Generating Electricity

Electricity is a very convenient form of energy that can be generated using different energy

resources. Some of these resources are renewable and some are non-renewable. Each

resource has advantages and disadvantages.

Fossil fuels

Fossil fuels for power stations

Energy transfer for the generation of electricity from a fossil fuel

The fossil fuels are coal, oil and natural gas. They are fuels because they release heat energy

when they are burned. They are fossil fuels because they were formed from the remains of living

organisms millions of years ago.

About three-quarters of the electricity generated in the UK comes from power stations fuelled by fossil

fuels. To the right is an energy transfer diagram for the generation of electricity from a fossil fuel such

as coal.

Disadvantages of using fossil fuels

Fossil fuels are non-renewable energy resources. Their supply is limited and they will eventually

run out. Fossil fuels do not renew themselves, while fuels such as wood can be renewed endlessly.

Fossil fuels release carbon dioxide when they burn, which adds to the greenhouse effect and

increases global warming. Of the three fossil fuels, for a given amount of energy released, coal

produces the most carbon dioxide and natural gas produces the least.

Coal and oil release sulfur dioxide gas when they burn, which causes breathing problems for living

creatures and contributes to acid rain.

Nuclear fuels

You should be able to outline how electricity is generated using nuclear fuels.

Uranium and plutonium

The main nuclear fuels are uranium and plutonium. These are radioactive metals. Nuclear fuels are

not burnt to release energy. Instead, the fuels are involved in nuclear reactions in the nuclear

reactor, which leads to heat being released.

The rest of the process of generating electricity is then identical to the process using fossil fuels [fossil

fuels: Fossil fuels, such as coal, gas and oil, are mined from the earth. They are formed from broken

down animals and plants that died a very long time ago. Fossil fuels are burnt to produce energy.

Often they are used to create electricity. As they can only be used once, these are called finite fuels.]

The heat energy is used to boil water. The kinetic energy in the expanding steam spins turbines, which

then drive generators to produce electricity.

Advantages

Unlike fossil fuels, nuclear fuels [nuclear fuels: Radioactive materials, usually uranium or plutonium,

used in nuclear reactors. ] do not produce carbon dioxide or sulfur dioxide.

Disadvantages

Like fossil fuels, nuclear fuels are non-renewable energy resources. If there is an accident, large

amounts of radioactive material could be released into the environment. In addition, nuclear waste

remains radioactive and is hazardous to health for thousands of years. It must be stored safely.

Make sure that you understand radiation by visiting Radioactive substances.

Wind energy

You should be able to outline how electricity is generated using wind energy.

Big convection currents

The wind is produced as a result of giant convection [convection: The transfer of heat energy through

a moving liquid or gas. ] currents in the Earth's atmosphere, which are driven by heat energy from the

sun. This means that the kinetic energy in wind is a renewable energy resource: as long as the sun

exists, the wind will too.

Wind turbines

A wind turbine

Wind turbines have huge blades mounted on a tall tower. The blades are connected to a nacelle

[nacelle: The part at the top of the tower of a wind turbine. The blades of the turbine are joined to the

nacelle, which contains gears linked to a generator. ] or housing that contains gears linked to a

generator [generator: An 123 electromagnetic device that produces electricity when it is turned. ]. As

the wind blows, it transfers some of its kinetic energy to the blades, which turn and drive the generator.

Several wind turbines may be grouped together in windy locations to form wind farms.

Advantages

Wind is a renewable energy resource and there are no fuel costs. No harmful polluting gases are

produced.

Disadvantages

Wind farms are noisy and may spoil the view for people living near them. The amount of electricity

generated depends on the strength of the wind. If there is no wind, there is no electricity.

Check that you understand convection currents by visiting Heat transfer and efficiency.

Water energy

You should be able to outline how electricity is generated using water.

Wave energy

The water in the sea rises and falls because of waves on the surface. Wave machines use the

kinetic energy in this movement to drive electricity generators.

Tidal barrage

Huge amounts of water move in and out of river mouths each day because of the tides. A tidal barrage

is a barrier built over a river estuary to make use of the kinetic energy in the moving water. The

barrage contains electricity generators, which are driven by the water rushing through tubes in the

barrage.

Hydroelectric power (HEP)

Like tidal barrages, hydroelectric power stations use the kinetic energy in moving water. But the water

comes from behind a dam built across a river valley. The water high up behind the dam contains

gravitational potential energy [gravitational potential energy: The energy stored by an object lifted

up against the force of gravity. ]. This is transferred to kinetic energy as the water rushes down

through tubes inside the dam. The moving water drives electrical generators, which may be built inside

the dam.

Advantages

Water power in its various forms is a renewable energy resource and there are no fuel costs. No

harmful polluting gases are produced. Tidal barrages and hydroelectric power stations are very

reliable and can be turned on quickly.

Disadvantages

It has been difficult to scale up the designs for wave machines to produce large amounts of electricity.

Tidal barrages destroy the habitat of estuary species, including wading birds. Hydroelectricity dams

flood farmland and push people from their homes. The rotting vegetation underwater releases

methane, which is a greenhouse gas.

Geothermal energy

You should be able to outline how electricity is generated from geothermal energy.

Volcanic areas

Several types of rock contain radioactive substances such as uranium. Radioactive decay of these

substances releases heat energy, which warms up the rocks. In volcanic areas, the rocks may heat

water so that it rises to the surface naturally as hot water and steam. Here the steam can be used to

drive turbines [turbines: Revolving machinery with many blades turned by wind, water or steam.

Turbines in a power station turn the generators. ] and electricity generators. This type of geothermal

power station exists in places such as Iceland, California and Italy.

Hot rocks

In some places, the rocks are hot, but no hot water or steam rises to the surface. In this situation, deep

wells can be drilled down to the hot rocks and cold water pumped down. The water runs through

fractures in the rocks and is heated up. It returns to the surface as hot water and steam, where its

energy can be used to drive turbines and electricity generators. The diagram below shows how this

works.

How a generating station creates energy

Advantages

Geothermal energy is a renewable energy resource and there are no fuel costs. No harmful polluting

gases are produced.

Disadvantages

Most parts of the world do not have suitable areas where geothermal energy can be exploited.

Solar energy

You should be able to outline how solar energy is used to generate electricity and to produce hot

water.

Solar cells

Solar cells are devices that convert light energy directly into electrical energy. You may have seen

small solar cells in calculators. Larger arrays of solar cells are used to power road signs in remote

areas, and even larger arrays are used to power satellites in orbit around Earth.

Solar panels

Solar panels do not generate electricity, but rather they heat up water. They are often located on the

roofs of buildings where they can receive heat energy from the sun. The diagram outlines how they

work.

Cold water is pumped up to the solar panel, there it heats up and is transferred to a storage tank.

A pump pushes cold water from the storage tank through pipes in the solar panel. The water is heated

by heat energy from the sun and returns to the tank. In some systems, a conventional boiler may be

used to increase the temperature of the water.

Solar panel

Advantages

Solar energy is a renewable energy resource and there are no fuel costs. No harmful polluting gases

are produced.

Disadvantages

Solar cells are expensive and inefficient, so the cost of their electricity is high.

Solar panels may only produce very hot water in very sunny climates, and in cooler areas may

need to be supplemented with a conventional boiler.

Although warm water can be produced even on cloudy days, neither solar cells nor solar

panels work at night.

Resources compared

You should be able to compare and contrast the different energy resources used to produce electricity.

