AQA Core Science: Energy and 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.

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.

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.

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. The heat energy is used to boil water. The kinetic energy in the expanding steam spins turbines, which then drive generators to produce electricity. Unlike fossil fuels, nuclear fuels do not produce carbon dioxide or sulfur dioxide.

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.

The wind is produced as a result of giant convection 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 have huge blades mounted on a tall tower. The blades are connected to a nacelle or housing that contains gears linked to a generator. 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.

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

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. 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. 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. 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.

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. 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.

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 and electricity generators. This type of geothermal power station exists in places such as Iceland, California and Italy.

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.

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

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

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 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. 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 energy is a renewable energy resource and there are no fuel costs. No harmful polluting gases are produced however 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 and although warm water can be produced even on cloudy days and neither works at night.

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. The type of fuel in order of start of time going from short to long is gas-fired station (shortest start-up time), oil-fired station, coal-fired station and 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 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 can be transferred from place to place by conduction, convection and radiation. 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 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.

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.

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.

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. 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.

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.

Dark colours with a dull or matt finish have a good ability to emit thermal radiation and a good ability to absorb thermal radiation. Wheras light colours with a shiny finish have a poor ability to emit thermal radiation and a poor ability to absorb thermal radiation.

Heat energy is transferred from homes by conduction through the walls, floor, roof and windows. It is also transferred from homes by convection. For example, cold air can enter the house through gaps in doors and windows, and convection currents can transfer heat energy in the loft to the roof tiles. Heat energy also leaves the house by radiation through the walls, roof and windows.

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. The gap between the two panes of glass is filled with air. Heat loss through conduction is reduced, as air is a poor conductor of heat. Heat transfer by convection currents is also reduced by making the gap is very narrow.

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.

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

• 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.

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. 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.

Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved. The Sankey diagram for an electric lamp below shows that most of the electrical energy is transferred as heat rather than light.

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. Ordinary electric lamps contain a thin metal filament that glows when electricity passes through it. However, most of the electrical energy is transferred as heat energy instead of light energy.

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). The efficiency of a device will always be less than 100%.

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.

Power stations are built in order to generate electricity. There are four main stages: the fuel is burned to boil water to make steam, the steam makes a turbine spin, the spinning turbine turns a generator which produces electricity, 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.

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.

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 amount of electrical energy 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) 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. ime 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.

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.