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by Maggie Twaddle

This report describes the use of solar panels to make electricity and includes an example of a school that has been using solar power since 2008. Students from the school share what they have learnt about solar power, including the fact that solar power is a form of renewable energy.

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Your browser does not support the audio element. by Maggie Twaddle This report describes the use of solar panels to make electricity and includes an example of a school that has been using solar power since 2008. Students from the school share what they have learnt about solar power, including the fact that solar power is a form of renewable energy.

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Electricity for Kids | What is Electricity? Where does Electricity come from?

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Electricity is the flow of tiny particles called electrons and protons. It can also mean the energy you get when electrons flow from place to place. Electricity can be seen…

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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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Related articles on our site

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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Electricity for kids - and everyone else: A simple introduction!

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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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Britannica Kids - Electricity second link

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Britannica Kids - Electricity second link

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Electricity is a form of energy associated with the atomic particles called electrons and protons. In particular, electricity involves the movement or accumulation of negatively charged electrons in relation to positively charged protons. The world's modern economies, with their industrial, transportation, and communication systems, were made possible by electricity. Old energy forms, such as water and steam, imposed limitations on production-limitations on where goods could be produced and on how much could be produced. Electricity has few such limits: it can go anywhere, even far into space.

The development of electricity has resulted in the total transformation of civilization. It brings power into homes to operate lights, kitchen appliances, television sets, radios, furnaces, computers, garage doors, and more. So common are its uses that one cannot imagine today's world without it. Streets would not be lit. Telephones would not work. Storefronts and factories would be dimmed.

Electrical forces are also responsible for holding body cells together in the shape they have. In fact, electrical forces are fundamental in holding all matter together. As printed words are being read, electric currents speed along nerve cells from eye to brain. The effect of an electric current can be seen in the flash of lightning between thundercloud and Earth as well as in the spark that can be produced when one walks on a carpet in a dry room.

Understanding electricity begins with describing its effects. One way to begin is to examine interactions that occur when electricity is at rest, in a form called static electricity. Static electricity can be seen at work when hair is combed on a cold, dry day. As the comb is pulled through the hair, strands of hair stand out stiffly. Some kind of force seems to pull the hair upward toward the comb. To understand the nature of this force it is necessary to know something about the concept of electric charge and the structure of atoms.

Simple experiments can illustrate how electric charge works. If a glass rod is rubbed with silk and touched to a small sphere of aluminum foil suspended by a thread, the sphere moves away from the glass rod. The rod and sphere repel each other. If the process is repeated with a second sphere and the spheres are brought near each other, they too repel. If a plastic rod is rubbed with wool and brought near either sphere, the spheres move toward the plastic rod. These objects attract each other. If two new aluminum foil spheres are touched to the plastic rod, they are repelled by the plastic, as well as by each other. But they are attracted to the glass rod and the spheres touched by the glass rod.

These experiments can be explained by a two-charge model. Rubbing the glass with silk causes the glass to acquire a positive charge. When touched to the spheres it shares some of its positive charge with the spheres and these objects repel. Rubbing the plastic with wool causes the plastic to acquire a negative charge. When touched to the spheres it shares some of its negative charge with the spheres and these objects repel.

This interpretation of the experiment leads to the conclusion that like charges repel. But when a positively charged sphere is brought near a negatively charged plastic rod or sphere, the objects attract each other. And when a negatively charged sphere is brought near a positively charged glass rod, the objects attract each other. This leads to the conclusion that unlike charges attract. No matter how an object is charged, if it attracts the negatively charged sphere, it will also repel the positively charged sphere, and vice versa. This leads to the belief that there are only two kinds of electric charge.

Electric Charges in Atoms
A model of matter is needed to explain how only two kinds of charged rods can be produced. Atoms contain two kinds of charge, which are arbitrarily called positive and negative. Every atom is composed of a positively charged nucleus around which are distributed negatively charged electrons. Each nucleus contains a specific number of protons-particles that carry the positive charge. (With the exception of the hydrogen atom, nuclei also contain uncharged neutrons.) In an uncharged atom there are equal numbers of protons and electrons, and such an atom is said to be neutral. If a neutral atom loses one or more of its electrons, it has an excess number of protons and it is positively charged. If a neutral atom gains one or more electrons, it has an excess number of electrons and it is negatively charged. In either case it is called an ion.

How does the simple atomic model relate to the static electricity experiments? Rubbing action creates charged objects because it tears electrons loose from some kinds of atoms and transfers them to others. In the case of plastic rubbed with wool, electrons are taken from the wool and pile up on the plastic, giving the plastic a net negative charge and leaving the wool charged positively. When glass is rubbed with silk, the glass loses electrons and the silk gains electrons, making the glass positively charged and the silk negatively charged.

Electric Force

Encyclopædia Britannica, Inc.

Two electrically charged objects exert a force upon each other. This force, as well as the basic unit of electric charge, is named after the French physicist Charles-Augustin de Coulomb. In the 18th century Coulomb formulated a law for determining the strength of the electric force between two charged objects. For two point charges (a point charge is a charged sphere whose radius is very small compared to its distance from a second point charge), carrying charges q ₁ and q ₂, whose centers are separated by distance d, the electric force F is determined by:

In this equation, k is a constant, meaning that it is generally assumed to have the same value at all places and at all times. If the charges are measured in coulombs ( C) and the distance in meters ( m), the electric force can be calculated in newtons ( N) with the conversion constant:

Electrostatic Induction
The fact that electrical force decreases rapidly as the distance between the charges increases is important in explaining another observation about static electricity. If a charged rod (whether positive or negative) is brought near some uncharged bits of paper, the paper is initially attracted to the rod. How can this happen? When a positively charged rod is brought near a neutral scrap of paper, the electrons in each atom of the paper are drawn somewhat toward the rod, and the nuclei, which are positive, are pushed slightly away. This repositioning of the charges in the neutral scrap of paper is called electrostatic induction.

Because the electrons on the average will be slightly closer to the positive rod than to the nuclei, the force of attraction will be somewhat larger than the force of repulsion. Thus the paper experiences a net attractive force, and it is drawn toward the rod. A negatively charged rod will also attract uncharged bits of paper, but the repositioning of charges in the paper is reversed. Again, the attractive force will be somewhat larger than the repelling force.

The principle of electrostatic induction can be seen at work in industry and in nature. Electrostatic air cleaners attract neutral dust particles to a charged screen by induction. The electrostatic scrubbers used to clean the smoke produced by coal-burning power plants use the same principle on a larger scale. Charging by induction also occurs when the lower, negatively charged regions of thunderclouds induce a positive charge on Earth's surface. If the charges become large enough, the resistance of the air is overcome and lightning occurs.

Charged objects can exert forces on uncharged objects over a distance. The electric field provides a way to describe the effect of the electric force at points in space around an electric charge. The electric field strength at a point, E, is defined as the ratio of the electric force F on a test charge to the size of the test charge, q test, placed at that point:

(A test charge is an infinitesimal charge placed in an electric field to probe the strength of the field.) This formula defines a specific value for E at each point in space. Regardless of the size of the test charge, the ratio of electric force to q test will be a particular value at each position in the field.

Strength
The strength of an electric field is a vector quantity. Vectors have direction as well as magnitude. An arrow can be used to represent the electric field strength: the stronger the field, the longer the arrow. The direction of the electric field vector is taken to be the same as the direction of the electric force on a positive test charge placed in the field. If the separate electric field vectors for many points in space are joined, lines are obtained that give an overview of the electric field. These lines, called lines of force, were conceived by the 19th-century English scientist Michael Faraday. Where these lines are more concentrated, the field is stronger and the electric force on a test charge will be larger. If a positive test charge is placed somewhere in the field, the force on that charge will be directed along a line tangent to the field line.

An especially simple electric field occurs in the space between two oppositely charged flat plates. The field lines are equally spaced between the plates, showing that the electric field strength is the same everywhere. Such a field is called a uniform electric field. An electron placed in such a field at any spot in the field will accelerate at a constant rate toward the positive plate because the electrical force on it is constant. (It should be noted that the electron, which is negatively charged, moves in a direction opposite to that of the field lines.) If an electron enters a uniform field parallel to the plates, it will veer toward the positive plate. The stronger the field is, the more the deflection.

Potential Difference
A positive test charge placed near a fixed positive point charge will accelerate away, increasing in velocity and kinetic energy (energy of motion). Conversely, to move this positive test charge back toward the fixed positive charge, work must be done on the test charge. The energy put into this process is stored as electric potential energy by virtue of the new position of the test charge in the field. Electric potential difference is a measure of this change in energy as the charge moves from one place to another in an electric field. Mathematically it is given by:

The unit for measuring electric potential difference is the volt, which is a joule of energy for every coulomb of charge. Sometimes the electric potential difference is called voltage. When there is no difference in electrical potential-that is, when there is zero voltage-between points in a field, there is no tendency for electric charge to move between those points. On the other hand, if there is a large potential difference between two points in a field, positive electric charge will tend to move from higher to lower potential; negative charge would move the opposite way.

Moving Charges in Electric Fields
An electrochemical cell, or battery, provides a familiar example to illustrate the movement of charges in an electric field. A dry cell battery rated at 1.5 volts has an electric potential at the positive terminal that is 1.5 volts above that of the negative. Frequently this potential difference is called the emf, or electromotive force, of the battery, but this name is misleading because the electric potential difference is not really a force; the 1.5-volt rating indicates the energy change per coulomb of electric charge moved between the terminals. Each coulomb of charge moved between the terminals acquires 1.5 joules of energy.

When connected by a conductor-a material that does not inhibit the motion of electrons-electrons move away from the negative terminal (-) toward the positive terminal (+) through the conductor. The electrons move in response to the electric field set up in the conductor. When the terminals of the battery are connected by an insulator-a material in which electron motion is inhibited-the electrons in the insulating medium are not moved very much. Because air is an insulator, electric charge does not move between the terminals of the dry cell until they are connected by a conductor.

If the potential difference is very high, electric charge may be moved through the field even without a good conductor. In the picture tube of a traditional television set, for example, electrons ejected from a heated electrode, called the cathode, are accelerated by a very high voltage (10,000 to 50,000 volts) and fly through an evacuated tube, crashing into a screen coated with a fluorescent material to produce the bursts of light that are seen as the picture. Such a tube is known as a cathode-ray tube, or CRT. (Moving electrons can be called "cathode rays.") Cathode-ray tubes have many other applications-for example, in the oscilloscopes used by medical personnel for displaying heartbeat and brain-wave data.

A Simple Battery
A simple battery can be made from strips of zinc (Zn) and copper (Cu) metal suspended in a salt solution, which is a conductor. Prior to connecting the strips by a conductor, a dynamic equilibrium exists at each metal surface. Some zinc atoms lose a pair of electrons, becoming Zn ²⁺ ions. The electrons remain on the zinc metal, while the Zn ²⁺ moves into the solution. The reverse process occurs at an equal rate: Zn ²⁺ ions gain two electrons and adhere to the zinc strip as zinc atoms. A similar equilibrium exists at the copper surface, involving copper metal and Cu ²⁺ ions.

When the metals are connected by a conductor, these equilibria are thrown out of balance. Because zinc atoms lose electrons easier than copper atoms do, electrons are forced through the conductor from the zinc strip to the copper strip. As electrons leave the zinc, net formation of Zn ²⁺ ions occurs at the zinc strip. At the copper strip, Cu ²⁺ ions gain electrons, becoming copper metal. As electrons move between the zinc and copper strips through a wire outside the cell, positive ions in the solution migrate away from the zinc strip, and negative ions move away from the copper strip. This keeps the current, or flow of charges, going.

Batteries can be constructed using a variety of chemicals. Any two substances with different affinities for electrons can be suspended in a medium that allows ions to migrate, producing a battery.

The concepts of current and circuits are central to the understanding of electricity. Electric current is a flow of electric charges. The charges may be electrons, protons, ions, or even positive "holes" (absences of electrons that may be thought of as positive particles). A circuit is a path through which electric current is transmitted.

Electric Charges in a Current
A current can be described as either direct or alternating based on the way that charges move within it. In direct current (DC), the charges always move in the same direction through the device receiving power. Batteries and fuel cells produce direct current. In alternating current (AC), the charges move back and forth in the device and in the wires connected to it. For many purposes either type of current is suitable, but alternating current is customarily used because it can be generated and distributed with greater efficiency. The power sent out by power plants is alternating current.

When the charge q is measured in coulombs and the time t in seconds, the current see I will be expressed in amperes, which is defined as coulombs per second:

A one-ampere current means that one coulomb of electric charge passes each point in the circuit each second. Because each electron carries only 1.6 × 10 ⁻¹⁹ (1.6 10-quintillionths, or 1.6/10,000,000,000,000,000,000) coulomb of charge, the one-ampere current-normally used in the operation of a 120-watt incandescent bulb-implies that in one second about 6 × 10 ¹⁸ (6 quintillion, or 6,000,000,000,000,000,000) electrons pass each point in the filament of the bulb.