You may be given information in the examination for you to discuss.

Power stations

Power stations fuelled by fossil fuels or nuclear fuels are reliable sources of energy. This means

they can provide power whenever it is needed. However, their start-up times vary according to the

type of fuel used.

This list shows the type of fuel in order of start of time going from short to long.

1. gas-fired station (shortest start-up time)

2. oil-fired station

3. coal-fired station

4. nuclear power station (longest start-up time)

Nuclear power stations and coal-fired power stations usually provide 'base load' electricity - they are

run all the time because they take the longest time to start up. Oil-fired and gas-fired power stations

are often used to provide extra electricity at peak times, because they take the least time to start

up.

The fuel for nuclear power stations is relatively cheap, but the power stations themselves are

expensive to build. It is also very expensive to dismantle old nuclear power stations and to store

their radioactive waste, which is a dangerous health hazard.

Renewable resources

Renewable resources of fuel do not cost anything, but the equipment used to generate the power

may be expensive to build. Certain resources are reliable, including tidal barrages and hydroelectric

power. Others are less reliable, including wind and solar energy.

Heat transfer and efficiency

Heat can be transferred from place to place by conduction [conduction: The transfer of heat

energy through a material - without the material itself moving. ], convection [convection: The transfer

of heat energy through a moving liquid or gas. ] and radiation [infrared radiation: Electromagnetic

radiation emitted from a hot object. ]. Dark matt surfaces are better at absorbing heat energy than

light shiny surfaces. Heat energy can be lost from homes in many different ways and there are

ways of reducing these heat losses.

There are several different types of energy, and these can be transferred from one type to

another. Energy transfer diagrams show the energy transfers in a process. More efficient

devices transfer the energy supplied to them into a greater proportion of useful energy than

less efficient devices do.

Heat transfer by conduction and convection

Heat is thermal energy. It can be transferred from one place to another by conduction, convection and

radiation. Conduction and convection involve particles, but radiation involves electromagnetic waves.

Conduction

Heat energy can move through a substance by conduction. Metals are good conductors of heat, but

non-metals and gases are usually poor conductors of heat. Poor conductors of heat are called

insulators. Heat energy is conducted from the hot end of an object to the cold end.

The electrons in piece of metal can leave their atoms and move about in the metal as free electrons.

The parts of the metal atoms left behind are now charged metal ions. The ions are packed closely

together and they vibrate continually. The hotter the metal, the more kinetic energy these vibrations

have. This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons.

These move through the structure of the metal, colliding with ions as they go.

Convection

Liquids and gases are fluids. The particles in these fluids can move from place to place. Convection

occurs when particles with a lot of heat energy in a liquid or gas move and take the place of particles

with less heat energy. Heat energy is transferred from hot places to cooler places by convection.

Liquids and gases expand when they are heated. This is because the particles in liquids and gases

move faster when they are heated than they do when they are cold. As a result, the particles take up

more volume. This is because the gap between particles widens, while the particles themselves stay

the same size.

The liquid or gas in hot areas is less dense than the liquid or gas in cold areas, so it rises into the cold

areas. The denser cold liquid or gas falls into the warm areas. In this way, convection currents that

transfer heat from place to place are set up.

Radiation

All objects give out and take in thermal radiation, which is also called infrared radiation. The hotter

an object is, the more infrared radiation it emits.

Light from the sun reaching earth

Infrared radiation is a type of electromagnetic radiation that involves waves. No particles are involved,

unlike in the processes of conduction and convection, so radiation can even work through the vacuum

of space. This is why we can still feel the heat of the Sun, although it is 150 million km away from the

Earth.

Some surfaces are better than others at reflecting and absorbing infrared radiation.

Comparison of surfaces abilities to reflect and absorb radiation

colour finish ability to emit thermal radiation ability to absorb thermal radiation

dark dull or matt good good

light shiny poor poor

If two objects made from the same material have identical volumes, a thin, flat object will radiate heat

energy faster than a fat object. This is one reason why domestic radiators are thin and flat. Radiators

are often painted with white gloss paint. They would be better at radiating heat if they were painted

with black matt paint, but in fact, despite their name, radiators transfer most of their heat to a room by

convection.

Reducing heat loss

You should be able to describe how heat energy is lost from buildings and to explain how these losses

can be reduced.

Ways to reduce heat loss

There are some simple ways to reduce heat loss, including fitting carpets, curtains and draught

excluders.

Heat loss through windows can be reduced using double glazing. There may be air or a vacuum

[vacuum: A volume that contains no matter - space is almost a vacuum. ] between the two panes of

glass. Air is a poor conductor of heat, while a vacuum can only transfer heat energy by radiation.

Heat loss through walls can be reduced using cavity wall insulation. This involves blowing insulating

material into the gap between the brick and the inside wall, which reduces the heat loss by conduction.

The material also prevents air circulating inside the cavity, therefore reducing heat loss by convection.

Heat loss through the roof can be reduced by laying loft insulation. This works in a similar way to

cavity wall insulation.

Forms of energy

You should be able to recognise the main types of energy. One way to remember the different types of

energy is to learn this sentence:

Most Kids Hate Learning GCSE Energy Names

Each capital letter is the first letter in the name of a type of energy.

Types of energy

Type of energy Description

Magnetic Energy in magnets and electromagnets

Kinetic The energy in moving objects. Also called movement energy.

Heat Also called thermal energy

Light Also called radiant energy

Gravitational potential Stored energy in raised objects

Chemical Stored energy in fuel, foods and batteries

Sound Energy released by vibrating objects

Electrical Energy in moving or static electric charges

Elastic potential Stored energy in stretched or squashed objects

Nuclear Stored in the nuclei of atoms

Energy transfer

You should recall from your Key Stage 3 studies how to draw and interpret an energy transfer

diagram.

Different types of energy can be transferred from one type to another. Energy transfer diagrams show

each type of energy, whether it is stored or not, and the processes taking place as it is transferred.

Sankey diagrams also show the relative amounts of each type of energy.

Energy transfer diagrams

This energy transfer diagram shows the useful energy transfer in a car engine. You can see that a car

engine transfers chemical energy, which is stored in the fuel, into kinetic energy in the engine and

wheels.

Process of using chemical energy

This diagram shows the energy transfer diagram for the useful energy transfer in an electric lamp. You

can see that the electric lamp transfers or converts electrical energy into light energy.

Process of using electrical energy

Notice that these energy transfer diagrams only show the useful energy transfers. However, car

engines are also noisy and hot, and electric lamps also give out heat energy.

Efficiency

You should know that energy can be 'wasted' during energy transfers, and you should be able to

calculate the efficiency [efficiency: The fraction of the energy supplied to a device which is transferred

in a useful form. ] of a device.

'Wasted' energy

Energy cannot be created or destroyed. It can only be transferred from one form to another or moved.

Energy that is 'wasted', like the heat energy from an electric lamp, does not disappear. Instead, it is

transferred into the surroundings and spreads out so much that it becomes very difficult to do anything

useful with it.

Calculating efficiency

The efficiency of a device such as a lamp can be calculated using this equation:

efficiency = ( useful energy transferred ÷ energy supplied ) × 100

The efficiency of the filament lamp is (10 ÷ 100) × 100 = 10%.

This means that 10% of the electrical energy supplied is transferred as light energy (90% is transferred

as heat energy).

The efficiency of the energy-saving lamp is (75 ÷ 100) × 100 = 75%. This means that 75% of the

electrical energy supplied is transferred as light energy (25% is transferred as heat energy).

