurring. Power plants can work with less purified, much more ordinary uranium providing they add another substance called a moderator. The moderator, typically made of carbon or water, effectively "converts" the less pure uranium so it will allow a chain reaction to happen. (I won't go into the details here, but it works by slowing down neutrons so they are less readily absorbed by any uranium-238 impurities and have a greater chance of causing fission in the all-important uranium-235.) All we really need to know about the moderator is that it makes a chain reaction possible in relatively impure uranium-and without it the reaction stops.

So what happens if the reaction inside a power plant starts to run out of control? If that happens, so much energy is released that the reactor overheats and may even explode-but in a relatively small, entirely conventional explosion, not an apocalyptic nuclear bomb. In that situation, the moderator burns or melts, the reactor is destroyed, and the nuclear reaction stops; there is no runaway chain reaction. The worst situation is called a meltdown: the reactor melts into a liquid, producing a hot, radioactive glob that drops deep down into the ground, potentially contaminating water supplies.

There are various other important differences that stop nuclear power plants from turning into nuclear bombs. In particular, nuclear bombs have to be assembled in a very precise way and detonated so that they implode (pushing the nuclear material together so it reacts properly). These conditions don't occur in a nuclear power plant.

A different kind of power plant called a fast-breeder reactor works a different way, producing its own plutonium fuel in a self-sustaining process. Its chain reaction is much closer to what happens in a nuclear bomb and it doesn't work through a moderator. That's why a fast-breeder reactor could, theoretically, run out of control and cause a nuclear explosion.

Photo: Nuclear nightmare: In the days following the Chernobyl nuclear power explosion in the Ukraine in 1986, a cloud of radioactive "fallout" spread throughout Europe. In this sequence of pictures, you can see the cloud (the pink area) on day 2, day 6, and day 10 after the accident. It's important to note that what happened here was a conventional explosion that threw radioactive material high into the air: it wasn't anything like a nuclear bomb. Pictures by Lawrence Livermore National Laboratory courtesy of US Department of Energy.

Nuclear power-good or bad?

"There are currently 449 operating nuclear reactors in 31 countries, with a total installed generating capacity of more than 390,000 megawatts (MW)" (390GW)US Energy Information Administration, 2017

There are plenty of people who support our use of nuclear power, and at least as many who oppose it. Supporters say it's a less environmentally destructive way of producing electrical energy because, overall, it releases fewer greenhouse emissions (less carbon dioxide gas) than burning fuels such as coal, oil, and natural gas. But opponents are concerned about the dangerous, long-lasting waste that nuclear power stations make, the way nuclear-energy byproducts help people build nuclear bombs, and the risk of catastrophic nuclear accidents. Here's a quick summary of the pros and cons. What do you think? Do the advantages outweigh the disadvantages?

Pros

One nuclear plant will make 2-3GW of power-as much as a large coal plant or about 1000-1500 large wind turbines working at full capacity. No-one disagrees that nuclear is a very effective way to generate enormous amounts of energy.

Nuclear plants produce much lower carbon emissions than fossil fuel plants (coal, oil, and natural gas).

It's much more efficient to release energy by smashing atoms apart than by "burning them" (releasing energy through the chemical reaction we call combustion). That's why nuclear plants need tiny amounts of fuel (compared to fossil fuel plants).

Nuclear plants can help to reduce a country's dependence on imported oil from unstable regions such as the Middle East. Countries without large fossil fuel supplies find nuclear an attractive option.

Chart: Nuclear plants (orange slice) supply about 8 percent of the energy used in the United States (that's all energy, not just electricity). Fossil fuels (gray slices) supply 10 times more. Source: Energy Information Administration, US Department of Energy, 2015 figures.

Cons

Waste from nuclear plants remains dangerously radioactive for many years, so it's difficult to dispose of safely.

Nuclear byproducts can be used to make bombs and there's a risk of nuclear material being acquired by terrorists.