Traditionally, the direction of electric current has been described as the direction of positive charge motion (conventional current), even though in most circuits it is the electrons that actually move (in the opposite direction). Even though electric charge moves through the filament of the bulb, the filament itself is not charged. The amounts of positive and negative charge in the filament are equal. The positive and negative charges are simply moving in opposite directions relative to each other.

It is mainly the quantity of electric current, or the amperage-not the potential difference, or voltage-that can produce a lethal shock (though higher voltages generally cause higher currents). Currents of less than 0.005 ampere that pass through the heart are not likely to cause damage. Currents of about 0.1 ampere are usually fatal, even if endured for only one second.

Conductivity and Resistance in a Circuit
A circuit is produced when the terminals of a battery are connected with a conductor. As described above, chemical reactions within the battery create a potential difference between the terminals, and electrons flow in the conductor in one direction, away from the negative terminal toward the positive.

The 19th-century German physicist Georg Simon Ohm showed that the current in such a circuit was directly related to the voltage of the battery but noted that the amount of current also depended on the nature of the conductor. Different kinds of conductors differed in the degree to which they resisted movement of electrons with a given voltage. He defined the resistance of a conductor as the ratio of the potential difference across the conductor (in volts) to the current (in amperes) through the conductor:

The unit of resistance is now known as the ohm, usually abbreviated as the Greek letter omega-Ω

The greater the potential difference across a circuit, the more electric current is made to flow through it and the greater the heat effects produced as electrons force their way through the wire. A wire in which a current generates a substantial quantity of heat energy is called a resistor. In a circuit diagram resistors are represented by the symbol

. For example, in the filament of an incandescent lightbulb and in the heating element of an electric toaster or clothes dryer, heat energy is generated as electrons are forced through these resistors. If the current is large enough, the heat generated can be used for welding metals or for smelting in electric furnaces. Two or more resistors can be connected in a circuit in series or, more often, in parallel ( see below).

A thicker wire offers less resistance to current than a thinner one of the same material. This is because current consists of electrons flowing through the metal of the wire. The electrons jump from atom to atom in the metal in response to the electric field in the circuit. A conductor with a larger cross section allows more electrons to interact with the field. Because there is more current with a given voltage, a conductor with a larger cross section has lower resistance.

Under special conditions, materials become superconductive, meaning all resistance disappears because electrons pair up and do not collide; current flows without losing power. Some conductors must be cooled to temperatures near −273° C, or absolute zero, before they become superconductive. Because of the high cost of cooling such superconductors, progress in the commercial application of superconductivity was impeded. In the 1980s, however, a new class of higher-temperature superconductor was discovered. These materials are rather brittle and are difficult to form into wire, but progress is being made.

Series Circuits

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When resistors are connected in series, the electric current that flows through one resistor will flow through the next resistor and so on. Everywhere in a series circuit the current is the same. The total resistance in such a circuit is simply the sum of the resistances of the separate resistors-R ₁, R ₂, and so on:
If the total potential difference across these resistors is V t, then the current in the circuit will be limited by the total resistance: The voltage drop across any particular resistor is given by:
Wiring in series is satisfactory if the devices need only low amounts of power for operation since each added resistor will cause the current in the circuit to drop. However, if one element of the circuit burns out, the entire circuit is broken.

Parallel Circuits
Parallel wiring allows the electric current to move through different pathways. Each branch can be switched on or off independently, allowing some or all of the devices to be used. In general, if the resistance along a given parallel branch is increased, a smaller amount of current will flow through that branch. The more branches that are available in parallel wiring, however, the lower the total resistance in the circuit becomes, and thus the higher the total current. This may seem to be a contradiction, but it is very similar to what happens when people leave a crowded theater that has several exits. Although there is resistance to movement at each exit, there will be a larger overall rate of movement with more exits. The total resistance for resistors connected in parallel is given by the equation:

The total current that flows into the branches will equal the sum of the currents in each branch:

Regardless of which branch the charges follow, they all move between points of equal potential as they move through the parallel resistors. Thus the voltages across each resistor wired in parallel are equal:

Besides the advantage of being able to use resistors along each branch independently, the parallel scheme of wiring allows the addition of extra branches without changing the current in the branches already in use, thus keeping the energy consumption in each branch unchanged. In the home each additional device that is plugged into a given circuit adds another parallel branch. But with each device added, the total resistance drops and the total current increases. If too much current flows through the conducting wires, they may overheat and a fire may occur. A fuse or a circuit breaker can be included in a parallel circuit to prevent overheating. If the current increases to a dangerous point, a filament in the fuse overheats, burns out, and the circuit is broken. In order for the parallel circuit not to be overloaded, it is necessary to remove one or more of the branches to increase the overall resistance and decrease the current.

Switches

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If a circuit is complete, electrons will flow as long as the cell acts. Usually it is desirable to be able to turn current on and off. This can be done with switches, which act like a drawbridge. If the bridge is open, traffic cannot move along the road. When the bridge is closed, traffic can move. If the switch is open, current cannot flow. Closing the switch makes it possible for the current to flow. Circuits are called open or closed according to the position of the switch.

Electric Power
When electrons flow through a circuit, they lose the extra energy given to them by the battery before returning to it. This energy commonly is manifested as heat (for example, in a lightbulb filament), but it can be used to do mechanical work as well (for example, in a motor). The more energy each electron has, and the more electrons that pass by any point in the circuit per unit time, the greater the rate at which energy is converted from electrical to other forms. The rate of energy expenditure is called power. It can be measured in watts, which are joules per second, and found by multiplying the electric potential difference, in volts, by the current, in amperes:

Electric power appears as heat when the electric field in a circuit acts on an electron and imparts to the electron kinetic energy. When the electron strikes an atom, it transfers most of the energy to the atom. The atom then vibrates faster. Faster vibration means more heat, since heat is energy of motion.

In turn, heat in metals reduces conductivity, or increases resistance, slightly. More vibration makes atoms "get in the way" of the electrons more often. The electrons then must spend extra time on deflected courses instead of going straight ahead. This cuts down current slightly. In modern theory, the atoms scatter the electron waves carried by the electrons.

Heat can also generate electricity by acting upon the joined ends of two different kinds of metal. The unheated ends must also be joined in order to complete a circuit. Voltage can be increased by joining several junctions, called thermocouples, in series to make a thermopile. A pile made of antimony and bismuth, for example, with unheated ends kept at a constant temperature, can be used to detect temperature changes of a hundred-millionth of a degree. Thermopiles are also used to measure high temperatures ( see thermometer).

Like electricity, magnetism is a fundamental force. Magnetism and electricity are closely related and are regarded as two expressions of a single force, the electromagnetic force. The region around a magnet in which magnetic forces can be seen is called a magnetic field. An electric current also creates a magnetic field.

How Magnetic Fields Form
In 1820 the Danish scientist Hans Christian Oersted found by accident that the magnetized needle of a compass would realign if brought near a current-carrying wire. The diagram shows how the needle would point if placed at various positions in a plane perpendicular to a conductor carrying electrons upward. If the direction of the current were reversed, the compass needle would reverse its orientation.

Apparently a magnetic field encircles the current-carrying wire. This magnetic field can be represented as a series of concentric field lines, which form closed loops, in planes perpendicular to the current. A simple rule allows the prediction of the field direction if the direction of electron motion is known: If the wire is encircled by the fingers of one's left hand, with the thumb pointing in the direction of electron motion, the magnetic field lines encircle the wire in the same direction as the fingers, and a compass needle will align itself tangent to these lines. For a moving positive charge, the right hand is used to predict the magnetic field direction. (The direction of a magnetic field is taken as the direction in which the north-seeking pole of a compass needle points.)

A magnetic field is produced around any moving electric charge, positive or negative, whether in a conductor or in free space. The German-born theoretical physicist Albert Einstein showed in his theory of relativity that the magnetic field produced by a moving electric charge is caused by a warping of the charge's electric field; this is caused by its relative motion. An observer moving along with the charge would not detect any magnetic effects. Thus an electric charge at rest relative to an observer does not produce a magnetic field.

Currents in a Magnetic Field
All magnetism arises from moving electric charge. If a current flows in a coil of wire, called a solenoid, the magnetic field will be directed through the solenoid and out one end. The field curves around and reenters the other end of the solenoid. This is similar to the shape of the magnetic field around a bar magnet with a south and north pole and led the French physicist André-Marie Ampère to speculate in the early 1820s that the magnetic field of a bar magnet is produced by circulating currents in the magnet. Today it is believed that those circulating currents are caused by the motions of electrons, particularly by their spin within individual atoms. The tiny magnetic fields of the individual atoms align themselves into domains in which the magnetic effects add together. (Physicists are reluctant to picture electrons as actually spinning, however, because quantum theory indicates that it is impossible to prove such motion by experiment.)

If an unmagnetized iron rod is inserted into a solenoid, the magnetic field inside the solenoid forces the electrons in the iron atoms to align their spins, producing domains that reinforce the magnetic field strength. This arrangement of solenoid and iron rod is an electromagnet and gives a magnetic field considerably stronger than that caused by the current in the solenoid alone.

Deflection in a Magnetic Field
Moving electric charges are surrounded by a magnetic field, and magnetic fields interact with magnets. Thus it is not surprising that charges moving through a magnetic field experience a force. What is surprising is the direction of the deflecting force. For deflection to occur, the charge must have some component of its motion perpendicular to the magnetic field. An electric charge-whether positive or negative-that is moving parallel to the lines of force of the magnetic field will not be acted on by any force from the field. However, a positive charge that moves across the field from left to right will be deflected by an outward force. If the positive charge moves from right to left, it is deflected inward. A negative charge moving across the field will be deflected oppositely.

The deflection for positive charges can be predicted by positioning the right hand so that the thumb points in the direction of the moving positive charge and the fingers point in the direction of the magnetic field. (Magnetic fields point away from north poles and toward south poles.) The positive charge will be deflected from its original path in a direction out of the palm of the hand. A similar left-hand rule applies to negative charges moving across magnetic fields.

Earth's Magnetic Field
Earth's magnetic field deflects and traps charged particles that travel from the Sun and other stars toward Earth. These trapped charged particles have formed two doughnut-shaped regions known as the Van Allen radiation belts. Some particles not trapped by Earth's magnetic field are steered by that field into the atmosphere near the poles. The aurora borealis is produced as these deflected charges crash into molecules of gas in Earth's atmosphere.

Using Magnetism for Measurement
Instruments designed to detect and measure electric current in circuits make use of the deflecting force on charges moving through magnetic fields. The galvanometer and ammeter detect and measure electric current as it flows in a coil pivoted between the poles of a permanent magnet. As the current in the coil increases, the moving charges are increasingly deflected by the magnetic field. By standardizing the scale, the quantity of current in amperes can be determined. In measuring current, the ammeter is connected in series in the circuit.

A voltmeter is used to measure the potential difference, or voltage drop, between points in the circuit. Like the ammeter it has a movable coil positioned in a magnetic field. But the voltmeter has an extremely high internal resistance. The trickle of current through the voltmeter depends on the potential difference between the terminals of the meter. The voltmeter is connected in parallel across a resistor to measure the potential difference in volts.

Everyday Applications of Magnetism
Magnetic tape recorders and video recorders demonstrate practical uses for the magnetic field produced by an electric current. In tape recording, as current varies in a tape head (itself an electromagnet), the magnetized particles on the tape are realigned to conform to the magnetic field produced by the changing current. Digital video recorders and computer hard drives use magnetically coated disks to record data in a similar way.

In the picture tubes of traditional televisions, magnetic fields are used to steer the electrons from the cathode. As the magnetic field strength is varied, the electrons are deflected so that they scan across the screen. In a loudspeaker, the current from the amplifier is fed to a coil of wire attached to the speaker cone. The coil is arranged so that it is in line with a permanent magnet. As current in the coil is varied, the moving charges are deflected by the field of the permanent magnet. As the coil moves, the cone of the speaker vibrates, causing sound waves to be produced. In addition, powerful magnetic fields keep charged particles moving in circles in the rings of high-energy accelerators used to investigate the substructure of protons and neutrons.

Solenoid are used in a variety of devices with a cylindrical piece of iron inserted. When a switch is closed, the current in the solenoid produces a magnetic field, which pulls the iron through the solenoid. The motion of the iron often is used to activate switches, relays, or other devices. Solenoids are used in doorbells, in the starter motor of automobiles, and in the water valves in washing machines, among many other applications.