Note that the efficiency of a device will always be less than 100%.

Using electricity

Electricity is supplied to consumers through the National Grid at a very high voltage to reduce

energy losses during transmission. Transformers are used to increase or decrease the voltage

of the supply. Electricity is charged in units. One unit is equivalent to one kilowatt of electricity

used for one hour.

The National Grid

At the power station

Power stations are built in order to generate electricity. The diagram shows the main steps involved.

Power station

There are four main stages:

1. the fuel is burned to boil water to make steam

2. the steam makes a turbine spin

3. the spinning turbine turns a generator which produces electricity

4. the electricity goes to the transformers to produce the correct voltage

The energy needed to boil the water comes from fossil fuels or nuclear fuels. Renewable energy

resources such as wind and wave power may drive the generators directly.

Transformers

A transformer is an electrical device that changes the voltage of an alternating current (ac) supply,

such as the mains electrical supply. A transformer changes a high-voltage supply into a low-voltage

one, or vice versa.

A transformer that increases the voltage is called a step-up transformer.

A transformer that decreases the voltage is called a step-down transformer.

The National Grid

Electricity is transferred from power stations to consumers through the wires and cables of the

National Grid. When a current flows through a wire some energy is lost as heat. The higher the

current, the more heat is lost. To reduce these losses, the National Grid transmits electricity at a

low current. This needs a high voltage.

Power stations produce electricity at 25,000V. Electricity is sent through the National Grid cables at

400,000V, 275,000V and 132,000V.

Step-up transformers are used at power stations to produce the very high voltages needed to transmit

electricity through the National Grid power lines. These high voltages are too dangerous to use in the

home, so step-down transformers are used locally to reduce the voltage to safe levels. The voltage of

household electricity is about 230V.

The cost of using electricity

You should be able to calculate the cost of using an electrical appliance when given enough

information about it.

The unit

The amount of electrical energy [electrical energy: Energy transferred by electricity. ] transferred to

an appliance depends on its power and the length of time it is switched on. The amount of mains

electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1kWh.

The equation below shows the relationship between energy transferred, power and time:

energy transferred (kWh) = power (kW) × time (h)

Note that power is measured in kilowatts here instead of the more usual watts. To convert from W to

kW you must divide by 1000.

For example, 2000W = 2000 ÷ 1000 = 2kW.

Also note that time is measured in hours here, instead of the more usual seconds. To convert from

seconds to hours you must divide by 3600.

For example, 1800s = 1800 ÷ 3600 = 0.5 hours.

The cost

Electricity meters measure the number of units of electricity used in a home or other building. The

more units used, the greater the cost. The cost of the electricity used is calculated using this equation:

total cost = number of units × cost per unit

For example, if 5 units of electricity are used at a cost of 8p per unit, the total cost will be 5 × 8 = 40p.

An introduction to waves

Light travels as waves. Waves can be described by their amplitude, wavelength and frequency.

The speed of a wave can be calculated from its frequency and wavelength.

What are waves?

Waves are vibrations that transfer energy from place to place without matter (solid, liquid or gas)

being transferred. Think of a Mexican wave in a football crowd. The wave moves around the stadium,

while each spectator stays in their seat only moving up then down when it's their turn.

Some waves must travel through a substance. The substance is known as the medium, and it can

be solid, liquid or gas. Sound waves and seismic waves [seismic waves: vibrations caused by earth

movements ] are like this. They must travel through a medium. It is the medium that vibrates as the

waves travel through.

Other waves do not need to travel through a substance. They may be able to travel through a medium,

but they do not have to. Visible light, infrared rays, microwaves and other types of electromagnetic

radiation [electromagnetic radiation: Energy travelling as waves in the form of changing electrical

and magnetic fields. ] are like this. They can travel through empty space. Electrical and magnetic fields

vibrate as the waves travel.

Amplitude, wavelength and frequency

You should understand what is meant by the amplitude, wavelength and frequency of a wave.

Amplitude

As waves travel, they set up patterns of disturbance. The amplitude of a wave is its maximum

disturbance from its undisturbed position. Take care, the amplitude is not the distance between the top

and bottom of a wave.

Amplitude and wavelength

Wavelength

The wavelength of a wave is the distance between a point on one wave and the same point on the

next wave. It is often easiest to measure this from the crest of one wave to the crest of the next wave,

but it doesn't matter where as long as it is the same point in each wave.

Frequency

The frequency of a wave is the number of waves produced by a source each second. It is also the

number of waves that pass a certain point each second. The unit of frequency is the hertz (Hz). It is

common for kilohertz (kHz), megahertz (MHz) and gigahertz (GHz) to be used when waves have very

high frequencies. For example, most people cannot hear a high-pitched sound above 20kHz, radio

stations broadcast radio waves [radio waves: Low frequency electromagnetic radiation used to

transmit information such as television and radio programmes. ] with frequencies of about 100MHz,

while most wireless computer networks operate at 2.4GHz.

Wave speed

You should know and be able to use the relationship between wave speed, frequency and wavelength.

How fast do waves travel?

The speed of a wave - its wave speed - is related to its frequency and wavelength, according to this

equation:

wave speed (metre per second) = frequency (hertz) × wavelength (metre)

For example, a wave with a frequency of 100Hz and a wavelength of 2m travels at 100 × 2 = 200m/s.

The electromagnetic spectrum

Electromagnetic radiation [electromagnetic radiation: Energy travelling as waves in the form of

changing electrical and magnetic fields. ] travels as waves and transfers energy from one place to

another. All electromagnetic waves can travel through a vacuum, and they all travel at the

same speed in a vacuum.

The electromagnetic spectrum is a continuous range of wavelengths. The types of radiation

that occur in different parts of the spectrum have different uses and dangers, which depend on

their wavelength and frequency.

What is a spectrum?

The visible spectrum

Refraction from a prism

White light can be split up using a prism to form a spectrum. A prism is a block of glass with a

triangular cross-section. The light waves are refracted as they enter and leave the prism. The shorter

the wavelength of the light, the more it is refracted. As a result, red light is refracted the least and violet

light is refracted the most, causing the coloured light to spread out to form a spectrum.

The electromagnetic spectrum

Visible light is just one type of electromagnetic radiation. There are various types of electromagnetic

radiation, some with longer wavelengths than visible light and some with shorter wavelengths than

visible light.

The electromagnetic spectrum

You should know the different types of electromagnetic radiation and their typical uses.

The main types of electromagnetic radiation

frequency type of electromagnetic radiation

typical use wavelength

highest gamma radiation killing cancer cells shortest

X-rays medical images of bones

ultraviolet detecting forged bank notes by fluorescence

visible light seeing

infrared optical fibre communication

microwaves cooking

lowest radio waves television signals longest

Gamma radiation

Gamma waves have a very high frequency [frequency: The number of repetitions per second of a

wave. The unit of frequency is the hertz, 'Hz'. ]. Gamma radiation cannot be seen or felt. It mostly

passes through skin and soft tissue, but some of it is absorbed by cells.

Gamma radiation is used, among other things, for the following purposes.

to sterilise surgical instruments

to kill harmful bacteria in food

to kill cancer cells (note that lower doses of gamma radiation could lead to cells becoming

cancerous)

X-rays

Chest X-ray

X-rays have a lower frequency than gamma radiation. Like gamma rays, they cannot be seen or felt.

X-rays mostly pass through skin and soft tissue, but they do not easily pass through bone or metal.

