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Charge, Current and Voltage | GCSE Physics | Doodle Science

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Follow me!: https://twitter.com/DoodleSci Doodle Science teaches you high school physics in a less boring way in almost no time! Script: There are three things to understand when we talk about electricity: Charge, Current and Voltage. All objects and materials have charge; it's created by the net of electrons or protons.

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The science of static electricity - Anuradha Bhagwat

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Changing Ideas •Henri Becquerel discovered radioactivity in 1896. When scientists first began to work with ionising radiation they did not take the safety precautions that we do now. They did not know about the dangers of radioactive materials at that time.

•Eventually the dangers became obvious. Scientists noticed that touching radioactive sources burned their skin. People who worked with ionising radiation for long periods developed cancers.
• We now know that ionising radiation can cause tissue damage (burns). Smaller amounts over a long period can cause mutations in the DNA, leading to cancer.
• To reduce the risks, radioactive sources are now handled with tongs and stored in lead-lined containers. They are never pointed at people and protective clothing is worn.
Nuclear Waste
•There are three types of radioactive waste: high level waste (HLW), intermediate level waste (ILW) and low level waste (LLW).

•HLW includes the fission products from nuclear power stations. It is very radioactive and stays that way for about 50 years. After this it becomes ILW, which is moderately radioactive but remains so for tens of thousands of years. LLW is only slightly radioactive.
• HLW is sealed in glass to stop the radiation escaping until it becomes ILW. ILW is stored in concrete and steel containers. LLW is buried in special landfill sites.
• Other possible options for disposing of radioactive waste include firing it into space, dumping it in barrels at sea or storing it deep underground.

• Nuclear power does not produce carbon dioxide so doesn’t add to the greenhouse effect. However, the radioactive waste produced is dangerous and needs to be carefully stored for thousands of years. There is also concern about the risk of accidents.
Half Life •The activity of a radioactive substance is the number of nuclear decays per second. It is measured in becquerel (Bq). 1 Bq means one nuclear decay per second. • The activity of a radioactive substance decreases over time as more of its unstable nuclei decay and become stable nuclei.
• The half-life of a radioactive substance is the time taken for half of the unstable nuclei to decay. Half-lives vary from a fraction of a second to millions of years depending on the substance.
• After one half-life the activity of a radioactive substance will have halved. After another half-life it will have halved again (so it is now a quarter of the original level). • A Geiger-Müller (GM) tube is used to measure radioactivity. The count rate is the amount of ionising radiation detected by the GM tube over a particular time.
Background Radiation
•Background radiation is the low level of ionising radiation that is around us all the time. The main source is radon gas.
•Radon gas is produced when uranium found naturally in rocks decays. Radon gas diffuses into the air and can build up in houses, which is dangerous. Radon has a half-life of 3.8 days and decays by giving out an alpha particle. The amount of radon given out by different rocks depends on their uranium level, which varies from place to place.

• Other sources of background radiation include cosmic rays, the ground and buildings (due to radioactive rocks), medical sources and food and drink.

• Background radiation levels can be measured by taking several readings with a GM tube (without a radioactive source being present) and then averaging them. This background count should then be subtracted when measuring the activity of a source.

Uses of Radiation

•Ionising radiation can be used to diagnose and treat cancer. Radioactive tracers can be used to show where there are abnormal activity levels in the body, such as in tumours. Beams of gamma rays can then be used to kill the cancer cells (radiotherapy).

•Ionising radiation can also be used to sterilise medical equipment. Gamma rays are used to kill any microorganisms. This is useful for plastic items that cannot be sterilised using heat.

• Food can also be irradiated with gamma rays to kill microorganisms and insect pests. This means that it can be stored for longer before it goes off and is safer. It does not make the food more radioactive.

More Uses of Radiation

•Smoke alarms contain a source of alpha particles. These ionise air particles, which allows a small electric current to flow across an air gap. If smoke gets into the air gap it absorbs the alpha particles and the current stops, causing the alarm to sound.
•Beta radiation can be used to control the thickness of paper. A detector detects beta particles coming through the paper. If the paper is too thick, the count rate decreases. If it is too thin, the rate increases. A computer uses this feedback to increase or decrease the pressure of the rollers to make the paper thicker or thinner. • Radioactive tracers are also used to detect leaks in underground pipes. A gamma source is added to the water flowing through the pipe. If there is a leak, the water leaks into the surrounding earth and a higher level of radiation will be detected in that area.

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Isotopes
•Atoms are made up of three sub-atomic particles: protons, neutrons and electrons.
•All atoms of the same element have the same number of protons (atomic number) but the number of neutrons can vary. This means atoms of the same element can have different mass numbers (protons + neutrons; also called nucleon number).
• Atoms of the same element that have the same number of protons (atomic number) but a different number of neutrons (mass number) are called isotopes.
• The element lithium has two isotopes: lithium-6 and lithium-7. Both isotopes have three protons. Lithium-6 has three neutrons, giving it a mass number of 6, while lithium-7 has four neutrons, giving it a mass number of 7.

Ionising Radiation

•If an atom loses (or gains) an electron, the numbers of protons and electrons no longer balance and so it has a charge and becomes an ion. Ionising radiation has enough energy to make atoms lose electrons and become ions.

•Radioactive substances have atoms with unstable nuclei. An unstable nucleus can decay to become more stable by emitting energy in the form of ionising radiation. This is a random process because it is not possible to predict when it will happen.