The interaction between moving charges and a magnetic field makes possible two very useful devices: the electric motor and the generator. In a motor, electrical energy is converted into energy of motion. In a generator, the reverse process takes place: mechanical energy is converted into electrical energy.

Motors
A simple motor can be represented as a loop of wire attached to a source of direct current (DC). The loop is pivoted to rotate in a magnetic field. As electric charge moves along the loop, magnetic forces deflect the charge, causing the loop to rotate. To keep the loop rotating, the direction of current in the loop must be reversed every 180 degrees. A device called a split-ring commutator is used for this purpose.

Another design is the induction motor, in which a magnetic field revolves around a piece of metal and creates eddy currents in the metal. These currents produce magnetic fields that interact with the revolving field. This makes the metal rotate if it is pivoted properly. The rotating metal constitutes a motor. The smallest motors of this type use a rotor (revolving part) made of metal disks notched at the edges to place the eddy currents properly. Larger types may use a squirrel-cage rotor. This is made of metal bars arranged to form a skeleton cylinder. The ends of the bars may be attached to disks, or the bars may be mounted on a cylinder of enameled iron and connected at the ends. The eddy currents flow through the bars and end connections.

The revolving fields are produced by using two-phase or three-phase current to energize the field coils. The phases amount to different alternating currents in the same circuit. Because different-phase currents can be used, induction motors are classified as polyphase. The currents reach maximum and minimum strengths in each direction of flow at different times. The field coils are connected to place maximums at different points in turn around a circle. This produces the revolving field.

Synchronous motors use a polyphase current to provide revolving fields in the stator (stationary part), and other current (sometimes direct) gives the rotor a field that follows the stator fields around. Such motors run at constant speeds, proportional to the frequency of the supplied current.

Generators
A generator is a motor working in reverse: a motor changes electrical energy into mechanical energy, but a generator produces electrical energy from mechanical energy. Superficially the diagram of a generator appears identical to that of a motor. Each consists of a loop of wire that can rotate in a magnetic field. In a motor, electric current is fed into the loop, resulting in rotation of the loop. In the generator, the loop is rotated, resulting in the production of electric current in the loop. For 180 degrees of the rotation, electron deflection produces an electric current in the loop that moves in one direction; for the next 180 degrees, the electron deflection is reversed. As the current leaves the loop to an external circuit, it moves in one direction and then the other. This is alternating current. For a generator to generate direct current it is necessary to use a split-ring commutator at the point where the generator feeds current to the external circuit. The current in the loop is still alternating, but it is direct in the external circuit.

Electromagnetic Induction
Michael Faraday, the English scientist, and Joseph Henry of the United States independently showed in 1831 that moving a magnet through a coil of wire would generate a current in the wire. If the magnet was plunged into the coil, current flowed one way. When the magnet was removed, the current direction was reversed. This phenomenon is called electromagnetic induction, and it is the principle underlying the operation of the generator. As long as the magnet and the coil move relative to each other, a potential difference is produced across the coil and current flows in the coil. A potential difference is also produced if the magnetic field through the coil grows stronger or weaker. The greater the rate at which the magnetic field changes, the greater the potential difference produced. The key is that the magnetic field must be changing.

In 1864 James Clerk Maxwell suggested: (1) If an electric field changes with time, a magnetic field is induced at right angles to the changing electric field. The greater the rate at which the electric field changes, the stronger the induced magnetic field. (2) If a magnetic field changes with time, an electric field is induced at right angles to the changing magnetic field. The greater the rate at which the magnetic field changes, the stronger the induced electric field.

Maxwell calculated that these electric and magnetic fields would propagate each other and travel through space as time-varying fields. The speed of these electromagnetic waves is 3.0 × 10 ⁸ (300,000,000) meters per second. That happens to be the same as the speed of light. In fact, visible light is merely a narrow range of frequencies in what is known as the electromagnetic spectrum. As people read a printed page, electromagnetic waves reflected from the page pass into their eyes. As the electric field of that wave reaches the eye's retina, electrons in molecules of the retina interact with the field, change position, and start the message to the brain that eventually allows a person to understand what has been read.

Lenz's Law
Whenever a changing magnetic field generates a current in a coil of wire, the current will generate its own magnetic field. That induced magnetic field will always tend to oppose the change in the magnetic field that induced it. This rule was first suggested by the Russian-born physicist Heinrich F.E. Lenz in 1834. The effects of the induced field can be observed during the operation of a hand-cranked generator. When the generator is cranked slowly, little current is produced and weak electromagnetic forces oppose the rotation. But as the cranking rate is increased and more current is produced, the forces on the rotating loop become stronger, and the loop is correspondingly more difficult to turn.

Lenz's law also applies to motors, where a current-carrying wire moves in a magnetic field. That movement, in turn, produces a current in the wire that opposes the original direction of current in the wire. Because electric current cannot occur without a potential difference, this opposition effect is sometimes called a back-emf. When a motor is started, a large current flows at first, and, as the motor begins to turn rapidly, a large back-emf is induced and the net current in the motor drops. If a large load is suddenly added to the motor, slowing it drastically, the back-emf will drop, and the sudden rise in current may cause overheating and burn out the motor.

Even a simple coil of wire in a DC circuit exhibits the effects of back-emf. As the current in the coil increases, the changing magnetic field produced around the coil will tend to produce a back-emf. This is called self-inductance. Normally the current in a circuit rises rapidly after the switch is closed. But in this circuit, the current rises relatively slowly. On the other hand, when the switch is opened, the current in the circuit normally falls to zero almost instantly. But as the magnetic field around the coil decreases, an emf is generated that tends to keep the current from decaying as rapidly. A coil like this is used in devices designed to prevent damage to electronic equipment caused by voltage spikes-sudden increases in potential difference that would tend to produce rapidly changing currents.

Generators do not create energy. To produce electricity either the loop or the magnets must be rotated relative to one another. The energy for this rotation can be provided by a variety of sources. In some sources water is converted to steam, which is used to drive turbines that operate generators. The energy to boil the water and convert it to steam comes from burning coal, oil, or natural gas, or from the heat released by controlled nuclear reactions. The rotation may be driven by the gravitational potential energy stored in water held behind the dam of a hydroelectric plant, by wind in wind turbines, or by the steam produced naturally within Earth.

Science in Seconds (www.scienceinseconds.com)

Almost everywhere on Earth, electric power is delivered to homes and businesses as alternating current, though different countries use different frequencies. Frequency, measured in hertz, indicates how many times the direction of the current reverses each second. In the United States, when an incandescent lamp is switched on, electrons move back and forth within the filament 60 times per second (a frequency of 60 hertz), matching the frequency at which the alternating current is produced by the generators at the power plant. In contrast, most European countries use a frequency of 50 hertz.

The electrons that move in the filament are not the same electrons that were deflected in the generator loop. When the lamp is turned on, it lights almost instantly because the changing electric field produced in the generator loop travels through the circuit at close to the speed of light. As the field passes along the conductor, electrons in the conductor interact with the field, but they cannot move very far unimpeded because they bump into vibrating atoms in the conductor. Thus the electrons themselves do not fly through the wires at close to the speed of light, nor do they move along by a domino effect. It is the electric energy carried by the field that moves the electrons in the circuit.

It is energy, not charge, that flows from the electrical outlet. The electric charges that move in a lamp are already in the lamp. Thus electric companies bill their customers not for electrons but for electrical energy. The charges on an electric bill are based on kilowatt-hours. A kilowatt is a unit of power, or energy consumed per time unit. When power is multiplied by time, the result is a measure of energy:

So the kilowatt-hour is a unit of energy.

The companies that supply electrical power try to deliver that power as efficiently as possible. This necessitates transmitting the energy in a way that minimizes power losses due to heating within the transmission lines from power station to home, school, or workplace. If the transmission lines obey Ohm's law, in the expression

resistance x current can be substituted for voltage to obtain:

This relationship is important in considering the power loss during transmission. Doubling the current means four times the power loss. Alternatively, if the current is too low for a given power, the voltage will be too high for safe use in the home. The problem of having sufficiently low voltage at the point of delivery and low currents for efficient transmission is solved by using transformers.

A simple transformer consists of a coil of wire fed by a voltage source such as a generator. The coil is wound around one side of an iron frame. This is the primary coil. The other side of the iron frame is wound with another coil, the secondary, that feeds electricity to a separate circuit. As alternating current from the generator flows in the primary coil, the magnetic field in that coil strengthens, weakens, and then reverses direction as the alternating current changes. The iron core intensifies these magnetic-field changes. As the magnetic field in the secondary coil changes with time, electric charges in the secondary are deflected and current is produced. The alternating voltage produced in the secondary depends on the relative number of turns in the secondary compared to the primary coil. If V ₁ is the voltage across the primary, V ₂ the voltage across the secondary, N ₁ the number of turns of coil in the primary, and N ₂ the number of turns of coil in the secondary, then

If the secondary coil, for example, is wound with 100 times as many turns as the primary, the voltage across the secondary will be 100 times larger than that across the primary. A transformer used in this way is said to be a step-up transformer. If there are fewer turns on the secondary, it is a step-down transformer.

It might seem that a step-up transformer gives more energy than it uses: after all, the larger secondary voltage means a larger energy change per charge moved in the field. But that voltage increase comes at the expense of a reduced current. If the transformer were 100 percent efficient, then

If I ₁ and I ₂ are the currents in the primary and secondary coils, then

So increasing the voltage in the secondary coil by a factor of 100 results in a current reduction by a factor of 100.

By using a step-up transformer between generator and transmission lines, the current can be reduced to a low value, thereby reducing power losses due to heating of the transmission lines. The voltage across these power lines will be correspondingly large. By using a step-down transformer between the transmission lines and the point of delivery, the voltage will be reduced and the current increased to a level allowing effective operation of electrical devices.

In a battery-driven circuit, the flow of electric current is produced by spontaneous chemical changes that occur at each battery terminal. In a battery, stored chemical energy is converted to electrical energy.

In electrolysis the process is reversed. By forcing an electric current through some substances, it is possible to change electrical energy into stored chemical energy. The process of electrolysis causes chemical reactions that do not occur spontaneously. For example, when common table salt, or sodium chloride (NaCl), is heated to 1,486 °F (808 °C), the solid turns to a stable melt consisting of sodium ions (Na⁺) and chloride ions (Cl ⁻). If inert electrodes are immersed in the melt and an electric current is forced through the molten salt by a sufficiently high voltage, sodium metal will be produced at one electrode and chlorine gas at the other. A similar electrolytic process is used to obtain pure aluminum from solutions of aluminum oxide.

Electrolysis is important in silverplating. In this process an electric current is passed through an object that is immersed in an appropriate solution of a silver compound. If the voltage is sufficient, silver ions (Ag⁺) will accept electrons from the object being plated. The ions thereby change to silver atoms (Ag), which plate the surface. A similar technique is used in electroplating copper, chromium, and gold.

The earliest recorded observations about electricity date from about 600 bc and are attributed to the Greek philosopher Thales of Miletus. He noted that when amber, a fossil resin, was rubbed it would attract feathers or bits of straw. The Roman author Pliny the Elder wrote about similar experiments in ad 70 in his Natural History. He also mentions shocks given by torpedo fish.

In 1600 William Gilbert, an English scientist who was physician to Queen Elizabeth I, published De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure (1600; On the Magnet, Magnetic Bodies, and the Great Magnet of the Earth). He studied what was already known about amber and lodestone, a mineral that attracts iron. He gave a proof that the attraction exhibited by amber was not magnetic. He also proposed that Earth behaved as though it were a spherical lodestone.

In 1672 the German physicist Otto von Guericke reported the invention of an electric machine: A ball of sulfur on a shaft was rotated; if he touched the rotating sulfur ball with his hand, he noted that sparks were produced. He also proved that electrified objects can transfer some of their ability to attract, called charge, to nonelectrified objects.

In about 1736 the French chemist Charles-François Du Fay learned that rubbing glass and rubbing resinous substances seemed to produce charges of different kinds. He found that two charges of the same kind repel while unlike charges attract. He suggested that electricity may consist of two kinds of "invisible fluid," which he named "vitreous" and "resinous."

In about 1745 a German clergyman, E. Georg von Kleist, and a professor at the University of Leiden (sometimes spelled Leyden), Pieter van Musschenbroek, discovered independently that a glass vessel filled with water and charged by a friction source could store the charge for later use. The device became known as the Leyden jar. Sir William Watson and Dr. John Bevis of England improved the jar by coating the inside and outside with tinfoil. This vessel could store enough charge to make sparks that would explode gunpowder or set alcohol afire. Watson's most important discovery was that electricity traveled almost instantaneously along a wire about 2 miles (3.2 kilometers) in length. In 1746 he suggested that electricity was only one kind of fluid and that an excess or lack of that fluid would account for the two kinds of electricity proposed earlier by Du Fay.