X-rays are used to produce photographs of bones to check for damage such as fractures. They are

also used in industry to check metal components and welds for cracks or other damage.

Lower doses of X-rays can cause cells to become cancerous, so precautions are taken in hospitals to

limit the dose received by patients and staff when X-ray photographs are taken.

Ultraviolet radiation

Ultraviolet radiation is found naturally in sunlight. We cannot see or feel ultraviolet radiation, but our

skin responds to it by turning darker. This happens in an attempt to reduce the amount of ultraviolet

radiation that reaches deeper skin tissues. Darker skins absorb more ultraviolet light, so less ultraviolet

radiation reaches the deeper tissues. This is important because ultraviolet radiation can cause normal

cells to become cancerous.

Ultraviolet radiation is used in:

sun beds

security pens

fluorescent lights (coatings inside the tube or bulb absorb the ultraviolet light and re-emit it as

visible light)

Infrared radiation

Infrared radiation is absorbed by the skin and we feel it as heat. It is used in heaters, toasters and

grills. It is also used for television remote controls and in optical fibre [optical fibre: A fine glass fibre,

through which light travels by total internal reflection, from one end to the other. ] communications.

Cooking

Microwave radiation has lower frequencies and longer wavelengths than visible light. Microwaves with

certain wavelengths are absorbed by water molecules and can be used for cooking. Water in the food

absorbs the microwave radiation, which causes the water to heat up and cook the food. The water in

living cells can also absorb microwave radiation. As a result, they can be killed or damaged by the

heat released.

Communications

Microwave radiation can also be used to transmit signals such as mobile phone calls. Microwave

transmitters and receivers on buildings and masts communicate with the mobile telephones in their

range.

Certain microwave radiation wavelengths pass through the Earth's atmosphere and can be used to

transmit information to and from satellites in orbit.

Television and radio

Radio waves have lower frequencies and longer wavelengths than microwaves. They are used to

transmit television and radio programmes. Television uses higher frequencies than radio.

A radio programme receiver does not need to be directly in view of the transmitter to receive

programme signals. For low frequency radio waves diffraction can allow them to be received behind

hills, although repeater stations are often used to improve the quality of the signals.

The lowest frequency radio waves are also reflected from an electrically charged layer of the upper

atmosphere, called the Ionosphere. This means that they can reach receivers that are not in the line of

sight because of the curvature of the Earth's surface.

Microwaves and radio waves in the atmosphere

Radioactive substances

An atom of any given element consists of a nucleus containing a number of protons and

neutrons. The nucleus is surrounded by electrons.

The half-life of a radioactive isotope is the time taken for half its radioactive atoms to decay.

There are three main types of radiation, called alpha, beta and gamma radiation, which all have

different properties. Radiation can damage cells and make them cancerous. Very high doses of

radiation can kill cells. It can be detected using photographic film or a Geiger-Muller tube.

Radiation badges are used to monitor the level of radiation that people who work with

radioactive sources are exposed to.

Radiation has many practical uses. It can be used in medicine to trace where certain chemicals

collect in the body, indicating disease, and also in industry, where it can be used to control

measuring equipment.

Atoms and isotopes

The nuclear model

Structure of the atom

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, and the electrons are arranged in energy

levels or shells around the nucleus.

Isotopes

All the atoms of a given element have the same number of protons and electrons. However, the

number of neutrons can vary. Atoms of the same element that have different numbers of neutrons are

called isotopes of that element. The diagram shows three hydrogen isotopes.

Hydrogen-1

1 proton, 0 neutron, 1 electron

Radioactivity

The different isotopes of an element have identical chemical properties. Some isotopes, however, are

radioactive. This means that they give out radiation from their nuclei. This happens all the time,

whatever is done to the substance. For example, the radiation is still given out if the substance is

cooled down in a freezer, or takes part in a chemical reaction.

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.

Helium atom

Alpha particle

2 protons

2 neutrons

2 electrons

2 protons

2 neutrons

0 electrons

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.

Penetrating properties of radiation

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

Alpha radiation is the least penetrating. It can be stopped - or absorbed - by just a sheet of paper.

Beta radiation

Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.

Gamma radiation

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.

Penetrative properties of different types of radiation

Deflecting radiation

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.

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.

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.

A typical radiation badge

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.

Hazards of radiation

Radiation and living cells

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.

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.

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.

Notice that these effects are opposites and make sure you get them the right way around.

Half-life

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

Different radioactive isotopes have different half-lives. For example, the half-life of carbon-14 is 5,715

years, but the half-life of francium-223 is just 20 minutes.

Graphs

It is possible to find out the half-life of a radioactive substance from a graph of the count rate against

time. The graph shows the decay curve for a radioactive substance.

The decay curve for a radioactive substance

The count rate drops from 80 to 40 counts a minute in two days, so the half-life is two days. In the next

two days, it drops from 40 to 20 - it halves. In the two days after that, it drops from 20 to 10 - it halves

again - and so on.

Using radiation

Here are some examples of how radiation is used:

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

Tracers

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.

Representing motion

The slope on a distance-time graph represents the speed of an object.

The velocity of an object is its speed in a particular direction. The slope on a velocity-time

graph represents the acceleration of an object. The distance travelled is equal to the area

under a velocity-time graph.

Speed, distance and time

You should recall from your Key Stage 3 studies how to calculate the speed of an object from the

distance travelled and the time taken.

The equation

When an object moves in a straight line at a steady speed, you can calculate its speed if you know

how far it travels and how long it takes. This equation shows the relationship between speed, distance

travelled and time taken:

For example, a car travels 300 metres in 20 seconds.

Its speed is 300 ÷ 20 = 15m/s.

Background information

The vertical axis of a distance-time graph is the distance travelled from the start. The horizontal axis is

the time from the start.

Features of the graphs

When an object is stationary, the line on the graph is horizontal. When an object is moving at a steady

speed, the line on the graph is straight, but sloped.

The diagram shows some typical lines on a distance-time graph.

Distance - time graph

Note that the steeper the line, the greater the speed of the object. The blue line is steeper than the red

because it represents an object moving faster than the one represented by the red line.

The red lines on the graph represent a typical journey where an object returns to the start again.

Notice that the line representing the return journey slopes downwards.

Velocity-time graphs

You should be able to explain velocity-time graphs for objects moving with a constant velocity or

constant acceleration [acceleration: The rate of change of velocity, measured in metres per second

squared. Acceleration = change of velocity ÷ time taken. ].

Background information

The velocity of an object is its speed in a particular direction. This means that two cars travelling at the

same speed, but in opposite directions, have different velocities.

The vertical axis of a velocity-time graph is the velocity of the object. The horizontal axis is the time

from the start.

Features of the graphs

When an object is moving with a constant velocity, the line on the graph is horizontal. When an object

is moving with a constant acceleration, the line on the graph is straight, but sloped. The diagram

shows some typical lines on a velocity-time graph.

Speed - time graph

The steeper the line, the greater the acceleration of the object. The blue line is steeper than the red

line because it represents an object with a greater acceleration.

Notice that a line sloping downwards - with a negative gradient - represents an object with a constant

deceleration - slowing down.

The equation

When an object moves in a straight line with a constant acceleration, you can calculate its acceleration

if you know how much its velocity changes and how long this takes. This equation shows the

relationship between acceleration, change in velocity and time taken:

For example, a car accelerates in 5s from 25m/s to 35m/s.

Its velocity changes by 35 - 25 = 10m/s.