Nuclear plants aren't sustainable or renewable forms of energy, because they rely on mining limited reserves of uranium. They're not zero-carbon either, because it takes a lot of energy to mine that uranium.

Nuclear plants are expensive and take many years to construct, usually in the face of fierce public opposition.

Nuclear plants can produce long-range air pollution andwater pollution.

Since nuclear plants need huge amounts of cooling water, they're often built by the coast-but that makes them dangerously susceptible to rising sea levels and earthquake tsunamis.

At the end of their life, nuclear plants are very expensive to decommission safely.

Find out more

On this website

You might like these other articles on our site covering related topics:

Other websites

The world's largest nuclear plants differ by age, number of reactors, and utilization: A good summary of the current state of world nuclear power from the US Energy Information's Today in Energy blog, February 6, 2017.

Nuclear power stations and reactors operational around the world: listed and mapped: Where are all the world's nuclear power plants? What type of reactor do they use? When were they first opened? This March 2011 article from The Guardian presents a list of all the world's (non-scientific) reactors and plots them on a Google Map.

Nuclear Reactors and Nuclear Bombs: What Defines the Differences?: PBS Newshour, April 6, 2011. Explains the differences in the uranium fuel used by reactors and bombs.

Nuclear Reactors, the China Syndrome, and Waste Storage by Richard Muller. This is a brief online introduction to nuclear energy-effectively a shortened version of the ideas covered in Richard's book "Physics for Future Presidents," listed below. (Archived link via the Wayback Machine.)

Hyperphysics: Nuclear: A series of good, short introductions to nuclear physics and its various applications.

Articles

Books

It's relatively easy to find books and websites that are stridently pro- or anti- nuclear power-so I've left those off my reading list. The ones listed here are much more neutral, hopefully more objective and educational, and present a fairly balanced mixture of the various arguments based on science and evidence. Don't let people tell you what to think: learn the science-and make up your own mind!

For older readers

Nuclear Power: Both Sides by Michio Kaku and Jennifer Trainer (eds). W.W. Norton, 1984. Arguments for and against nuclear power in 21 essays packed into one handy book.

Nuclear Or Not?: Does Nuclear Power Have a Place in a Sustainable Energy Future? by Prof. David Elliott (ed). Palgrave Macmillan, 2009. An objective review of the arguments for and against nuclear power, presented in a series of essays.

Nuclear Power: A Very Short Introduction by Maxwell Irvine. Oxford University Press, 2011. A slim (144-page) volume designed to cut through the heated arguments about nuclear power.

Nuclear?: In this chapter from his book Sustainable Energy Without the Hot Air, physicist David MacKay considers how nuclear power stacks up as a sustainable power source.

Physics for Future Presidents by Richard Muller. New York, W.W. Norton, 2008. Quite a lot of Richard Muller's excellent book is devoted to different nuclear topics, from weapons and waste to fission and fusion. Although pro-nuclear, it sets out its arguments in a very measured way and you can take them or leave them. There is a very good assessment of whether nuclear energy and waste is as dangerous as some people think when you consider other types of risks.

For younger readers

Nuclear Power: Is it too risky? by Jim Pipe. Franklin Watts, 2010. A short (32-page) book with arguments for and against all the different types of energy, including nuclear, presented side by side.

Nuclear Power: Energy Debate by Ewan Mcleish. Wayland, 2009. A short (48-page) library-style volume presenting another balanced look at the pros and cons of nuclear power.

Energy by Chris Woodford. New York/London, England: Dorling Kindersley, 2007: This is my own very colorful little introduction to the world of energy for ages 9-12 or so. Nuclear is considered as one of many different types of energy.

Power and Energy by Chris Woodford. New York: Facts on File, 2004. Another one of my energy books. This is a more detailed and more wordy one suitable for ages from about 10-16, and with more emphasis on how humans have harnessed different energy sources through the ages.

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