The statesman and inventor Benjamin Franklin of the United States, who is credited with the invention of the lightning rod, was an advocate of Watson's one-fluid model. Apparently the enormous respect commanded by Franklin was significant in the widespread acceptance of Watson's model. It was not until the 1890s, however, that a clear understanding of what electricity is finally emerged, showing that both Du Fay and Watson were correct in some ways.

In 1753 the Englishman John Canton discovered electrostatic induction. Henry Cavendish, another Englishman, found that the force of electric attraction varies inversely with the square of the distance between the charges. He did not publish his findings, and the law is named for Charles-Augustin de Coulomb, who also discovered the relationship.

The study of electricity was greatly aided by the invention of a device for producing a steady current of electricity. In 1780 Luigi Galvani, an Italian anatomist, noted that the legs of a dead frog hung from an iron hook twitched when touched with different kinds of metals. He thought a special "animal electricity" caused this, but an Italian professor of physics, Count Alessandro Volta, suspected a chemical cause. He placed unlike metals in piles between pads moistened with acid or salt solutions. The piles produced a steady electrical current: Volta had invented the battery. In 1807 Sir Humphry Davy of England used current from a powerful battery to obtain pure sodium and potassium from molten soda and potash. The Frenchmen Siméon-Denis Poisson, Joseph-Louis Lagrange, André-Marie Ampère, and Dominique Arago and the Englishman George Green worked out many fundamental laws of electrodynamics. In 1826 the German physicist Georg Simon Ohm announced discoveries concerning voltage, current, and resistance in circuits.

In 1820 the Danish physicist Hans Christian Oersted discovered that an electric current caused deflection of a compass needle. This was followed by the invention of the electromagnet by the Englishman William Sturgeon. In the United States Joseph Henry improved this device and made other discoveries. Unfortunately, he lacked contact with European scientists, and his findings remained almost unknown for many years.

In 1821 the English chemist Michael Faraday, who had no formal training in mathematics or science, undertook a survey of the experiments and theories of electromagnetism that had appeared in the previous year. Faraday started by repeating Oersted's experiment. He began to develop his own ideas for describing electric and magnetic fields using lines of force. By 1831 he had discovered the principle of electromagnetic induction and the working basis for motors and generators.

In 1873 the Scottish physicist James Clerk Maxwell published a profound mathematical analysis predicting that any changing electric or magnetic field would generate electromagnetic waves that would be propagated through space at the speed of light. In 1888 Heinrich Hertz of Germany produced the predicted waves and confirmed the speed Maxwell had calculated. In 1901 Pyotr N. Lebedev of Russia confirmed the existence of radiation pressure, also predicted by Maxwell.

From Joseph Thomson, African Explorer by Rev. J.B. Thomson, 1897

In 1897 the Englishman Sir Joseph J. Thomson discovered that a negatively charged particle was ejected from the cathode in high-voltage gas discharge tubes. This particle was the electron. Scientists became convinced that electric current in a conductor consisted of electrons flowing from negative to positive. Between 1907 and 1911 Ernest Rutherford, a New Zealander working in Canada, discovered that the positive charges of the atom were clustered in an incredibly tiny space-the nucleus-which occupied only about 10 ⁻¹⁵ (1 quadrillionth, or 1/1,000,000,000,000,000) of the volume of the entire atom.

In 1909 Robert Millikan of the United States determined that the basic unit of charge was 1.6 × 10 ⁻¹⁹ (1.6 10-quintillionths, or 1.6/10,000,000,000,000,000,000) coulomb. He then concluded that this is the electric charge carried by each proton and electron.

Thomson's discovery of the electron became the working basis for new discoveries and advancements in the United States. Marvelous developments in electronics were opened up in 1907 when Lee De Forest provided the needed technical instrument by developing John A. Fleming's diode tube into a triode. Charles P. Steinmetz developed high-tension transmission of current largely by mathematical studies. Nikola Tesla did much the same for the induction motor. Practical inventors such as Thomas Edison and Alexander Graham Bell applied the principles of electricity and magnetism in a wide variety of devices.

In 1925 the Austrian physicist Erwin Schrödinger combined the theoretical work of Niels Bohr, Louis de Broglie, Max Planck, and Werner Heisenberg to develop quantum mechanics. This theory treats electrons and protons as wavelike particles that can exist only in certain allowable energy states and whose location and momentum are describable in terms of probability and not certainty.

Since 1965, when the American physicist Murray Gell-Mann first proposed that protons and neutrons are actually composed of quarks, physicists have explored high-energy particle collisions to find support for their theories of quantum chromodynamics. These theories have already pointed to a convincing unity between what once had been thought to be three different forces in nature: electromagnetism, the nuclear strong force, and the nuclear weak force. This work offers the hope of uniting these forces and the force of gravity into a single unified theory: a theory that will account for all physical phenomena.

Bodanis, David. Electric Universe (Crown, 2005).Buban, Peter, and others. Electricity and Electronics Technology, 7th. ed. (Glencoe/McGraw, 1999).Dreier, David. Electrical Circuits: Harnessing Electricity (Compass Point, 2008). Gibilsco, Stan. Electricity Demystified (McGraw, 2005).Kirkland, Kyle. Electricity and Magnetism (Facts On File, 2007). Panofsky, W.K., and Phillips, Melba. Classical Electricity and Magnetism, 2nd ed. (Dover, 2005).Parker, Steve. Electricity, rev. ed. (DK, 2005). Parker, Steve. Fully Charged: Electricity (Heinemann, 2005).Solway, Andrew. Generating and Using Electricity (Heinemann, 2009). Tucker, Tom. Bolt of Fate: Benjamin Franklin and the Electric Kite Hoax (Public Affairs, 2003).

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Electricity is a form of energy associated with the atomic particles called electrons and protons. In particular, electricity involves the movement or accumulation of...

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A parents’ guide to electricity: what children learn in KS2 science and the electrical terminology introduced in the primary-school classroom.

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Electricity 101: How Power Gets to Your Home

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How Amazon's Super-Complex Shipping System Works

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World’s Most Satisfying Machines And Tools...

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The Life Cycle of a Frog | Frog Life Cycle | Video for Kids

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The water cycle for kids - What is the water cycle? - Why does it rain? - Science for children

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The Water Cycle | Educational Video for Kids

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Pour yourself a glass of water and take a sip. Did you know that the water you've just swallowed is the same water that wooly mammoths, King Tutankhamun and the first humans drank? That's because Earth has been recycling water for over 4 billion years!

The world's water moves between lakes, rivers, oceans, the atmosphere and the land in an ongoing cycle called - you guessed it! - the water cycle. As it goes through this continuous system, it can be a liquid (water), a gas (vapour) or a solid (ice).

So, are all you budding young geographers ready to learn some splashing new facts? Then join NG KiDS as we take a look at the different stages of the wonderful water cycle...

Energy from the sun heats up the surface of the Earth, causing the temperature of the water in our rivers, lakes and oceans to rise. When this happens, some of the water "evaporates" into the air, turning into a gas called " vapour". Plants and trees also lose water to the atmosphere through their leaves. This process is known as " transpiration ".

As water vapour rises up high into the sky, it cools and turns back into a liquid, forming clouds. This process is called " condensation ". Currents high up in the air move these clouds around the globe.

The water cycle is also known as the "hydrologic cycle".

Precipitation

When too much water has condensed, the water droplets in the clouds become too big and heavy for the air to hold them. And so they fall back down to Earth as rain, snow, hail or sleet, a process known as " precipitation ".

Collection

The fallen precipitation is then "collected" in bodies of water - such as rivers, lakes and oceans - from where it will eventually evaporate back into the air, beginning the cycle all over again. How it is collected, depends on where it lands...

Some will fall directly into lakes, rivers or the sea, from where it will evaporate and begin the cycle all over again.

If the water falls on vegetation, it may evaporate from leaves back into the air, or trickle down to the ground. Some of this water may then be taken up by the plant roots in the earth.

In cold climates, the precipitation may build up on land as snow, ice or glaciers. If temperatures rise, the ice will melt to liquid water and then soak into the ground, or flow into rivers or the ocean.

Water that reaches land directly may flow across the ground and collect in the oceans, rivers or lakes. This water is called " surface run-off ". Some of the precipitation will instead soak (or "infiltrate") into the soil, from where it will slowly move through the ground until eventually reaching a river or the ocean.

And there you have it, gang - the ongoing water cycle!

Water can change from a solid to a gas, without becoming a liquid first. Through this process of "sublimation" our planet"s ice can evaporate directly into the air without melting!

Be sure to check out our water conservation primary resource. Great for teachers, homeschoolers and parents alike, show children 25 ways to help keep our oceans, rivers and streams clean...

Illustration: Getty Images UK

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The Water Cycle! - National Geographic Kids

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Pour yourself a glass of water and take a sip. Did you know that the water you've just swallowed is the same water that wooly mammoths, King Tutankhamun and the first humans drank? That's because Earth has been recycling water for over 4 billion years!

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Volcanoes for Kids | A fun and engaging introduction to volcanoes for children

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Volcano | The Dr. Binocs Show | Learn Videos For Kids

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The frog life cycle

Frogs are animal athletes, with strong powerful legs that allow them to leap out of harm's way. These powerful limbs are the secret to their success - and incredibly, it takes baby frogs weeks and weeks to grow them!

Like beautiful butterflies, frogs and toads undergo an astonishing transformation - which is called ' metamorphosis' - as they grow up. From jelly-like eggs laid in water, they grow into wriggling tadpoles, and finally grow legs suitable for land!

But how does this mesmerising metamorphosis work? Let's have a look at each stage of the frog life cycle in turn, to find out...

Stage 1: Extraordinary eggs

It all starts with adult frogs laying hundreds of tiny eggs, which clump together in groups known as frogspawn. This happens in early spring, when the weather is just starting to get warmer. Spotting frogspawn is often thought of as being a key sign of spring!

You can hunt for frogspawn in shallow, still water - a garden pond, for example! As the eggs are defenceless, they're usually laid among vegetation and just below the surface of the water, to give them some protection.

If you're lucky enough to spot some of these strange eggs, pay attention to their shape. Frogs lay 'frogspawn', which looks like a round cluster of eggs, while toads lay 'toadspawn', which comes in long ribbons.

DID YOU KNOW?
As far as scientists are concerned, there's no difference between frogs and toads! They have different characteristics, but share the same family tree.

In the frogspawn, the baby frogs start out as tiny black dots surrounded by a jelly-like substance. Over time, they become comma-shaped, as their tails begin to develop - amazing!

Stage 2: Teeny tadpoles!

After spending 1-3 weeks eating the yolk of their egg, the baby frog hatches into the big, wide world.

Now, the baby frogs are known as tadpoles. They have gills, a mouth, and a long tail, which they need for swimming!

DID YOU KNOW?
The tadpole stage of the frog life cycle is also known as the larval stage. Tadpoles are frog larvae, in the same way that caterpillars are butterfly larvae!

For the first week or two after hatching, tadpoles won't move around very much, as they're still absorbing some nutrition from the yolk of their egg. However, once the yolk is all gone, the tadpoles are big and strong enough to venture out into their watery world!

Unlike adult frogs, tadpoles can't go on land - so they feed on plant material filteredfrom the water, and tear off tiny chunks of nearby vegetation.

The legless, water-bound tadpoles slowly metamorphose into frogs over the next 14 weeks.

First, they grow back legs, then front legs too! Soon after, their body starts to change shape, and they're able to start eating insects.

Next, the tadpoles' tails shrink away, and skin grows over their gills, as they develop lungs and eardrums! These are super important steps, as they prepare the tadpole for life on land.

Once their gills and tails are gone forever, tadpoles undergo one last 24-hour push, where the metamorphosis completes. Once this stage is finished, the baby frogs emerge from the water as tiny adults!

Amazingly, tadpoles are able to control the timing of their transformation. If they're living in a dangerous environment, like a pond full of hungry fish, they will metamorphose more quickly, to make their escape! On the other hand, if their pond has lots of plants to munch through, and few predators - or if it's extra cold on land - tadpoles can delay their metamorphosis, and remain in the water for up to a year. Wow!

Stage 3: Fully-grown frog!

And there you have it - the frogs are all grown up, and ready to head out into the big wide world!

As adults, frogs and toads are much less reliant on water. So long as they stick to the shade and don't dry out, they can live on land - but they often return to ponds and lakes for a splash!

Before too long, the females will look for water to lay her own eggs. Once that's happened, the life cycle is complete - what a frog-tastic life they lead!

Thanks to S. Hermann & F. Richter; Jill Wellington; PollyDot; David Clode, and Venita Oberholster for the images used in this article.