So its acceleration is 10 ÷ 5 = 2m/s2.

Background

To calculate the gradient of the line on a graph, divide the change in the vertical axis by the change in

the horizontal axis.

Distance-time graphs

The gradient of a line on a distance-time graph represents the speed of the object. Study this distance-

time graph.

Distance - time graph

Question

What is the speed represented by the blue line?

Answer

The object travels 10m in 2s.

Its speed is 10 ÷ 2 = 5m/s.

The gradient

The gradient of a line on a velocity-time graph represents the acceleration of the object. Study this

velocity-time graph.

Velocity - time graph

Question

What is the acceleration represented by the sloping line?

Answer

The object increases its velocity from 0m/s to 8m/s in 4s.

Its acceleration is 8 ÷ 4 = 2m/s2.

The area

The area under the line in a velocity-time graph represents the distance travelled. To find the distance

travelled in the graph above, we need to find the area of the light-blue triangle and the dark-blue

rectangle.

1. Area of light-blue triangle

o The width of the triangle is 4 seconds and the height is 8 metres per second. To find

the area, you use the equation:

o area of triangle = 1⁄2 × base × height

o so the area of the light-blue triangle is 1⁄2 × 8 × 4 = 16m.

2. Area of dark-blue rectangle

o The width of the rectangle is 6 seconds and the height is 8 metres per second. So the

area is 8 × 6 = 48m.

3. Area under the whole graph

o The area of the light-blue triangle plus the area of the dark-blue rectangle is:

o 16 + 48 = 64m.

o This is the total area under the distance-time graph. This area represents the distance

covered.

Summary

the gradient of a velocity-time graph represents the acceleration

the area under a velocity-time graph represents the distance covered

Force, mass and acceleration

A stationary object remains stationary if the sum of the forces acting upon it - resultant force -

is zero. A moving object with a zero resultant force keeps moving at the same speed and in the

same direction.

If the resultant force acting on an object is not zero, a stationary object begins to accelerate in

the same direction as the force. A moving object speeds up, slows down or changes direction.

Acceleration depends on the force applied to an object and the object's mass.

The resultant force

An object may have several different forces acting on it, which can have different strengths and

directions. But they can be added together to give the resultant force. This is a single force that has

the same effect on the object as all the individual forces acting together.

When the resultant force is zero

When all the forces are balanced, the resultant force is zero. In this case:

a stationary object remains stationary

a moving object keeps on moving at the same speed in the same direction

For example, in the diagram of the weightlifter, the resultant force on the bar is zero, so the bar does

not move. Its weight acting downwards is balanced by the upward force provided by the weightlifter.

The longer the arrow, the bigger the force. In this diagram, the arrows are the same length, so we

know they are the same size.

When the resultant force is not zero

When all the forces are not balanced, the resultant force is not zero. In this case:

A stationary object begins to move in the direction of the resultant force.

A moving object speeds up, slows down or changes direction depending on the direction of

the resultant force.

In this diagram of the weightlifter, the resultant force on the bar is not zero. The upwards force is

bigger than the downwards force. The resultant force acts in the upwards direction, so the bar moves

upwards.

In this next diagram of the weightlifter, the resultant force on the bar is also not zero. This time, the

upwards force is smaller than the downwards force. The resultant force acts in the downwards

direction, so the bar moves downwards.

Forces and acceleration

You should know that objects accelerate when the resultant force is not zero, and understand the

factors that affect the size of the acceleration [acceleration: The rate of change of velocity, measured

in metres per second squared. Acceleration = change of velocity ÷ time taken. ].

Size of the force

An object will accelerate in the direction of the resultant force. The bigger the force, the greater the

acceleration.

Doubling the size of the (resultant) force doubles the acceleration.

The mass

An object will accelerate in the direction of the resultant force. A force on a large mass will accelerate it

less than the same force on a smaller mass.

Doubling the mass halves the acceleration.

Forces and acceleration calculations

You should know the equation that shows the relationship between resultant force, mass and

acceleration, and be able to use it.

The equation

resultant force (newton, N) = mass (kg) × acceleration (m/s2)

You can see from this equation that 1N is the force needed to give 1kg an acceleration of 1m/s2.

For example, the force needed to accelerate a 10kg mass by 5m/s2 is:

10 x 5 = 50N

The same force could accelerate a 1kg mass by 50m/s2 or a 100kg mass by 0.5m/s2.

Putting it simply, we can say that it takes more force to accelerate a larger mass.

Question

A truck has a mass of 2,000kg.

The driving force created by the engine is 3,000 newtons.

Calculate the acceleration caused by this force.

Answer

1. Write down the equation.

o force = mass × acceleration

2. Rearrange the equation.

o acceleration = force⁄mass

3. Put in the values.

o acceleration = 3,000N⁄ 2,000kg

4. Work out the answer and write it down.

o acceleration = 1.5m/s2

Questions and answers

Here are four typical forces on which you could be asked questions:

1. Air resistance - drag

o When an object moves through the air, the force of air resistance [air resistance: A

force of friction produced when an object moves through the air. ] acts in the opposite direction

to the motion. Air resistance depends on the shape of the object and its speed.

2. Contact force

o This happens when two objects are pushed together. They exert equal and opposite

forces on each other. The contact force from the ground pushes up on your feet even as you

stand still. This is the force you feel in your feet. You feel the ground pushing back against your

weight pushing down.

3. Friction

o This is the force that resists movement between two surfaces which are in contact.

4. Gravity

o This is the force that pulls objects towards the Earth. We call the force of gravity on an

object its weight. The Earth pulls with a force of about 10 newtons on every kilogram of mass.

Question

Look at the animation of the parachutist falling at a steady speed. Name the forces acting on

the parachutist and state how they are acting.

Answer

There are just two forces acting on the parachutist. Gravity (weight) pulls the parachutist

down. Air resistance - drag - pushes up on the canopy of the parachute.

Question

Look at the animation of the car moving at a steady speed. Name the forces acting on the car

and state how they are acting.

Answer

There are several forces acting on the car. Gravity pulls down on the car. The contact force

from the road pushes up on the wheels. And the driving force from the engine pushes the car

along. Also, there is friction between the road and the tyres. There is friction in the wheel

bearings. And air resistance acts on the front of the car.

Weight and friction

Gravity is a force that attracts objects with mass towards each other. The weight of an object is

the force acting on it due to gravity. The gravitational field strength of the Earth is 10 N/kg.

The stopping distance of a car depends on two things: the thinking distance and the braking

distance.

Weight

Weight is not the same as mass. Mass is a measure of how much stuff is in an object. Weight is a

force acting on that stuff.

You have to be careful. In physics, the term weight has a specific meaning, and is measured in

newtons. Mass is measured in kilograms. The mass of a given object is the same everywhere, but its

weight can change.

Gravitational field strength

Weight is the result of gravity. The gravitational field strength of the Earth is 10 N/kg (ten newtons per

kilogram). This means an object with a mass of 1kg would be attracted towards the centre of the Earth

by a force of 10N. We feel forces like this as weight.

You would weigh less on the Moon because the gravitational field strength of the Moon is one-sixth of

that of the Earth. But note that your mass would stay the same.

Weight

On Earth, if you drop an object it accelerates towards the centre of the planet. You can calculate the

weight of an object using this equation:

weight (N) = mass (kg) × gravitational field strength (N/kg)

Question

A person has a mass of 60kg. How much do they weigh on Earth, if the gravitational field

strength is 10N/kg?