• There are three types of ionising radiation: alpha particles, beta particles and gamma rays.
• Alpha particles contain two protons and two neutrons and so have a charge of +2. They are strongly ionising but are stopped by a few centimetres of air or a sheet of paper.
• Beta particles are electrons so have a charge of -1. They are moderately ionising and travel further than alpha particles. They are stopped by a few millimetres of aluminium.

• Gamma rays are electromagnetic waves so have no mass and no charge. They are ten times less ionising than beta particles. They are partly stopped by several centimetres of lead but need several metres of concrete to be completely absorbed.

Nuclear Reactions

•In some large, unstable nuclei, the nucleus can split into two daughter nuclei. This is known as nuclear fission.
• A uranium-235 nucleus will undergo fission if it absorbs an extra neutron. It splits into two daughter nuclei and releases two or three neutrons. This gives out huge amounts of energy, mainly in the form of kinetic energy, as the daughter nuclei and neutrons are all moving at high speeds. Thermal energy is released too.
• The neutrons released in fission can be absorbed by other uranium-235 nuclei. If all the neutrons are absorbed and cause more fissions, there is an uncontrolled chain reaction. This is what happens in an atomic bomb.
• The chain reaction can be controlled if only one neutron from each fission is absorbed by another uranium-235 nucleus. This is what happens in a nuclear reactor, where other materials are used to absorb some of the neutrons.
Nuclear Power

•Nuclear power stations use uranium-235 pellets as fuel. The pellets are placed in fuel rods in the nuclear reactor core.

•The chain reaction is controlled using control rods. These absorb neutrons produced by fission. The control rods are placed in between the fuel rods. If the rate of fission needs to be increased, the control rods are moved out of the core a bit so fewer neutrons are absorbed. If the rate needs to be reduced, they are moved further in.

• The reactor core also contains a moderator. This slows down the neutrons released by fission to make it more likely that they will be absorbed by a uranium-235 nucleus.

• The thermal energy released by fission is transferred to a coolant. The coolant is pumped to a heat exchanger where it is used to produce steam. The steam drives a turbine, which turns a generator. This transfers the kinetic energy to electrical energy.

• Over time, radioactive waste builds up in the reactor core. This is made up of the radioactive daughter nuclei and radioactive isotopes formed when materials in the core absorb neutrons.

Fusion – Our Future?

•Nuclear fusion happens when small nuclei combine to form larger ones. This is the energy source for stars – hydrogen nuclei combine in stars to form helium.

• The fusion reaction between two isotopes of hydrogen (hydrogen-2 and hydrogen-3) is being investigated on Earth. It produces helium, a neutron and a lot of energy. The helium is not radioactive but the materials used to contain fusion do become radioactive.

• New scientific discoveries have to be validated by the scientific community before they are accepted. A report of the discovery is peer-reviewed (checked) by other scientists and they must be able to get the same results using the same method.

• In nuclear fusion the hydrogen nuclei have to be forced together before they will join. This is difficult because they are both positively charged so they repel each other.

• In stars the strong gravity creates high densities of nuclei, which helps to overcome the repulsion. On Earth very high temperatures and pressures are used instead. So far all the experimental fusion reactors have used more energy than they produced.

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Static Electricity

• Atoms contain protons (positive charge) and neutrons (no charge) in their nuclei. They also contain electrons (negative charge) moving around the nucleus.

• Electrons can be transferred if you rub two insulating materials together. One material will gain electrons and get a negative charge. The other will lose electrons and get a positive charge. Protons can not be transferred because they are fixed in the nucleus

• When objects are charged like this it is known as static electricity. If the charges are the same (both positive or both negative) then the objects will repel. If the charges are different the objects will attract.

• If a negatively charged object is held near a wall, it repels the electrons in the wall away from the surface. The surface is then positive and attracts the object. This is known as an induced charge. Positive objects have the same effect but by attracting extra electrons in the wall towards the surface to make it negative.

Uses and Dangers of Static Electricity

• You can build up an electrostatic charge in your body just by walking around. If you then touch a metal object you feel a small shock as the built-up electrons move from you to earth (or the other way around) through the metal.

• Static electricity can also build up on clouds and cause lightning.

• Electrostatic charges can build up on aircraft and refuelling pipes, which is dangerous. A spark caused by the static electricity discharging could ignite the fuel. To prevent this, a bonding line is used to earth aircraft before refuelling starts, and fuel hoses are made from conducting materials.

• Static electricity can be useful. If a sprayer is connected to one terminal of an electricity supply, all the droplets get the same charge and spread out. This is used in insecticide sprayers and paint sprayers. The object to be painted can also be given the opposite charge so that it attracts the paint and less is wasted.

Electric Currents

• All materials contain electrons. In conducting materials, like metals, some of the electrons are free to move about.
• In a complete electric circuit the cell pushes all the free electrons in the same direction at once around the circuit. This flow of electrons is called an electric current. Cells and batteries make the electrons flowing in the circuit move in the same direction all of the time, producing a direct current. Generators produce alternating current, where the electrons change direction many times per second.
• A current is a rate of flow of charged particles. Its size depends on how much charge passes a particular point each second.

• Charge and current are related by the equation:
Q  =  I × t   charge =   current ×   time   coulombs (C)   amperes (A)   seconds (s)