Have you ever found frogspawn, or spotted a frog? Let us know in the comments below!

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The Frog Life Cycle for Kids - National Geographic Kids

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Frogs are animal athletes, with strong powerful legs that allow them to leap out of harm's way. These powerful limbs are the secret to their success - and incredibly, it takes baby frogs weeks and weeks to grow them! Like beautiful butterflies, frogs and toads undergo an astonishing transformation - which is called ' metamorphosis' - as they grow up.

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Natural Disasters for Kids | Learn about tornados, hurricanes, and more!

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Natural disasters are violent events that are outside the control of humans. They are caused by the forces of nature and may result in loss of life, injury, and damage to...

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Get Ready - Emergency preparedness in New Zealand

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What's the Plan, Stan? has suggestions for teaching and learning programmes for students in years 1-8. They focus on emergency events and the impacts they could have on your community.

At years 1-4, students will be able to:

explore the impact of emergency events in New Zealand on the environment and people

investigate an emergency event that could happen in their local area and the scientific explanation for why it happens

take action to prepare for an emergency and lessen the impact on themselves and others, and

understand the role of Civil Defence and the Natural Hazards Commission Toka Tū Ake.

At years 4-8, students will be able to:

expand on earlier learnings

explore emergency events in a local context

understand the local and historical impact

understand the science behind the phenomenon, and

know preparation strategies and tips.

These learning experiences can be adapted to your local area and school curriculum. The aim is for the learning to be student-led, so the resource is designed to allow flexibility.

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What's the Plan, Stan? is a free resource to support schools, teachers and students to develop the knowledge and skills to prepare for emergency events.

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What is a Disaster? Disasters can be natural or man-made. Natural disasters are caused by nature, such as tornadoes, ice storms, severe weather and…

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Download FREE teacher-made resources covering 'Natural Disasters'

How do you explain natural disasters for kids?

So, what are natural disasters for kids? Your kids have probably heard the term 'natural disaster' in the classroom or on the news. A simple definition of a natural disaster is an adverse event initiated by natural processes, such as a sudden change in the earth's crust or climate. There are plenty of categories that we can use to distinguish natural disasters, but here are the fundamental three:

Geological disasters

Hydrological disasters

Meteorological disaster

These basic categorisations, however, have been blurred in recent years with the growing threat of climate change. Natural disasters that scientists predict may not have happened either with as much frequency, or intensity has been caused by the warmer climate, the rising sea levels, and the breakdown of ecosystems. In this way, the definition of what is a natural disaster and a man-made disaster in Geography is less clear than it has been previously.

Geologicalnatural disasters for kids

Avalanches and Landslides - This is when material runs down a slope of some kind. In the case of an avalanche, this will be snow that has either built up in a vulnerable position or fallen because of a sudden warm climate that has weakened the structure of the ice underneath. It is the same principle for landslides, which tend to occur because of weakened material on mountainsides. This can be because of rain, or excavation. Mines also tend to experience landslides because of the removal of supporting material when harvesting metal or other minerals.

Earthquakes - When the earth's crust moves, and two plates either converge or shift over one another, a huge amount of energy is released. An earthquake is a result of that energy transferring into seismic waves, which move everything around it. Depending on where the epicentre (the point directly above the collision) is, often determines the consequences. Earthquakes, themselves, are typically not the reason for the damage, but secondary events such as tsunamis, building collapses, and volcanic eruptions are hugely disastrous.

Volcanic eruptions - As mentioned, volcanic eruptions are also a result of the energy that is generated by the movement of tectonic plates. When the earth's crust generates this energy, some of this travels in the form of heat. Under the earth's surface, it becomes hot enough for the rock to melt and become magma. This molten rock is lighter than the surrounding rock, so starts travelling to the surface, where it eventually explodes out in the form of a volcano. The molten lava, however, is often not the primary concern. The extremely hot gases and clumps of rock that take flight create a deadly pyroclastic flow. The speed with which this travels is typically how the disastrous effects are caused.

Hydrologicalnatural disasters for kids

Floods - Floods can be caused for a number of reasons, and are not always disasters, therefore making the definition of this natural disaster hard to establish in Geography. Floods are an influx of water in an area that is typically dry and is inhabited by humans, such as a city, town or village. This may be caused by unprecedented levels of rain that either flood valleys, or burst boundaries where water is ordinarily contained. It can also be caused by the sudden melting of glaciers, where water travels from high latitudes to lower ground. Sometimes floods are welcomed, especially in natural or rural landscapes, because they can enrich the soil. Check out this page on floodplains for more information about how floods in certain areas of the world are not disasters at all.

Tsunamis - This kind of natural disaster is the earthquake's seismic waves having a repercussion on the water. Tsunamis are often far more devastating than earthquakes, in that the waves can travel as far as 16 km on land after reaching the coast. And tsunamis do not consist solely of one wave, they usually possess a wave train where four or five waves may arrive after the initial wave hits. There can be up to 100 miles between a seismic wave, meaning the subsequent waves can arrive anywhere between five minutes and two hours later. Check out our tsunami wiki for more information.

Limnic eruptions - These are far less common than the other hydrological natural disasters, as only two events of this kind have been observed. Limnic eruptions or a lake overturn as it is sometimes called, are when pockets of carbon dioxide that have long been sequestered under a body of water, then erupt, travel to the surface, and asphyxiate the humans, animals, and wildlife nearby.

Meteorologicalnatural disasters for kids

Cyclones and hurricanes - These names are used interchangeably, depending on which sea or ocean the event is taking place. Fundamentally, a hurricane occurs when warm moist air begins to rise over the water. When this process continues, large swathes of heavy clouds, a storm, start to swirl. This is because of the earth's Coriolis Effect, which dictates that anything suspended or in motion over long distances starts to detour from a straight line. When a cyclone reaches land it can be dangerous because it inflicts extreme winds, rainfall, and flooding that can have detrimental effects on the area.

Heatwaves and droughts - These are significant periods of excessively hot weather, which can cause a number of issues, such as deaths from overheating, crop failure, and wildfires. In hot areas of woodland, trees are often dried up by the sun, making them easily flammable and capable of spreading fire to neighbouring trees and buildings.

Blizzards and hail storms - This is when high winds stir up snow that has already fallen, obstructing travel, medical care, and agriculture. Sometimes precipitation, rain, will freeze while in very cold conditions and remains frozen after hitting the ground.

Twinkl Newsroom: Headlinenatural disasters for kids

Natural disasters can cause major disruptions to buildings, environments and people's lives. For this reason, major natural disasters are heavily reported in the news. Read on to learn about a few of the recent natural disasters for kids that have been covered by our very own Twinkl NewsRoom.

Three Storms Hit UK in One Week: In 2022, the UK was battered by 3 storms in one week, causing destruction and flooding up and down the country. Storm Franklin is the 3rd named storm in a week, following the appearance of Storm Dudley and Storm Eunice.You can teach your children about the latest news with Three Storms Hit UK in One Week - KS2 Reading Comprehension. Our experienced teachers have designed 2 articles to explain the weather-related news story in LKS2 and UKS2 appropriate language.

La Palma Volcano Continues Erupting: The volcano make headlines in 2021 asCumbre Vieja continued its latest eruption. Two new fissures opened adding to the amount of molten rock flowing down the side of the mountain. Use our La Palma Volcano - KS2 Reading Comprehensionto find everything you need to deliver an informative and engaging lesson about the ongoing eruption of the La Palma volcano

Storm Ciara Hits UK Soil: Storm Ciara is a storm that hit the UK over a weekend in 2020 and caused a lot of disruption. People worked hard to help others and engineers worked hard to get electricity back to houses that had no power because the storm had cut them off. Use ourLKS2 Storm Ciara Hits UK Daily News Resource Pack to learn more.

How are natural disasters for kids monitored?

Nowadays, the technology for monitoring natural disasters is what allows us to minimise the impact they have - with more time, more safety precautions can be taken. There are two main ways of monitoring for natural disasters:

Seismic monitoring - Seismographs are machines that pick up the vibrations that occur when tectonic plates slide past one another. They are able to pick up on the thousands of earthquakes that occur each year, even the minuscule ones. Using the Gutenberg-Richter diagram, scientists can develop averages and graphs that allow them to predict how many more may occur in a year and with what force.

Weather satellites - While satellites were previously a revelatory invention for understanding weather, they can be used to tell us lots about the earth's crust, too. They can monitor wind speeds and the water content of clouds, as well as predict where floods may occur by assessing sea levels.

Measuring natural disasters for kids

Natural disasters are measured in all kinds of ways - some are measured by the number of fatalities, injuries, and damage they cause. But all of them are measured with units for quantifying their force. Here are some of the main ones scientists use:

The Saffir-Simpson Hurricane Wind Scale (hurricanes)

The Richter scale (earthquakes)

Tsunami magnitude, or Mt (tsunamis)

History's worst natural disasters for kids

The multiple ways in which natural disasters are measured means that producing a definitive list of the most destructive natural events is difficult. And many disasters, though recorded in some way, took place when we didn't have the scientific tools we have today - meaning it is hard to compare disasters across history.

The Yangzi River Flood of 1931 is one of the most deadly, if not the most deadly natural disaster to have occurred in modern times. Spring snow, which had melted, burst the banks of the Chinese river and turned it into a small ocean. Research suggests that the death toll was somewhere between two and three million.

Historic disasters such as the Shaanxi earthquake in 1556 is, by modern calculation, one of the strongest earthquakes recorded. Researchers suggest it reached an eight on the Richter scale, killing almost 60% of the population in the region. The disaster, along with countless aftershocks and floods, took the lives of 830,000 people.

In 1970, the Bhola Cyclone travelled up the east coast of Pakistan reaching winds of 115 miles per hour. The event killed just under 500,000 people and devastated coastal towns across the country.

Resources for teaching about natural disasters for kids

We have a wide range of activities for teaching children the geographical definition of natural disasters. Take a look at the Earthquakes Differentiated Reading Comprehension Activity for a detailed explanation of earthquakes that you can suit to the abilities of your children.

Make this subject into a creative task with the Volcano Experiment Resource Pack. Teach your children about chemical reactions while demonstrating what a volcanic eruption looks like. You can get them to make their own model volcanoes with the Volcano Cross Section Paper Model.

For a fascinating activity on hydrological natural disasters, take a look at the Tsunami Experiment Instructions Sheet. This is a great way of showing how devastating this event can be for the land it impacts.

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What are Natural Disasters For Kids? | Geography Definition | Twinkl

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What are natural disasters for kids? Learn more about natural disasters for kids using this handy Twinkl teaching wiki guide full of information, guidance, and teaching assistance for your students.

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This timeline lists New Zealand's worst post-1840 natural disasters, transport accidents, fires, mining accidents and other tragedies that have caused major loss of life. Follow the links for more information on NZHistory, Te Ara and other sites. See also: map showing location of these disasters

This list does not include loss of life due to enemy action in wartime, such as the First World War battle of Passchendaele, where, on 12 October 1917, 843 New Zealanders were killed on a single day, or the sinking of the liner Wimmera, which hit a German mine off Cape Maria van Diemen on 26 June 1918. Nor does it include major disease outbreaks, such as the 1918 influenza pandemic, which claimed more than 8600 lives over several months.

1846 Taupō landslide

On 7 May a massive landslide on the shores of Lake Taupō overwhelmed the kāinga (Māori village) of Te Rapa, killing around 60 people, including Ngāti Tūwharetoa leader Mananui Te Heuheu Tūkino II. Find out more.

1855 Wairarapa earthquake

On 23 January a magnitude 8.2 earthquake struck the lower North Island. It killed between five and nine people in Wellington, Manawatū and Wairarapa and radically altered the landscape of the Wellington region. Find out more on Te Ara.

1863 HMS Orpheus shipwreck

On 7 February the Royal Navy steam corvette HMS Orpheus, carrying British troops, foundered at the entrance to Auckland's Manukau Harbour. Of the 259 men on board, 189 died in the worst maritime disaster in New Zealand waters. Find out more about the Orpheus disaster

1863 Central Otago floods

Twenty-five gold miners lost their lives as flash floods raged along the Shotover River, north-east of Queenstown, on 26 July 1863. Find out more on Te Ara.

1865 Fiery Star shipwreck

On 11 May the sailing ship Fiery Star caught fire and sank south of Cuvier Island, off the Coromandel Peninsula, with the loss of 79 lives. Find out more on Te Ara.

1865 City of Dunedin shipwreck

Leaving Wellington on 20 May, the paddle steamer City of Dunedin and its 39 passengers and crew disappeared without trace. The ship is presumed to have foundered in Cook Strait. Find out more

1866 General Grant shipwreck

On 14 May the sailing ship General Grant was wrecked in the Auckland Islands, south of New Zealand, with the loss of (ultimately) 73 lives. Ten survivors were finally rescued 18 months later. Find out more on Te Ara.

