Electric current is the flow of the electrical charges. To transfer energy, it doesnt matter which way the charges are going. thats why an alternating current works.

• The mains supply is AC and the battery supply is DC
• The UK mains electricity supply is about 230V and can kill if not used safely.
• Electrical circuits, cables, plugs and appliances are designed to reduce the chances of receiving an electric shock.
• The more electrical energy used, the greater the cost.
• Electrical supplies can be direct current (d.c.) or alternating current (a.c.).

A mains electricity cable contains two or three inner wires. Each has a core of copper, because copper is a good conductor of electricity. The outer layers are flexible plastic, because plastic is a good electrical insulator. The inner wires are colour coded:

colour  wire bluen neutral brown live green and yellow stripes earth

The features of a plug are:

• The case is made from tough plastic or rubber, because these materials are good electrical insulators.
• The three pins are made from brass, which is a good conductor of electricity.
• There is a fuse between the live terminal and the live pin.
• The fuse breaks the circuit if too much current flows.
• The cable is secured in the plug by a cable grip. This should grip the cable itself, and not the individual wires inside it.

Where does each wire go?

There is an easy way to remember where to connect each wire. Take the second letters of the words blue, brown and striped. This reminds you that when you look into a plug from above:

blue goes left, brown goes right and striped goes to the top.

Many electrical appliances have metal cases, including cookers, washing machines and refrigerators. The earth wire creates a safe route for the current to flow through if the live wire touches the casing.

You will get an electric shock if the live wire inside an appliance, such as a cooker, comes loose and touches the metal casing. However, the earth terminal is connected to the metal casing so that the current goes through the earth wire instead of causing an electric shock. A strong current surges through the earth wire because it has a very low resistance. This breaks the fuse and disconnects the appliance.

The circuit breaker

The circuit breaker does the same job as the fuse, but works in a different way. A spring-loaded push switch is held in the closed position by a spring-loaded soft iron bolt. An electromagnet is arranged so that it can pull the bolt away from the switch. If the current increases beyond a set limit, the electromagnet pulls the bolt towards itself, which releases the push switch into the open position.

The fuse breaks the circuit if a fault in an appliance causes too much current flow. This protects the wiring and the appliance if something goes wrong. The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.

Fuses in plugs are made in standard ratings. The most common are 3A, 5A and 13A. The fuse should be rated at a slightly higher current than the device needs:

• if the device works at 3A, use a 5A fuse
• if the device works at 10A, use a 13A fuse

Cars also have fuses. An electrical fault in a car could start a fire, so all the circuits have to be protected by fuses.

Power is a measure of how quickly energy is transferred. The unit of power is the watt (W). You can work out power using this equation:

The more energy that is transferred in a certain time, the greater the power. A 100W light bulb transfers more electrical energy each second than a 60W light bulb

Direct current and alternating current

You should know the differences between direct current (d.c.) and alternating current (a.c.) electrical supplies.

Direct current

If the current flows in only one direction it is called direct current, or d.c. Batteries and cells supply d.c. electricity, with a typical battery supplying maybe 1.5V

Alternating current

If the current constantly changes direction, it is called alternating current, or a.c.. Mains electricity is an a.c. supply, with the UK mains supply being about 230V. It has a frequency of 50Hz (50 hertz), which means it changes direction, and back again, 50 times a second.

• An atom is made from a nucleus surrounded by electrons. The nucleus contains protons and neutrons. Isotopes are atoms that have the same number of protons, but different numbers of neutrons.

• An early model of the atom was the plum pudding model. It was disproved by Rutherford's scattering experiment and replaced by the nuclear model.

• Atoms contain three sub-atomic particles called protons, neutrons and electrons. The protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is very much smaller than the atom as a whole. The electrons are arranged in energy levels around the nucleus.