Answer

weight = mass × gravitational field strength

weight = 60kg × 10N/kg

weight = 600N

Question

How much would the same person weigh on the Moon, if the gravitational field strength is

1.6N/kg?

Answer

weight = mass × gravitational field strength

weight = 60kg × 1.6 N/kg

weight = 96N

Falling objects

You should be able to describe the forces affecting a falling object at different stages of its fall.

Usually, you need to think about two forces:

1. The weight of the object. This is a force acting downwards, caused by the object’s mass the

Earth’s gravitational field.

2. Air resistance. This is a frictional force acting in the opposite direction to the movement of the

object.

Three stages of falling

When an object is dropped, we can identify three stages before it hits the ground:

1. At the start, the object accelerates downwards because of its weight. There is no air

resistance. There is a resultant force acting downwards.

2. As it gains speed, the object’s weight stays the same, but the air resistance on it increases.

There is a resultant force acting downwards.

3. Eventually, the object’s weight is balanced by the air resistance. There is no resultant force

and the object reaches a steady speed, called the terminal velocity.

Terminal velocity

What happens if you drop a feather and a coin together? The feather and the coin have roughly the

same surface area, so when they begin to fall they have about the same air resistance.

As the feather falls, its air resistance increases until it soon balances the weight of the feather. The

feather now falls at its terminal velocity. But the coin is much heavier, so it has to travel quite fast

before air resistance is large enough to balance its weight. In fact, it probably hits the ground before it

reaches its terminal velocity.

On the Moon

An astronaut on the Moon carried out a famous experiment. He dropped a hammer and a feather at

the same time and found that they landed together. The Moon's gravity is too weak for it to hold onto

an atmosphere, so there is no air resistance. When the hammer and feather were dropped, they fell

together with the same acceleration.

Stopping distances

You should know some of the factors affecting the stopping distance of a car.

Thinking distance

It takes a certain amount of time for a driver to react to a hazard and start applying the brakes. During

this time, the car is still moving. The faster the car is travelling, the greater this thinking distance will

be.

The thinking distance will also increase if the driver's reactions are slower because they are:

under the influence of alcohol

under the influence of drugs

tired

Braking distance

The braking distance is the distance the car travels from where the brakes are first applied to where

the car stops. If the braking force is too great, the tyres may not grip the road sufficiently and the car

may skid. The faster the car is travelling, the greater the braking distance will be.

The braking distance will also increase if:

The brakes or tyres are worn.

The weather conditions are poor, such as an icy or wet road.

The car is more heavily laden, for example, with passengers and luggage.

Stopping distance

The stopping distance is the thinking distance added to the braking distance. The graph shows some

typical stopping distances.

Stopping distances

Kinetic energy and momentum

Work done and energy transferred are measured in joules (J). The work done on an object can

be calculated if the force and distance moved are known.

A change in momentum happens when a force is applied to an object that is moving or is able

to move. The total momentum in an explosion or collision stays the same.

Background

Work and energy are measured in the same unit, the joule (J). When an object is moved by a force,

energy is transferred and work is done. But work is not a form of energy - it is one of the ways in which

energy can be transferred.

The equation

This equation shows the relationship between work done, force applied and distance moved:

work done (joule, J) = force (newton, N) × distance (metre, m)

The distance involved is the distance moved in the direction of the applied force.

Question

A force of 10N is applied to a box to move it 2m along the floor. What is the work done on the

box?

Answer

The work done is 10 × 2 = 20J.

Potential energy and kinetic energy

Gravitational potential energy

Any object that is raised against the force of gravity stores gravitational potential energy. For

example, if you lift a book up onto a shelf, you have to do work against the force of gravity. The book

has gained gravitational potential energy.

Elastic potential energy

Elastic objects such as elastic bands and squash balls can change their shape. They can be stretched

or squashed, but energy is needed to change their shape. This energy is stored in the stretched or

squashed object as elastic potential energy.

Kinetic energy

Every moving object has kinetic energy (sometimes called movement energy). The more mass an

object has, and the faster it is moving, the more kinetic energy it has. You should be able to discuss

the transformation of kinetic energy to other forms of energy.

Example 1 - The bouncing ball

Several energy transfers happen when a squash ball is dropped onto a table and bounces up again.

When the ball is stationary above the table, its gravitational potential energy (GPE) is at a maximum. It

has no kinetic energy (KE), or elastic potential energy (EPE).

As the ball falls, its GPE is transferred to KE and the ball accelerates towards the table.

When the ball hits the table, the KE is transferred to EPE as the ball squashes. As the ball regains its

shape, the EPE is transferred to KE and it bounces upwards.

When the ball reaches the top of its travel, all the KE has been transferred to GPE again. Note that

the ball will be lower than it was when it was first dropped, because some energy is also

transferred as heat and sound to the surroundings.

High up

GPE - maximum

KE - none

EPE - none

Falling

GPE - decreasing

KE - increasing

EPE - none

On table

GPE - minimum

KE - none

EPE - maximum

Example 2 - The pendulum

The pendulum is a simple machine for transferring gravitational potential energy to kinetic energy, and

back again.

When the bob is at the highest point of its swing, it has no kinetic energy, but its gravitational potential

energy is at a maximum. As the bob swings downwards, gravitational potential energy is transferred to

kinetic energy, and the bob accelerates.

At the bottom of its swing, the bob’s kinetic energy is at a maximum and its gravitational potential

energy is at a minimum.

As the bob swings upwards, its kinetic energy is transferred to gravitational potential energy again. At

the top of its swing, it once again has no kinetic energy, but its gravitational potential energy is at a

maximum.

Note that the bob’s swing will become lower with each swing, because some energy is also transferred

as heat to the surroundings.

Momentum

A moving object has momentum. This is the tendency of the object to keep moving in the same

direction. It is difficult to change the direction of movement of an object with a lot of momentum.

You can calculate momentum using this equation:

momentum (kg m/s) = mass (kg) × velocity (m/s)

Notice that momentum has:

magnitude - an amount because it depends on the object’s mass

direction - because it depends on the velocity of the object

Question

What is the momentum of a 5 kg object moving at 2 m/s?

Answer

momentum = mass x velocity

= 5 kg × 2 m/s

= 10 kg m/s

Conservation of momentum

So long as no external forces are acting on the objects involved, the total momentum stays the same

in explosions and collisions. We say that momentum is conserved. You can use this idea to work out

the mass, velocity or momentum of an object in an explosion or collision.

Example

A bullet with a mass of 0.03 kg leaves a gun at 1000 m/s. If the gun’s mass is 1.5 kg, what is the

velocity of the recoil on the gun?

momentum of bullet = mass × velocity

= 0.03 kg × 1,000 m/s

= 30 kg m/s

Rearrange the equation: velocity = momentum ÷ mass

velocity of recoil on gun = 30 kg m/s ÷ 1.5 kg

= 20 m/s

Safety features in vehicles

When there is a car crash, the car, its contents, and the passengers, decelerate rapidly. They

experience great forces because of the change in momentum, which can cause injury. If the time

taken for the change in momentum on the body is increased, the forces on the body are reduced too.

Seat belts and crumple zones are designed to reduce the forces on the body if there is a collision.

Seat belts

Seat belts stop you tumbling around inside the car if there is a collision. However, they are designed to

stretch a bit in a collision. This increases the time taken for the body’s momentum to reach zero, so

reduces the forces on it.

Air bags

Air bags increase the time taken for the head’s momentum to reach zero, so reduce the forces on it.