1868Great storm

On 3/4 February a violent storm swept across much of the country, wrecking 12 ships - including the Star of Tasmania and Water Nymph at Oamaru - and causing flash floods. More than 40 lives were lost. Read contemporary newspaper account.

1874 Cospatrick shipwreck

On the night of 17/18 November the emigrant ship Cospatrick, sailing from England to Auckland, was destroyed by fire off the Cape of Good Hope. Of the 473 people on board, only three survived. Although this tragedy occurred far from New Zealand and involved migrants who had yet to live in this country, the burning of the Cospatrick could be considered our worst civilian disaster. Find out more.

1879 Kaitangata mine accident

On 21 February, 34 miners were killed in an explosion at the Kaitangata coal mine in Otago. Find out more.

1881 Tararua shipwreck

On 29 April the steamer Tararua was wrecked off Waipapa Point, Southland. Of the 151 passengers and crew on board, 131 were lost in the worst civilian shipwreck in New Zealand waters. Find out more

1882 Timaru harbour tragedy

On 14 May a sudden storm wrecked two large sailing ships, the City of Perth and Ben Venue, in Timaru's exposed roadstead. Nine lives were lost. Among the dead were the port's harbourmaster and five local watermen, who had tried to rescue the ships' crews. See memorial for this event and more information on Te Ara

1886 Taiaroa shipwreck

On 11 April the steamer Taiaroa struck rocks near the mouth of the Clarence River, north of Kaikōura, and sank with the loss of 34 lives. Read a contemporary newspaper account.

1886 Tarawera eruption

On 10 June the volcanic Mt Tarawera, south-east of Rotorua, erupted spectacularly, killing perhaps 120 people and burying the famed Pink and White Terraces on Lake Rotomahana. Find out more on this site and Te Ara

1894 Wairarapa shipwreck

On 29 October, in a heavy fog, the liner Wairarapa steamed into cliffs on Great Barrier Island, with the loss of 121 of its 251 passengers and crew. Read contemporary newspaper account.

1896 Brunner mine accident

On 26 March an explosion at Brunner, West Coast, killed 65 coal miners in New Zealand's worst mining disaster. Find out about this and other mining accidents on Te Ara.

1900 Mōtū River accident

On 5 August, 16 children and two adults were drowned after their canoe capsized as they crossed the Mōtū River. Te Whānau-ā-Apanui placed a rāhui (ban) on fishing in the area. Find out about this accident. (Sooty's)

1902 Loch Long shipwreck

In late May the three-masted sailing ship Loch Long was wrecked off the Chatham Islands, with the loss of 24 lives. Find out more

1902 Elingamite shipwreck

On 9 November the steamer Elingamite was wrecked on the Three Kings Islands, north of Cape Rēinga, with the loss of 45 lives. Find out more on Te Ara.

1909 Penguin shipwreck

On 12 February the Cook Strait ferry Penguin struck rocks off Cape Terawhiti and sank with the loss of 72 lives. Find out more.

1914 Landslide on White Island

On 10 September 10 sulfur miners were killed on White Island when part of the crater wall collapsed, causing a landslide. Find out more

1914 Huntly mine accident

On 12 September 43 coal miners were killed in an explosion at Ralph's Mine, Waikato. Find out more

1923 Ongarue railway accident

On 6 July the North Island main trunk express slammed into a huge landslide at Ongarue, north of Taumarunui. With 17 deaths, this was the first major loss of life on New Zealand's railways. Find out more.

1926 Dobson mine accident

On 3 December an explosion at the Dobson coal mine on the West Coast killed nine miners. Read contemporary newspaper report

1929 Murchison earthquake

On 17 June an earthquake measuring 7.8 on the Richter scale struck the north of the South Island, killing 17 people. The shock was felt throughout New Zealand but centred on the Murchison area, where it caused massive landslides. Find out more on Te Ara.

1931 Hawke's Bay earthquake

On 3 February New Zealand's deadliest earthquake, measuring 7.8 on the Richter scale, devastated much of Napier and Hastings. The official death toll was 256, but 258 is likely to be a more accurate figure. Find out more on Te Ara. See also: images of the Hawke's Bay earthquake

1938 Kōpuawhara flood

On 19 February a flash flood swept away a Public Works railway construction camp at Kōpuawhara on the East Coast, killing 21 workers. Find out more.

1939 Huntly mine accident

On 24 September 11 men were asphyxiated by carbon monoxide at the Glen Afton coal mine, Huntly. Read more

1942 Seacliff Mental Hospital fire

On 9 December a fire at Seacliff Mental Hospital, north of Dunedin, killed 37 of the 39 female patients in Ward 5. Find out more

1943 Hyde railway accident

On 4 June the Cromwell-Dunedin express derailed near Hyde, Central Otago, with the loss of 21 lives. Find out more

1943 Paekākāriki maritime accident

On 19 June a LCVP (landing craft, vehicle and personnel) from the troop transport USS American Legion was swamped during an amphibious landing exercise at Paekākāriki, north of Wellington. Ten US Navy sailors drowned. Find out more

1943 US Liberator crash

On 2 August a US Liberator transport aircraft carrying enemy internees to Australia crashed into a mangrove swamp adjacent to Whenuapai airfield, killing 15 of the 30 people on board: nine Japanese (three women, two men and four children), three Thai students, and three members of the crew. Find out more

1947 Ballantyne's fire

On 18 November 41 people were killed in New Zealand's deadliest fire, in the Ballantyne's Department Store in Christchurch. Find out more.

1948 Mt Ruapehu air crash

On 23 October a Lockheed Electra airliner crashed near Mt Ruapehu, with the loss of all 13 passengers and crew. Find out more.

1949 Waikanae air crash

On 18 March a Lockheed Lodestar airliner crashed near Waikanae on the Kapiti Coast. All 15 passengers and crew were killed. Find out more from Te Ara.

1950 Ranui shipwreck

On 28 December the passenger launch Ranui, returning from a holiday trip to Mayor Island, was wrecked on North Rock, Mt Maunganui. Of the 23 people on board, only one survived. Read more

1951 Wellington to Lyttelton yacht race

On 23 January, 20 yachts left Wellington bound for Lyttelton in an ocean yacht race to celebrate Canterbury's centenary. Following a severe southerly storm only one yacht officially finished the race. Two others were lost along with their 10 crew members. Find out more.

1953 Tangiwai railway accident

On 24 December a North Island main trunk express plunged off the Tangiwai bridge into the Whangaehu River. The bridge had been fatally weakened by a lahar from Mt Ruapehu's crater lake. Of the 285 people on board, 151 were killed. This is New Zealand's worst rail disaster. See Tangiwai disaster for more information.

1959 Holmglen shipwreck

On 24 November the coaster Holmglen foundered north of Oamaru. All 15 crew were lost. Find out more.

1963 Northland bus crash

Fifteen people were killed in New Zealand's worst bus accident. A party was returning from Waitangi Day celebrations when shortly after lunchtime on 7 February the bus failed to take a bend as it descended Pilbrow Hill, in the Brynderwyn Hills, south of Whāngārei. Find out more.

1963 Kaimai air crash

On 3 July a DC-3 airliner crashed in the Kaimai Range, Bay of Plenty. All 23 passengers and crew were killed in what remains the worst air crash within New Zealand. Find out more.

1966 Kaitawa shipwreck

On 23 May near Cape Rēinga the collier Kaitawa was lost with all 29 hands. Read more

1967 Strongman mine accident

On 19 January an explosion at the Strongman coal mine, near Greymouth, killed 19 miners. Find out more.

1968 Wahine shipwreck

On 10 April the Lyttelton-Wellington ferry Wahine struck Barrett Reef at the entrance to Wellington Harbour in atrocious conditions caused by tropical cyclone Giselle. Of the 734 passengers and crew on board, 51 died (a 52nd victim died several weeks later, and a 53rd of related causes in 1990). See Wahine disaster for more information.

1968 Inangahua earthquake

On 24 May an earthquake measuring 7.1 on the Richter scale struck the Inangahua area on the West Coast. Three people were killed. Find out more on Te Ara.

1968 Maranui shipwreck

On 13 June the wheat-carrying coaster Maranui sank east of the Coromandel Peninsula when its cargo shifted in a storm. Nine of the crew drowned.

1979 Mt Erebus air crash

On 28 November an Air New Zealand DC-10 airliner, on a sightseeing flight to Antarctica, crashed into Mt Erebus. All 257 passengers and crew were killed in New Zealand's worst air disaster. See Mt Erebus disaster and the related entry on Te Ara for more information.

1985 Tory Channel air crash

Only one passenger survived when an Air Albatross Cessna flew into electrical wires strung across Tory Channel on 4 October. The pilot and seven members of two families were killed. Read more (Aviation Safety wiki).

1988 Whanganui air crash

On 12 May 10 people died when a charter plane crashed in Ahu Ahu Valley, near Whanganui. Read more

1989 Milford Sound air crash

On 8 August 10 people died when a scenic flight from Wanaka to Milford Sound crashed into the side of a mountain in the Upper Dart Valley. Read more

1993 Franz Josef Glacier air crash

On 25 October nine people died when a sightseeing plane crashed into Franz Josef Glacier on the West Coast. Read more

1995 Cave Creek disaster

On 28 April a Department of Conservation viewing platform built over a cliff at Cave Creek in the West Coast's Paparoa National Park collapsed, killing 14 people. Read more

2000 Lindis Pass air crash

On 24 April a Cessna aircraft whose passengers had been attending the Warbirds Over Wanaka airshow crashed into the side of a hill in Central Otago. The pilot and five passengers were killed. Read more

2008 Tongariro tragedy

On 15 April six students and their teacher from Auckland's Elim Christian College drowned in a canyoning accident while participating in an outdoor education programme near the Tongariro National Park. Read more

2010 Fox Glacier air crash

On 4 September nine people died when a skydiving plane crashed after taking off from Fox Glacier airfield on the West Coast. Read more.

2010 Canterbury (Darfield) earthquake

Although there were no deaths, this 4 September earthquake was - at the time - the largest to affect a major urban area since the 1931 Hawke's Bay quake. Read more

2010 Pike River mine accident

Two explosions on 19 and 24 November resulted in the deaths of 29 coalminers at the Pike River mine on the West Coast. It was this country's worst mining disaster since 1914. Read more

2011 Christchurch earthquake

At 12.51 p.m. on 22 February a 6.3 magnitude earthquake struck 10 km south-east of central Christchurch at a depth of only 5 km. The death toll was 185*, making it New Zealand's worst natural disaster in terms of loss of life since 1931. Read more

2012 Carterton balloon tragedy

At approximately 7.30 a.m. on 7 January 11 people - five couples and the pilot - were killed in a balloon accident near Carterton. A fire ignited on board causing the hot air balloon to crash in farmland. It is New Zealand's worst ballooning accident. Read more on NZHerald website

2012 Foveaux Strait fishing tragedy

Around midnight on 14 March The Easy Rider, a fishing boat, capsized off the northern tip of Stewart Island. Eight of those on board drowned. The single survivor said the boat was swamped by a rogue wave. Read more on NZHerald website

2016 Kaikōura earthquake

At 12.02 a.m. on 14 November a magnitude 7.8 earthquake struck New Zealand, causing significant damage to buildings and infrastructure in southern Marlborough and northern Canterbury. Landslides cut off road and rail links to Kaikōura, stranding large numbers of visitors in the popular tourist town. Two people lost their lives: one at a property in Mt Lyford as a result of a heart attack and another when a homestead collapsed in Kaikōura. Read more on Wikipedia

2016 Kaipara boating tragedy

On 26 November Francie, a chartered fishing boat, capsized while attempting to cross the Kaipara Harbour bar. Eight of the 11 men on board drowned. Read more on RNZ website

* The official toll was initially given as 181 but four further victims were confirmed by the coroner in February 2012.

See also:

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This timeline lists New Zealand's worst post-1840 natural disasters, transport accidents, fires, mining accidents and other tragedies that have caused major loss of life. Follow the links for more information on NZHistory, Te Ara and other sites.

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Natural Hazards and Risks in Aotearoa New Zealand

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Many people probably know marshmallows from toasting them over a campfire. Kraft Food's JET-PUFFED marshmallows are widely available, and involve some interesting technology, so I will try and explain the basic procedures involved in making these soft and airy products. But, I have to admit that I do not know the exact recipe used for them.