Properties of sub-atomic particles

ParticleRelative massRelative charge Proton 1 +1 Neutron 1 0 Electron Almost zero –1

The number of electrons in an atom is always the same as the number of protons, so atoms are electrically neutral overall. Atoms can lose or gain electrons. When they do, they form charged particles called ions:

• If an atom loses one or more electrons, it becomes a positively charged ion
• If an atom gains one or more electrons, it becomes a negatively charged ion

Evidence for atomic structure

You should be able to interpret information about Rutherford's scattering experiment.

The 'plum pudding' model of the atom

The plum pudding model

An early model (scientific idea) about the structure of the atom was called the plum pudding model. In this model, the atom was imagined to be a sphere of positive charge with negatively charged electrons dotted around inside it - like plums in a pudding.

Scientific models can be tested to see if they are wrong by doing experiments. An experiment carried out in 1905 showed that the plum pudding model could not be correct

The plum pudding model

Rutherford's scattering experiment

A scientist called Ernest Rutherford designed an experiment to test the plum pudding model. It was carried out by his assistants Hans Geiger and Ernest Marsden.

A beam of alpha particles was aimed at very thin gold foil and their passage through the foil detected. The scientists expected the alpha particles to pass straight through the foil, but something else also happened.

Some of the alpha particles emerged from the foil at different angles, and some even came straight back. The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom.

As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.

Isotopes

Atomic number and mass number

The number of protons in the nucleus of an atom is called its atomic number:

• The atoms of a particular element all have the same number of protons
• The atoms of different elements have different numbers of protons

The total number of protons and neutrons in an atom is called its mass number.

Isotopes are the atoms of an element with different numbers of neutrons. They have the same proton number, but different mass numbers.

Background radiation

Background radiation is all around us. Some of it comes from natural sources and some comes from artificial sources.

Natural sources

Natural sources of background radiation include:

• Cosmic rays - radiation that reaches the Earth from space
• Rocks and soil - some rocks are radioactive and give off radioactive radon gas
• Living things - plants absorb radioactive materials from the soil and these pass up the food chain

Artificial sources

There is little we can do about natural background radiation. After all, we cannot stop eating, drinking or breathing to avoid it!

However, human activity has added to background radiation by creating and using artificial sources of radiation. These include radioactive waste from nuclear power stations, radioactive fallout from nuclear weapons testing and medical x-rays.

Artificial sources account for about 15 per cent of the average background radiation dose. Nearly all artificial background radiation comes from medical procedures such as receiving x-rays for x-ray photographs.

detecting radiation

Human senses cannot detect radiation, so we need equipment to do this.

Photographic film

• Photographic film goes darker when it absorbs radiation, just like it does when it absorbs visible light. The more radiation the film absorbs, the darker it is when it is developed.
• People who work with radiation wear film badges, which are checked regularly to monitor the levels of radiation absorbed. The diagram shows a typical radiation badge when it is closed and what the inside looks like when it is opened.
• There is a light-proof packet of photographic film inside the badge. The more radiation this absorbs, the darker it becomes when it is developed. To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film. These may include aluminium, copper, lead-tin alloy and plastic. There is also an open area at the centre of the badge.

Geiger-Muller tube

The Geiger-Muller tube detects radiation. Each time it absorbs radiation, it transmits an electrical pulse to a counting machine. This makes a clicking sound or displays the count rate. The greater the frequency of clicks, or the higher the count rate, the more radiation the Geiger-Muller tube is absorbing.

• There are three main types of radiation emitted from radioactive atoms. These are alpha, beta and gamma radiation.

• Alpha radiation

• Alpha radiation consists of alpha particles. An alpha particle is identical to the nucleus of a helium atom, which comprises two protons and two neutrons.

• Beta radiation

• Beta radiation consists of high energy electrons emitted from the nucleus. These electrons have not come from the electron shells or energy levels around the nucleus. Instead, they form when a neutron splits into a proton and an electron. The electron then shoots out of the nucleus at high speed.