They also act a soft cushion and prevent cuts.

Crumple zones are another safety feature in cars

The equation

This equation shows the relationship between kinetic energy (J), mass (kg) and speed (m/s):

kinetic energy = 1⁄2 × mass × speed2

Question

What is the kinetic energy of a 1,000kg car travelling at 10m/s?

Answer

kinetic energy = 1⁄2 × mass × speed2

kinetic energy = 1⁄2 × 1,000 × 102 = 1/2 × 1,000 × 100 = 50,000J (or 50kJ)

Check your understanding of this section by having a go at the activity.

Momentum - higher

You need to be able to calculate the force involved in changing the momentum of an object. Here is

the equation you need:

The force is measured in newtons, N. The time is measured in seconds, s.

Question

A 25 kg bicycle is travelling at 12 m/s. What force is needed to bring it to a halt in 5 s?

Answer

Momentum at start = 25 × 12 = 300 kg m/s

Momentum at end = 0 × 12 = 0 kg m/s

Change in momentum = 300 - 0 kg m/s

So a force of 60N is needed.

You should see that, for a given change in momentum, the longer the time taken, the smaller the force

needed. This is the idea behind many car safety features. It also explains why it takes a long time to

stop a super-tanker at sea, or to change its direction.

Static electricity

Some insulating materials become electrically charged when they are rubbed together.Charges

that are the same repel, while unlike charges attract.

Electrostatic precipitators, photocopiers and laser printers make practical use of electrostatic

charges.

Attraction and repulsion

You should know how and why insulators can be electrically charged.

Moving charges

When you rub two different insulating materials against each other they become electrically charged.

This only works for insulated objects - conductors lose the charge to earth.

When the materials are rubbed against each other:

negatively charged particles called electrons move from one material to the other

the material that loses electrons becomes positively charged

the material that gains electrons becomes negatively charged

both materials gain an equal amount of charge, but the charges are opposite

Detecting charge

If two charged objects with the same type of charge are brought close together, they will repel each

other - that is, if they are both positive or both negative. They will attract each other if they have

opposite charges.

The only way to tell if an object is charged is to see if it repels another charged object. This is

because charged objects will also attract small uncharged objects.

Discharge

A charged object can be discharged by connecting it to earth with a metal wire or other conductor. If

the potential difference (voltage) is very large, a spark may jump across the gap between the charged

object and the conductor. This can be dangerous. For example, it could cause an explosion in a petrol

station.

Using static electricity

You need to be able to explain how static electricity can be useful.

Electrostatic precipitators

Many power stations burn fossil fuels such as coal and oil. Smoke is produced when these fuels burn.

Smoke comprises tiny solid particles, such as unreacted carbon, which can damage buildings and

cause breathing difficulties. To avoid this, the smoke is removed from waste gases before they pass

out of the chimneys. The electrostatic precipitator is the device used for this job.

The flow chart outlines how an electrostatic precipitator works.

1. Smoke particles pick up a negative charge.

2. Smoke particles are attracted to the collecting plates.

3. Collecting plates are knocked to remove the smoke particles.

Photocopiers

The flow chart outlines how a photocopier works. A laser printer works in a similar way.

Circuits

Electrical circuits can be represented by circuit diagrams. The various electrical components

are shown by using standard symbols in circuit diagrams. Components can be connected in

series, or in parallel. The characteristics of the current [current: Moving electric charges, for

example, electrons moving through a metal wire. ] and potential difference (voltage) are different in

series and parallel circuits.

Circuit symbols

You need to be able to draw and interpret circuit diagrams.

Standard symbols

The diagram below shows the standard circuit symbols you need to know.

Open SwitchClosed Switch Lamp Cell

Battery Voltmeter Resistor Fuse

Ammeter Variable resistor Thermistor Light dependent resistor (LDR)

Circuit diagrams

Two things are important for a circuit to work:

there must be a complete circuit

there must be no short circuits

To check for a complete circuit, follow a wire coming out of the battery with your finger. You should be

able to go out of the battery, through the lamp and back to the battery.

To check for a short circuit, see if you can find a way past the lamp without going through any other

component. If you can, there is a short circuit and the lamp will not light.

Series and parallel connections

You should know the difference between series and parallel connections in circuits.

Series connections

Components that are connected one after another on the same loop of the circuit are connected in

series. The current [current: Moving electric charges, for example, electrons moving through a metal

wire. ] that flows across each component connected in series is the same.

Two lamps connected in series

The circuit diagram shows a circuit with two lamps connected in series. If one lamp breaks, the other

lamp will not light.

Parallel connections

Components that are connected on separate loops are connected in parallel. The current is shared

between each component connected in parallel.

Two lamps connected in parallel

The circuit diagram shows a circuit with two lamps connected in parallel. If one lamp breaks, the other

lamp will still light.

Current and potential difference

You need to know how to measure the current that flows through a component in a circuit. You also

need to know how to measure the potential difference, also called voltage, across a component in a

circuit.

Current

A current flows when an electric charge [electric charge: The electrical state of an object, which can

be positively charged or negatively charged. ] moves around a circuit. No current can flow if the circuit

is broken, for example, when a switch is open.

Measuring current:

current is measured in amperes

amperes is often abbreviated to amps or A

the current flowing through a component in a circuit is measured using an ammeter [ammeter:

A device used to measure electric current. ]

the ammeter must be connected in series with the component

Potential difference - voltage

A potential difference, also called voltage, across an electrical component is needed to make a current

[current: Moving electric charges, for example, electrons moving through a metal wire. ] flow through

it. Cells or batteries often provide the potential difference needed.

Measuring potential difference:

potential difference is measured in volts, V

potential difference across a component in a circuit is measured using a voltmeter

the voltmeter must be connected in parallel with the component

Cells and circuits

You should know what happens to the potential difference and current when the number of cells in a

circuit is changed.

Potential difference

A typical cell produces a potential difference of 1.5V. When two or more cells are connected in series

in a circuit, the total potential difference is the sum of their potential differences. For example, if two

1.5V cells are connected in series in the same direction, the total potential difference is 3.0V. If two

1.5V cells are connected in series, but in opposite directions, the total potential difference is 0V, so no

current will flow.

Current

When more cells are connected in series in a circuit, they produce a bigger potential difference across

its components. More current flows through the components as a result.

Series circuits

You should know the characteristics of the current and potential difference in series circuits.

Current

When two or more components are connected in series, the same current flows through each

component.

Potential difference

When two or more components are connected in series, the total potential difference of the supply is

shared between them. This means that if you add together the voltages across each component

connected in series, the total equals the voltage of the power supply.

Parallel circuits

You should know the characteristics of the current and potential difference in parallel circuits.

Current

When two or more components are connected in parallel, the total current flowing through the circuit is

shared between the components. Check your understanding of this by answering the questions about

the circuit seen here. Assume that both lamps are identical.

Potential difference

When two or more components are connected in parallel, the potential difference across them is the

same. This means that if a voltage across a lamp is 12V, the voltage across another lamp connected

in parallel is also 12V.

Resistance and resistors

Resistance is measured in ohms. It can be calculated from the potential difference across a

component and the current flowing through it. The total resistance of a series circuit is the sum

of the resistances of the components in the circuit.

Resistors, filament lamps and diodes produce different current-potential difference graphs.

The resistance of thermistors depends on the temperature, while the resistance of light-

dependent resistors (LDRs) depends on the light intensity.