The main ingredients for this product listed in order are corn syrup, sugar, modified corn starch, water and gelatin. The first step is to hydrate and dissolve the gelatin in hot water. Then, the corn syrup, sugar, modified corn starch and the rest of the water are combined and concentrated by evaporation of water until the boiling point rises to somewhere around 240° to 245°F.

Next, the hot syrup and gelatin solution are combined and the marshmallow becomes a marshmallow when air is incorporated into the still hot molten mixture.

According to the company's web site - Kraftfoods.com - jet puffing is a patented process used since 1953 to incorporate air into these particular marshmallows [Note: there is more on the history of marshmallows on that site as well, including the fact that over 90 million pounds of marshmallows are purchased in the US annually!]. The marshmallow mixture is then shaped via a process called extrusion. According to the Kraftfoods.com web site, this involves pumping the mixture through tubular shaped pipes. The temperature of extrusion has to be just right so that the mixture will flow under pressure, but hold its shape after leaving the pipe. After further cooling, the marshmallow structure sets and they are cut into individual cylindrical pieces, dusted with corn starch to prevent sticking and bagged for sale.

We see the return to the molten state when we toast the marshmallows over the open fire because the gelatin gel part of the marshmallow melts back into the liquid state. Thus, gelatin-water gels in general are said to be "thermoreversible". These same ingredients are used to make rubbery gummy bear type products (minus the starch), chewy spice drops (minus the gelatin) or jelly candies (replace the starch and gelatin with pectin). It is magical how these different biopolymer gelling agents interact with the corn syrup and sugar to give so many variations of texture.

Scientist
Steven Mulvaney

Associate Professor

Food Science, Cornell University

Question From

Kris Corda
Hometown:
Ithaca
State:
NY
Interests:
Reading, skiing, baking, dancing, spending time with family and close friends
Future Career:
Higher Ed. Administrator, currently the director of the Big Red Barn Graduate and Professional Student Center at Cornell

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How Marshmallows Are Made | Unwrapped 2.0 | Food Network

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What is gravity or how does gravity work?

January 2001

Imagine two or more extra dimensions of space curled up into regions too small for us to directly sense. Imagine a chunk of gravity as a vibrating string that wanders through all of the 5+ dimensions of space. Imagine space itself twisted and curved near a black hole. This is the amazing world of gravitation research.

Models of gravity began with Aristotle (384-322 BC) who thought that the natural place for an object was "down" and this described gravity. Galileo (1564-1642), by experiment, showed that gravity caused all objects to fall to Earth at the same rate but was unable to explain why. Isaac Newton (1642-1727) provided us with his Universal Gravitational Law. While Newton greatly added to our understanding of how gravity works, we still don't know why gravity works.

Newton's law stated that every object in the universe with mass attracts every other object in the universe that has mass. This force is proportional to the product of the two masses and inversely proportional to the square of the distance between their centers. Thus two coffee cups have an attractive force on each other, and two planets also have an attractive force on each other, as will a planet and a coffee cup. All objects possess inertia (a tendency to maintain their state of motion). Objects with greater mass have greater inertia. So when the coffee cup and the planet are free to move, the cup does the moving because the planet has too much inertia to move any appreciable amount. Thus the cup "falls" to the planet. The force of gravity on an object on Earth is the "weight" of the object.

While gravitational forces act on things that are close together, they also act on things that are at great distances apart. For example the Earth and Moon are 240,000 miles apart but gravity creates an immense force (over a billion, trillion pounds) between the two bodies. This is sometimes referred to as action at a distance. Although it is hard philosophically to image a force exerted over such a distance through empty space, such is Newton's Gravitational Law. If it were not for gravity, the Moon would move off from Earth in a straight line because of its motion and inertia.

In this past century Einstein (1879-1955) explained gravity with his general relativity in terms of a warping or bending of space in the presence of an object. The more massive an object the more space is warped. This theory is arguably the most mathematically fierce in all of physics and cannot be understood without mathematics. If objects are not too massive, both Newton and Einstein's theories give similar results. However for very dense or massive objects, Einstein's theory provides a more accurate description. Scientists are still working on improving or further understanding the theory of gravity. The next level of understanding seems to be quantum gravity. Today considerable money and effort is being invested in an attempt to find gravity waves and/or particles called gravitons. Research is also under way to determine if Newton's theory must be modified because of the presence of extra dimensions. We will probably never have a perfect theory of gravity.

Scientists are essentially model builders. As data is collected and understanding expands, scientists constantly improve the model. Just as Galileo improved on Aristotle's theory, Newton improved on Galileo's, Einstein improved on Newton's, and so on it will go. New models are adopted if they better predict nature and are more aesthetically pleasing. People may someday regard our models of gravity as childish, but they work for us now and allow us to make predictions about the universe. As for a definitive answer, only the One who created gravity knows exactly how and why it works.

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What is gravity or how does gravity work? | Science Guys | Union University, a Christian College in Tennessee

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What is gravity or how does gravity work? Science Guys article by The Department of Physics at Union University

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Gravity for Kids | Learn all about how gravitational force works

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In the 1600s, an English physicist and mathematician named Isaac Newton was sitting under an apple tree -- or so the legend tells us. Apparently, an apple fell on his head, and he started wondering why the apple was attracted to the ground in the first place.

Newton publicized his Theory of Universal Gravitation in the 1680s. It basically set forth the idea that gravity was a predictable force that acts on all matter in the universe, and is a function of both mass and distance. The theory states that each particle of matter attracts every other particle (for instance, the particles of " Earth" and the particles of "you") with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

So the farther apart the particles are, and/or the less massive the particles, the less the gravitational force.

The standard formula for the law of gravitation goes [source: UT]:

Gravitational force = (G * m1 * m2) / (d2)

where G is the gravitational constant, m1 and m2 are the masses of the two objects for which you are calculating the force, and d is the distance between the centers of gravity of the two masses.

G has the value of 6.67 x 10E-8 dyne * cm ²/gm ². So if you put two 1-gram objects 1 centimeter apart from one another, they will attract each other with the force of 6.67 x 10E-8 dyne. A dyne is equal to about 0.001 gram weight, meaning that if you have a dyne of force available, it can lift 0.001 grams in Earth's gravitational field. So 6.67 x 10E-8 dyne is a miniscule force.

When you deal with massive bodies like the Earth, however, which has a mass of 6E+24 kilograms (see How much does planet Earth weigh?), it adds up to a rather powerful gravitational force. That's why you're not floating around in space right now.

The force of gravity acting on an object is also that object's weight. When you step on a scale, the scale reads how much gravity is acting on your body. The formula to determine weight is [source: Kurtus]:

weight = m * g

where m is an object's mass, and g is the acceleration due to gravity. Acceleration due to gravity on Earth, is 9.8 m/s² -- it never changes, regardless of an object's mass. That's why if you were to drop a pebble, a book and a couch off a roof, they'd hit the ground at the same time.

For hundreds of years, Newton's theory of gravity pretty much stood alone in the scientific community. That changed in the early 1900s.

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Gravity is a force that we experience every minute of our lives, but hardly notice or give a passing thought to in our daily routines. Have you ever wondered what gravity is and how it works? Learn about the force of gravity in this article.

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Gravity is one of the universe's fundamental forces and dominates every moment of our conscious experience. It keeps us close to the ground, drags baseballs and basketballs out of the air and gives our muscles something to struggle against. Cosmically, gravity is just as consequential.

From collapsing hydrogen clouds into stars to gluing galaxies together, gravity represents one of just a few players that determine the broad strokes of the universe's evolution.

In some ways, the story of gravity is also the story of physics, with some of the field's biggest names finding fame by defining the force that ruled their lives. But even after more than 400 years of study, the enigmatic force still lies at the heart of some of the discipline's greatest mysteries.
Related: How do you create lunar gravity in a plane? A veteran zero-G pilot explains
Gravity: A universal force

Four fundamental forces act upon us every day. The strong force and the weak force operate only inside the centers of atoms. The electromagnetic force rules objects with excess charge (like electrons, protons, and socks shuffling over a fuzzy carpet), and gravity steers objects with mass.

The first three forces largely escaped humanity's notice until recent centuries, but people have long speculated about gravity, which acts on everything, from raindrops to cannonballs.

Ancient Greek and Indian philosophers observed that objects naturally moved toward the ground, but it would take a flash of insight from Isaac Newton to elevate gravity from an inscrutable tendency of objects to a measurable and predictable phenomenon.

Newton's leap, which became public in his 1687 treatise " Philosophiae Naturalis Principia Mathematica", was to realize that every object in the universe - from a grain of sand to the largest stars - pulled on every other object. This notion unified events that appeared unrelated, from apples falling on Earth to the planets orbiting the sun. He also put numbers to the attraction: Doubling the mass of one object makes its pull twice as strong, he determined and bringing two objects twice as close quadruples their mutual tug. Newton packaged these ideas into his universal law of gravitation.

(Image credit: vchal via Getty Images)

Gravity Q&A with an expert

We asked Glenn Starkman, a Distinguished University Professor and co-Chair of Physics and Professor of Astronomy at Case Western Reserve University, a few frequently asked questions about gravity.

Distinguished University Professor and Co-chair Department of Physics

How does gravity work?

Now that's a straightforward question with a deep answer. Newton did an awfully good job at giving us an answer - the Law of Universal Gravitation that I quoted above. So good that we call the constant of proportionality, Newton's Gravitational Constant, and write it GN, or just G. In equation form I would write the gravitational force F between two objects as F = Gm1m2/r^2, where m1 and m2 are the two masses, and r is the distance between their centers.

Unlike g (lower case), which as I said varies with your location, G appears to be a constant of nature - the same in every place and at every time. People spend a lot of time trying to get very accurate measurements of G, but it is the most poorly measured constant of nature, known to only about 20 parts per million. (In contrast, the comparable constant for the electromagnetic force, called the fine-structure constant, alpha, is measured to about one part in 10 billion.)

So Newton's Law of gravitation is a very very good description of how gravity works in most circumstances. Without getting into all the controversies around who came up with what first, let's say that Newton discovered (or at least first published) this law in 1686. It is good enough for almost everything we do in everyday life. What is amazing is that it was also good enough to explain everything that astronomers were learning about the orbits of planets and moons. However, by the late 1850s, it was recognized that Mercury's orbit was ever so slightly wrong. (To be specific the orientation of the elliptical orbit was off by about 43 arcseconds - an arcsecond is 1/3600 of a degree - per century!)

This was the observational motivation that led to Einstein's Theory of General Relativity (GR), which he published in 1915. GR has a completely different take on the nature of gravity. Indeed, it has a completely different take on the nature of space and time. Or at least that is the most common interpretation of GR, called the geometric interpretation. (An interpretation is the story we tell in ordinary language about the mathematical equations of a theory. It is the mathematical equations that we compare with the measurements, but we use the story to explain the equations to fellow humans. The story also helps us think about the theory.) I will just say "according to GR," when what I mean is "according to the geometrical interpretation of GR."

According to GR's precursor, Special Relativity (published by Einstein in 1905), space and time are not separate in the way we normally think about them and in the way scientists treated them until then. They are part of a combined object/concept called spacetime. The division of spacetime into space and time depends on the person doing it, in particular, two people moving relative to one another will make that separation differently. Yeah, whoa. That's hard to wrap your head around.

According to GR, space (and time) are also not "static." In our normal life, we think of space as kind of like a giant "stage" that the actors - us, the planets, the stars - move around on. So according to our normal picture space doesn't change. GR upends that completely. It says that actually space(time) changes in response to the presence of objects (i.e. mass and energy) in it. These changes take the form of alterations in the geometry - the rules describing how distances and angles between points in space are related. The rules of geometry we learn in middle school, called Euclidean geometry, are not exactly right, because space(time) is not Euclidean (aka flat), instead it is "curved" and that curvature changes from place to place and from one time to another.

And then Einstein tells us objects move in that curved spacetime geometry. What we perceive as the effects of "gravity" (say, on falling pumpkins) is actually the motion of objects through the curved spacetime. So Einstein would say that if I throw a baseball from the outfield to the first baseman, and I see it following an arc, what is really happening is that the Earth has curved the spacetime around it, and the baseball is following a straight path through that curved spacetime from the outfielder to the first baseman.

To summarize, according to Einstein, gravity is the curving of spacetime by all the objects in it, combined with the "geodesic" (straight) motions of those objects through the spacetime.

A good analogy to help you think about this is to imagine a rubber sheet stretched taut in a frame, and you put a bowling ball in the center - the sheet stretches. Then you roll marbles on the surface of the curved rubber sheet. The marbles move in what appear to you to be curved paths, but actually, they are straight paths in the geometry of the curved sheet.

GR is quite well-tested, especially in the solar system.

What's the gravitational pull on Earth and how do we know?