• Gamma radiation

• Gamma radiation is very short wavelength - high frequency - electromagnetic radiation. This is similar to other types of electromagnetic radiation such as visible light and x-rays, which can travel long distances.

Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air and gamma radiation travels many metres. All types of radiation become less intense the further the distance from the radioactive material, as the particles or rays become more spread out.

The thicker the substance, the more the radiation is absorbed. The three types of radiation penetrate materials in different ways.

• Alpha radiation is the least penetrating. It can be stopped - or absorbed - by just a sheet of paper.
• Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.
• Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete.

Electric fields

Alpha particles are positively charged, beta particles are negatively charged and gamma radiation is electrically neutral. This means that alpha radiation and beta radiation can be deflected by electric fields, but gamma radiation is not deflected.

Remember that opposite charges attract. Beta particles are negatively charged so they will be attracted towards a positively charged plate. And positive alpha particles will be attracted towards a negatively charged plate.

Magnetic fields

Because they consist of charged particles, alpha radiation and beta radiation can also be deflected by magnetic fields. Just as with electric fields, gamma radiation is not deflected by magnetic fields.

When radiation collides with molecules in living cells it can damage them. If the DNA in the nucleus of a cell is damaged, the cell may become cancerous. The cell then goes out of control, divides rapidly and causes serious health problems.

Radiation warning symbol

The greater the dose of radiation a cell gets, the greater the chance that the cell will become cancerous. However, very high doses of radiation can kill the cell completely. We use this property of radiation to kill cancer cells, and also harmful bacteria and other micro-organisms.

The hazard symbol is shown on containers of radioactive substances to warn of the danger.

Alpha, beta and gamma radiation

The degree to which each different type of radiation is most dangerous to the body depends on whether the source is outside or inside the body.

If the radioactive source is inside the body, perhaps after being swallowed or breathed in:

• Alpha radiation is the most dangerous because it is easily absorbed by cells
• Beta and gamma radiation are not as dangerous because they are less likely to be absorbed by a cell and will usually just pass right through it

If the radioactive source is outside the body:

• Alpha radiation is not as dangerous because it is unlikely to reach living cells inside the body
• Beta and gamma radiation are the most dangerous sources because they can penetrate the skin and damage the cells inside

Alpha, beta and gamma radiation

The degree to which each different type of radiation is most dangerous to the body depends on whether the source is outside or inside the body.

If the radioactive source is inside the body, perhaps after being swallowed or breathed in:

• Alpha radiation is the most dangerous because it is easily absorbed by cells
• Beta and gamma radiation are not as dangerous because they are less likely to be absorbed by a cell and will usually just pass right through it

If the radioactive source is outside the body:

• Alpha radiation is not as dangerous because it is unlikely to reach living cells inside the body
• Beta and gamma radiation are the most dangerous sources because they can penetrate the skin and damage the cells inside

The nuclei of radioactive atoms are unstable. They break down and change into a completely different type of atom. This is called radioactive decay. For example, carbon-14 decays to nitrogen-14 when it emits beta radiation.

It is not possible to predict when an individual atom might decay. But it is possible to measure how long it takes for half the nuclei of a piece of radioactive material to decay. This is called the half-life of the radioactive isotope.

Two definitions

There are two definitions of half-life, but they mean essentially the same thing:

1. The time it takes for the number of nuclei of the isotope in a sample to halve
2. The time it takes for the count rate from a sample containing the isotope to fall to half its starting level

• In smoke detectors
• For sterilising medical instruments
• For killing cancer cells
• For dating rocks and materials such as archaeological finds
• In chemical tracers to help with medical diagnosis
• For measuring the thickness of materials in, for example, a paper factory

Doctors may use radioactive chemicals called tracers for medical imaging. Certain chemicals concentrate in different damaged or diseased parts of the body, and the radiation concentrates with it. Radiation detectors placed outside the body detect the radiation emitted and, with the aid of computers, build up an image of the inside of the body.