Calculating resistance

You should understand the relationship between potential difference, current and resistance.

Why do we get resistance?

An electric current flows when electrons [electrons: Sub-atomic particles, with a negative charge and

a negligible mass relative to protons and neutrons. ] move through a conductor. The moving electrons

can collide with the atoms of the conductor. This makes it more difficult for the current to flow, and

causes resistance.

Electrons collide with atoms more often in a long wire than they do in a short one. A thin wire has

fewer electrons to carry the current than a thick wire. This means the resistance in a wire increases as:

the length of the wire increases

the thickness of the wire decreases

Calculating resistance

Resistance is measured in ohms, Ω

You can calculate resistance using this equation:

potential difference (volt, V) = current (ampere, A) × resistance (ohm, Ω )

Question

The bulb in a bike light has a resistance of 3.0 Ω.

What is the potential difference across the bulb if a current of 0.6 A flows?

Answer

potential difference = current × resistance

                               = 0.6      × 3.0

                               = 1.8 V

Check that you understand the relationship between potential difference, current and resistance using

this activity.

Changing the resistance

You should know how to change the resistance [resistance: The opposition in an electrical

component to the flow of electricity through it. Resistance is measured in ohms. ] in a circuit, and how

to work out the resistance in a series circuit.

Series circuits

Sum of resistance is 6 ohms

When components are connected in series, their total resistance is the sum of their individual

resistances. For example, if a 2 Ω resistor, a 1 Ω resistor and a 3 Ω resistor are connected side by

side, their total resistance is 2 + 1 + 3 = 6 Ω.

If you increase the number of lamps in a series circuit, the total resistance will increase and less

current will flow.

Variable resistors

The resistance in a circuit can also be altered using variable resistors. For example, these

components may be used in dimmer switches, or to control the volume of a CD player. Adjust the

resistance in the simulation by clicking the + and - buttons to see the effect on the current.

The filament lamp

Current-potential difference graphs

A graph of current - vertical axis - against potential difference - horizontal axis - shows you how the

current flowing through a component varies with the potential difference across it.

You should be able to recognise these graphs for resistors at constant temperature, for filament

lamps, and for diodes.

Resistor at constant temperature

The current flowing through a resistor at a constant temperature is directly proportional to the potential

difference across it. A component that gives a graph like the one to the right is said to follow Ohm’s

Law.

The filament lamp

Lamp

The filament lamp is a common type of light bulb. It contains a thin coil of wire called the filament. This

heats up when an electric current passes through it, and produces light as a result.

The filament lamp does not follow Ohm’s Law. Its resistance increases as the temperature of its

filament increases. So the current flowing through a filament lamp is not directly proportional to the

voltage across it. This is the graph of current against voltage for a filament lamp.

The diode

You should be able to recognise the graph of current against voltage for a diode.

Background

Diode

Diodes are electronic components that can be used to regulate the potential difference in circuits and

to make logic gates. Light-emitting diodes (LEDs) give off light and are often used for indicator lights in

electrical equipment such as computers and television sets.

The diode has a very high resistance in one direction. This means that current can only flow in the

other direction. This is the graph of current against potential difference for a diode.

Thermistors and LDRs

You should be able to recognise the circuit symbols for the thermistor and the LDR (light-dependent

resistor), and know how the resistance of these components can be changed.

The thermistor

Thermistor

Thermistors are used as temperature sensors - for example, in fire alarms. Their resistance decreases

as the temperature increases:

At low temperatures, the resistance of a thermistor is high and little current can flow through

them.

At high temperatures, the resistance of a thermistor is low and more current can flow through

them.

The LDR

Light dependent resistor (LDR)

LDRs (light-dependent resistors) are used to detect light levels, for example, in automatic security

lights. Their resistance decreases as the light intensity increases:

In the dark and at low light levels, the resistance of an LDR is high and little current can flow

through it.

In bright light, the resistance of an LDR is low and more current can flow through it.

Mains electricity

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.).

Wiring a plug

You should know the features of a correctly wired three-pin mains electricity plug and be able to

recognise errors in the wiring of a plug.

The cable

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 [insulator: a material which does not conduct electricity. Opposite of conductor ].

The inner wires are colour coded:

Colours of inner wires within a cable

colour wire

blue neutral

brown live

green and yellow stripes earth

The plug

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 [fuse: An electrical component that protects circuits and electrical devices from

overload by melting when the current becomes too high. ] 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.

The inside of a plug

The diagram shows the key features of a correctly wired three-pin mains plug.

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.

Earthing

You should understand why electrical appliances are earthed.

Earthing

Earthing of an electric cooker

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.

Fuses and circuit breakers

Fuses and circuit breakers protect electrical circuits and appliances.

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.

Use this simulation to see how circuit breakers work.

The fuse

A 13A fuse with a low melting point wire

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.

Energy in circuits

Power

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:

Question

An electric lamp transforms 500 J in 5 s. What is its power?

Answer

The more energy that is transferred in a certain time, the greater the power. A 100W light bulb

transfers more electrical energy [electrical energy: Energy transferred by electricity. ] each second

than a 60W light bulb.

The equation below shows the relationship between power, potential difference (voltage) and current:

power (watts) = current (amps) x potential difference (volts)

Example

If the current is 5 A and the potential difference is 12 V, the power is 5 × 12 = 60 W.

This means that 60 J of energy is transferred per second.

Question

Which is the best fuse to use (3A, 5A or 13A) with a 1.15 kW electric fire at a potential

difference of 230 V?

Remember that 1.15 kW is 1,150 W

Answer

Re-arrange the equation:

The best fuse to use would be the 13A fuse. The 3A and 5A fuses would blow even when the

fire was working normally.

Check your understanding of the relationship between power, current and potential difference by trying

this activity.

Direct current and alternating current

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

supplies.

Direct current

Direct current

If the current [current: Moving electric charges, for example, electrons moving through a metal wire. ]

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. The diagram shows an oscilloscope screen displaying the

signal from a d.c. supply.

Alternating current

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 [frequency: The number

of repetitions per second of a wave. The unit of frequency is the hertz, 'Hz'. ] of 50Hz (50 hertz), which

means it changes direction, and back again, 50 times a second. The diagram shows an oscilloscope

screen displaying the signal from an a.c. supply.

Alternating current - higher

Alternating current

The potential difference of the live terminal varies between a large positive value and a large negative

value. However, the neutral terminal is at a potential difference close to earth, which is zero. The

diagram shows an oscilloscope screen displaying the signals from the mains supply. The red trace is

the live terminal and the blue trace the neutral terminal. Note that, although the mean voltage of the

mains supply is about 230V, the peak voltage is higher.

Energy in circuits - higher

Charge, current and time

Electrical charge is measured in coulomb, C. The amount of electrical charge that moves in a circuit

depends on the current flow and how long it flows for.

The equation below shows the relationship between charge, current and time:

charge (coulomb, C) = current (ampere, A) × time (second, s)

Question

How much charge moves if a current of 10 A flows for 30 s?

Answer

charge = current × time = 10 × 30 = 300 C

Energy transferred, potential difference and charge

For a given amount of electrical charge that moves, the amount of energy transformed increases as

the potential difference (voltage) increases.

The equation below shows the relationship between energy transformed, potential difference and

charge:

energy transformed (joule, J) = potential difference (volt, V) × charge (coulomb, C)

Question

How much energy is transformed when the potential difference is 120 V and the charge 2 C?

Answer

energy = potential difference × charge = 120 × 2 = 240 J