If you get up in the morning (or go to the doctor's office) and stand on a scale, you are measuring the gravitational pull of the Earth on you. We call that your weight. And what is happening when you are stepping on that scale is that the scale is balancing the Earth's gravitational pull down on you with the force of a bunch of springs pushing up on you. The more you compress the spring the harder it pushes up. The display on the scale shows how hard the springs are pushing.

There is another way that we can measure the gravitational pull - by dropping something, say a pumpkin, and seeing how much it accelerates. Newton has taught us that F = m a. This is Newton's Second Law of Motion. Here F is a force - in this case the gravitational force on the pumpkin; m is the mass of the pumpkin; a is the acceleration of the pumpkin. So if I measure the acceleration of the pumpkin, and I measure the mass of the pumpkin (let's leave aside how you do that), then I know the force of gravity on the pumpkin!

Interestingly, no matter what we drop - pumpkin, bowling ball, rock, Ming Vase - if we can compensate for/ignore/eliminate the effects of air resistance, we find it falls at the same acceleration: approximately 9.8 m/s/s. We call this g (lower case), the "acceleration due to gravity." This is the famous legend that has Galileo dropping objects from the Leaning Tower of Pisa. It tells us that the gravitational force is proportional to the mass of the object. This is such an important discovery that physicists call it a [rinciple, the Equivalence Principle. Why "equivalence"? Longer story.

The gravitational pull of the Earth on the surface of the Earth is m g, where m is the mass of the object, and g is 9.8 m/s/s. So the very simple answer to the question is "the gravitational pull on the Earth is (the force that causes) an acceleration of approximately 9.8 m/s/s."

Actually, this acceleration g isn't truly a constant. The answer depends on how far you are from the center of the Earth - it is a bit higher near the North and South pole, and a bit lower near the equator. (Also, if you measure it accurately, you have to take account of the centrifugal acceleration because your "laboratory" is on the surface of the Earth which is spinning on its axis once a day.)

Newton already understood this way back in the 1600s! He taught us Newton's Law of Universal Gravitation, which says that "every particle attracts every other particle in the universe with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centers." In this case, the two particles are the Earth and the pumpkin.

Does gravity affect time?

Yes. For example, clocks tick more slowly near a massive object than farther away. In other words, if you had two identical very accurate clocks and placed one in a laboratory at sea level, and the other on a high mountain, you would notice that the one at sea level is running more slowly than the one on the mountain.

You might worry that something had happened to the clocks, so you could switch their positions, and now you would find that again the one at sea level is running more slowly than the one on the mountain. Don't imagine that you can use this to, say, put off that next birthday. After a few billion years, the core of the Earth is a couple of years younger than the surface. But it is not too small an effect to measure. It was first confirmed experimentally in 1959 by Pound and Rebka.

Distinguished University Professor and co-Chair of Physics and Professor of Astronomy at Case Western Reserve University, Director of the Institute for the Science of Origins, Director of the Center for Education and Research in Cosmology and Astrophysics.

Gravity: A tool of discovery

The modern description of gravity so accurately predicts how masses interact that it has become a guide for cosmic discoveries.

American astronomers Vera Rubin and Kent Ford noticed in the 1960s that galaxies appear to rotate fast enough to spin off stars like a dog shakes off water droplets. But because the galaxies they studied weren't whirling apart, something appeared to be helping them stick together. Rubin and Ford's thorough observations provided strong evidence supporting Swiss astronomer Fritz Zwicky's earlier theory, proposed in the 1930s, that some invisible variety of mass was speeding up galaxies in a nearby cluster. Most physicists now suspect this mysterious " dark matter" warps spacetime enough to keep galaxies and galaxy clusters intact. Others, however, wonder if gravity itself might pull harder at galaxy-wide scales, in which case both Newton's and Einstein's equations would need adjustment.

Tweaks to general relativity would have to be delicate indeed, as researchers recently started detecting one of the theory's most subtle predictions: The existence of gravitational waves, or ripples in spacetime, caused by the acceleration of masses in space. Since 2016, a research collaboration operating three detectors in the United States and Europe has measured multiple gravitational waves passing through Earth. More detectors are on the way, launching a new era of astronomy in which researchers study distant black holes and neutron stars - not by the light they emit, but by how they rumble the fabric of space when they collide.

Yet general relativity's string of experimental successes gloss over what many physicists see as a fatal theoretical failure: It describes a classical spacetime, but the universe ultimately appears to be quantum or made up of particles (or "quanta") such as quarks and electrons.

The classical notion of space (and gravity) as one smooth fabric clashes with the quantum picture of the universe as a collection of sharp little pieces. How to extend the reigning Standard Model of particle physics, which spans all known particles as well as the other three fundamental forces (electromagnetism, the weak force, and the strong force), to cover space and gravity at the particle level remains one of the deepest mysteries in modern physics.

Additional resources

Watch this video explaining Newtonian gravity, from PBS Digital Media's Crash Course. Learn more about g ravity and how the warps and ripples in space work, from the Australian Academy of Science. Read astrophysicist Ethan Siegel's explanation of how gravity bends (and unbends) space, generating gravitational waves on Medium.

Bibliography

Head, Keith. "Gravity for beginners." University of British Columbia (2003).

Poisson, Eric, and Clifford M. Will. "Gravity." Cambridge University Press (2014).

Why does gravity pull us down and not up? Mario Borunda, The Conversation (2021).

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Gravity can be described in a variety of ways. Here's how Newton and Einstein took gravity from an observation to a measurable phenomenon.

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How Are Professional Video Games Made? See here!

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How Twinkies Are Really Made | The Follow | TODAY

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After a long day of school and an after-school club, you finally get home. You tackle your homework while you gobble down a healthy dinner. When you finally have some free time to , you grab a controller and get ready to go for a virtual drive or defend a planet from an invasion.

Playing video games is a popular for kids of all ages. For most kids, the video game journey begins at the video game store and ends at the . But have you ever given any thought to all the work that goes into creating the video games you love?

Some people see video games as simple electronic toys that do not take long to produce. In reality, though, the most popular modern video games are productions that the biggest summer blockbusters created by Hollywood movie studios. Video games can take a year or longer to be made!

Video games might have started out as chasing around a moving dot across a black screen, but today's modern games are interactive marvels that resemble movies and allow for game play across a wide variety of settings.

Every modern video game begins with a story. Whether it's a simulation of a historical or role-playing in a future fantasy world, the story drives the action of the video game and creates a strong between the player and the characters in the game.

Writers, artists, designers, and programmers must collaborate to create all the parts of the story, including the characters, the settings, the music and other sounds, and the possible paths the story can take. Unlike movies, which contain a single , video games must account for all the potential paths intense gameplay can take.

Today's gamers expect life-like video games with the best graphics possible. Characters are expected to move like real people would and objects are usually expected to loosely follow the laws of physics. Settings must look like the real world...or whatever fake world exists in the video game. The level of detail that artists and designers must achieve is challenging. That's why it could take over a hundred people to develop a single video game!

Once all the elements of the video game have been designed, it's up to computer experts to put it all together to make it work effectively as a video game. All the characters, settings, sounds, and gameplay elements must be programmed to work together seamlessly.

Computer programmers must write millions of lines of computer language instructions - called code - that control all aspects of the game. Code can be written in a variety of different computer languages, such as C++, Python, Visual Basic, and Perl.

If you think about controlling a particular in a game, there's a seemingly endless variety of things you can do with that . Code must be written to account for all the possible choices you can make with your . Each move you make leads to changes in the setting, as well as a whole new set of options for your next move. computer code controls all these factors and makes playing the game a seamless experience for the user.

Once a video game is nearly finished, it enters the testing phase, where players test every element of the game in order to find and fix any errors - called bugs - before the game is released. Upon release, the game is marketed to the general public and you get to enjoy the fruits of the thousands upon thousands of hours that went into making the game!

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We hope all you crafty miners will answer the call of duty and join us for today's Wonder of the Day!

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Join us in Wonderopolis today as we go up, up, up to learn about what brings us down, down, down!

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How do you like to communicate with people you know? Do you use email? Perhaps you like to text your friends. Maybe you use social media. Technology has made instant messaging -and slower physical mail is sometimes called "snail mail." Even if you're crazy about technology, though, there's still something about going out to the mailbox and finding an old-fashioned handwritten letter waiting for you.

What kinds of mail do you get in your mailbox at home? Does your family to any ? Maybe you get lots of bills and ads, too.

But what about notes from friends or relatives? On your birthday, do you get cards in the mail? How about get-well wishes when you've been ill?

Have you ever thought about how those pieces of mail get from their senders to your mailbox? When you stop to think about it, it's really amazing how so much mail can go to so many places in such a short time.

It all starts when the United States Postal Service (USPS) a piece of mail. This can happen when you put a letter in your mailbox. You can also put a letter in a drop box or give it to an employee at a post office.

From your post office, your mail travels to a mail processing plant. There, they sort it for shipment. The USPS uses advanced technology to mail for ZIP Codes. With this data, they print onto pieces of mail that are readable by routers-which separate mail into different areas for more sorting and then shipment.

Some mail may need to be sorted and re-routed several times, depending on how far it is traveling. It may end up on huge trucks or even airplanes. Eventually, it will reach a and finally a post office. There, they sort and route it for delivery to an address.

While computers do much of the work, people complete the last piece of the puzzle-hand delivery to a specific mailbox by a USPS mail carrier. And they carry a lot of mail! Over 500,000 U.S. Postal Service employees work hard to make sure that almost 129 billion pieces of mail get delivered every year!

Do you like to receive mail? What about sending a letter? Do you think you can make someone's day by sending them a note? Your mail is safe with the USPS-after all, their ads in the 1990s said, "We deliver!"

CCRA.R.1, CCRA.R.2, CCRA.R.3, CCRA.R.8, CCRA.R.9, CCRA.R.10, CCRA.L.1, CCRA.L.2, CCRA.L.3, CCRA.L.4, CCRA.L.5, CCRA.L.6, CCRA.SL.2, CCRA.SL.3, CCRA.W.2

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Join us in Wonderopolis today as we trace a letter!

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Dr. James Naismith was a Canadian physical instructor who invented the game of basketball in 1891 while working at the YMCA Training School in Springfield, Massachusetts.

Dr. Naismith had been challenged to a new game that could be played indoors in the cold Massachusetts winters to provide an " distraction " to a disruptive of students. With a two-week , Dr. Naismith decided to invent a game of skill, , and , rather than one that relied on pure .

He was inspired by a game he had played as a child called "duck on a rock," in which players a small rock at a "duck" placed on top of a large rock in an attempt to knock the "duck" off.

Using a soccer ball, two peach baskets placed 10 feet up in the air, nine players on each team, and a set of 13 basic rules, Dr. Naismith invented the game of "basket ball." The first game was played on December 21, 1891.

Initially, players could only advance the ball by passing it. Bouncing the ball along the floor - what we call "" today - did not become part of the game until later.

Players earned points by successfully tossing the soccer ball into the peach baskets. After each basket that was made, players had to climb a ladder to retrieve the ball from the basket. Iron hoops with open-ended nets didn't come along until 1913!
Interesting basketball facts:

Dr. Naismith was inducted into the Basketball Hall of Fame in 1959. The Basketball Hall of Fame is now called the Naismith Memorial Hall of Fame.

The first college basketball game was played on January 18, 1896, when the University of Iowa hosted a game with the University of Chicago. The final score was: Chicago 15, Iowa 12.

U.S. patent #1,718,305 was granted to G.L. Pierce on June 25, 1929, for the first version of what we now recognize as the "basketball."

"March Madness" began in 1939, when the first NCAA tournament took place at the University of Illinois.

Basketball became an official Olympic sport at the 1936 Olympic Games in Berlin, Germany.

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Dr. James Naismith was a Canadian physical instructor who invented the game of basketball in 1891 while working at the YMCA Training School in Springfield, Massachusetts. Dr. Naismith had been challenged to a new game that could be played indoors in the cold Massachusetts winters to provide an " distraction " to a disruptive of students.

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The time taken to get there will vary depending on where your item is being sent from and to.
Local/regional deliveries sent within a town/city/island will be delivered in the next working day by 9:30am and for residential addresses, you should allow up to 5pm. Read more

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If you do not have an account, you can send items in an express pack or a box with relevant tickets. You can purchase these tickets or express packs from your local branch. Read more

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As an express courier service provider, we specialise in the rapid delivery of smaller parcels, as opposed to the movement of pallets and other larger freight items performed by general logistics and trucking companies. Our maximum weight is 25kg, and the maximum length is 1.8 meters. Read more

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Couriers, Parcels, Logistics & Air Freight | New Zealand Couriers

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Looking For Express Courier Services? Whether You Need Domestic Shipping In NZ Or International Shipping, Post Haste Is The Best Choice. Find Out More Now!

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