When a radioactive chemical is used in this way it is not normally harmful, because:

• It has a short half-life and so decays before it can do much damage
• It is not poisonous

Emitters of beta radiation or gamma radiation are used because these types of radiation readily pass out of the body, and they are less likely to be absorbed by cells than alpha radiation.

Monitoring the thickness of materials

Radiation is used in industry in detectors that monitor and control the thickness of materials such as paper, plastic and aluminium. The thicker the material, the more radiation is absorbed and the less radiation reaches the detector. It then sends signals to the equipment that adjusts the thickness of the material.

Nuclear reactors use a type of nuclear reaction called nuclear fission. Another type of nuclear reaction - nuclear fusion - happens in the Sun and other stars.

Nuclear fission

• Nuclear power reactors use a reaction called nuclear fission. Two isotopes in common use as nuclear fuels are uranium-235 and plutonium-239. However, uranium-235 is used in most nuclear reactors.

Nuclear fission

Nuclear power reactors use a reaction called nuclear fission. Two isotopes in common use as nuclear fuels are uranium-235 and plutonium-239. However, uranium-235 is used in most nuclear reactors.

Nuclear fission

Nuclear power reactors use a reaction called nuclear fission. Two isotopes in common use as nuclear fuels are uranium-235 and plutonium-239. However, uranium-235 is used in most nuclear reactors.

Splitting atoms

'Fission' is another word for splitting. The process of splitting a nucleus is called nuclear fission. Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors because their nuclei are relatively large and easy to split.

For fission to happen, the uranium-235 or plutonium-239 nucleus must first absorb a neutron. When this happens:

1. The nucleus splits into two smaller nuclei
2. Two or three neutrons are released
3. Some energy is released

The additional neutrons released may be absorbed by other uranium or plutonium nuclei, causing them to split. Even more neutrons are then released, which in turn can split more nuclei. This is called a chain reaction. The chain reaction in nuclear reactors is controlled to stop it going too fast.

Nuclear fusion

Nuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens.

The Sun and other stars use nuclear fusion to release energy. The sequence of nuclear fusion reactions in a star is complex, but overall hydrogen nuclei join to form helium nuclei. Here is one nuclear fusion reaction that takes place:

Hydrogen-1 nuclei fuse with hydrogen-2 nuclei to make helium-3 nuclei

Stars form when enough dust and gas clump together because of gravitational forces. Nuclear reactions release energy to keep the star hot. Planets form when smaller amounts of dust and gas clump together because of gravitational forces.

Stable stars like the Sun change during their lifetime to form other types of stars, such as red giants, white dwarfs, neutron stars and black holes. The fate of a star depends upon how much matter it contains.

• Gracvity pulls the dust and gas together.
• As the gas falls together, it gets hot. A star forms when it is hot enough for nuclear reactions to start. This releases energy, and keeps the star hot.
• During its 'main sequence' period of its life cycle, a star is stable because the forces in it are balanced. The outward pressure from the expanding hot gases is balanced by the force of the star’s gravity. Our Sun is at this stable phase in its life.
• Gravity pulls smaller amounts of dust and gas together, which form planets in orbit around the star.

Stars about the same size as our sun

• These follow the left hand path:
• Main sequence star → red giant → white dwarf → black dwarf

Stars mich bigger than our sun

• These follow the right hand path:
• Main sequence star → red super giant → supernova → neutron star or black hole

Nuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens.

The Sun and other stars use nuclear fusion to release energy. The sequence of nuclear fusion reactions in a star is complex - but, in general, hydrogen nuclei join to form helium nuclei.

Nuclear fusion involves two atomic nuclei joining to make a large nucleus. Energy is released when this happens.

The Sun and other stars use nuclear fusion to release energy. The sequence of nuclear fusion reactions in a star is complex - but, in general, hydrogen nuclei join to form helium nuclei.