Physics · 1 Motion, forces and energy

1.7.3 Energy resources

Before you start

The energy stores: kinetic, gravitational potential, chemical, nuclear and internal (thermal), and that energy is transferred between them.
The principle of conservation of energy: energy is not created or destroyed, only transferred.
Using $Δ E_{p} = m g Δ h$ , with $g = 9.8$ N/kg as Paper 4 gives it.
Turning a percentage into a decimal (38% = $0.38$ ), and the prefixes k ( $10^{3}$ ) and M ( $10^{6}$ ).
That every atom has a tiny central nucleus.

Syllabus objectives covered (7)

Describe how useful energy may be obtained, or electrical power generated, from: (a) chemical energy stored in fossil fuels (b) chemical energy stored in biofuels (c) water, including the energy stored in waves, in tides and in water behind hydroelectric dams (d) geothermal resources (e) nuclear fuel (f) light from the Sun to generate electrical power (solar cells) (g) infrared and other electromagnetic waves from the Sun to heat water (solar panels) and be the source of wind energy including references to a boiler, turbine and generator where they are used
Describe advantages and disadvantages of each method in terms of renewability, availability, reliability, scale and environmental impact
Understand, qualitatively, the concept of efficiency of energy transfer
Know that radiation from the Sun is the main source of energy for all our energy resources except geothermal, nuclear and tidal
Know that energy is released by nuclear fusion in the Sun
Know that research is being carried out to investigate how energy released by nuclear fusion can be used to produce electrical energy on a large scale
Define efficiency as: (a) (%) efficiency = (useful energy output) / (total energy input) (× 100%) (b) (%) efficiency = (useful power output) / (total power input) (× 100%) recall and use these equations

Part 1 of 8

The chain inside a power station: boiler, turbine, generator

Most ways of generating electricity on a large scale finish the same way: something makes a turbine spin, and the turbine spins a generator. Learn this chain once and most of this subtopic becomes variations on it.
Start with a thermal power station, one that uses heat. Energy released from the fuel heats water in a boiler. The water boils and becomes steam at high pressure. Energy has moved from the fuel's store into the internal (thermal) store of the steam.
The steam is directed at the blades of a turbine and makes it spin. The energy is now in the kinetic store of the turbine.
The turbine is joined by a shaft to a generator, so the generator spins too. The generator is the part that produces the electric current. It transfers energy from the kinetic store electrically, out along the cables.
After the turbine, the steam is cooled in a condenser and turns back into water, which is pumped back to the boiler to be heated again.
apparatus diagram — not drawn yet
A block diagram of a thermal power station, left to right: a fuel box feeding a boiler (water in, steam out), an arrow labelled 'steam' to a turbine, a shaft from the turbine to a generator with cables leaving it labelled 'electricity', and a return path from the turbine through a condenser back to the boiler labelled 'water'.
Fuel heats water, steam spins the turbine, the turbine spins the generator. Only the generator makes electricity.
Keep the jobs separate. The boiler makes steam, the turbine spins, and the generator produces the electricity. A turbine on its own does not generate anything.
Not every method uses all three parts. Use the word boiler only where water is heated to make steam. Where moving air or moving water turns the turbine directly, there is no boiler. Solar cells use none of the three.

Part 2 of 8

Resources that heat water: fossil fuels, biofuels, nuclear fuel and geothermal

Fossil fuels are coal, oil and natural gas. Their energy is in a chemical store. In a power station the fuel is burned, the thermal energy released heats water in the boiler, and the chain boiler → turbine → generator does the rest.
Fossil fuels also give useful energy without making electricity: petrol and diesel (made from oil) are burned in vehicle engines, and gas is burned to heat homes.
Biofuels are fuels made from living things that grew recently. Examples are wood, ethanol made from crops such as sugar cane, and biogas (mostly methane) produced by rotting plant and animal waste. Their energy is also in a chemical store.
A biofuel is used exactly like a fossil fuel. It is burned in a power station to heat water in a boiler, or burned directly, for example ethanol in car engines.
Nuclear fuel, such as uranium, is not burned. In a nuclear reactor, uranium nuclei split. This is nuclear fission, and it releases energy from the nuclear store of the fuel. That energy heats water to make steam, and from there the chain is the same: turbine, then generator.
Geothermal energy uses hot rocks under the ground. In some places, such as volcanic regions, very hot rock lies fairly close to the surface. Cold water is pumped down, the rocks heat it, and it returns as steam or hot water.
The steam from a geothermal well drives a turbine and generator. The hot water can also heat buildings directly. In geothermal, the hot rock does the job that burning fuel does in a boiler: nothing is burned.

Part 3 of 8

No boiler needed: moving water, moving air and sunlight

Hydroelectric power uses water held behind a dam, high up. That water is in a gravitational potential store. When it is let out, it flows down through pipes, gaining kinetic energy, and spins turbines at the bottom of the dam. The turbines drive generators.
Tidal power uses the rise and fall of the sea. A barrage, a kind of dam, is built across an estuary. As the tide comes in and goes out, the water flows through turbines in the barrage, and these drive generators.
Wave power uses the up-and-down movement of the sea surface. A device on the water is moved by the waves, and this movement drives a generator. Waves and tides are different things: waves are made by wind over the sea, tides by the regular rise and fall of the whole sea level.
Wind power uses the kinetic store of moving air. The wind pushes on the blades of a wind turbine and makes them turn. The blades are connected to a generator.
The wind itself comes from the Sun. The Sun's radiation heats the Earth's surface unevenly. Air above warmer ground is heated, expands, becomes less dense and rises. Cooler, denser air moves in to take its place. That moving air is wind.
Solar cells transfer energy from light directly into electrical energy. There is no boiler, no turbine and no generator, and nothing moves. They power calculators, roofs, satellites and large solar farms.
Solar panels do a different job: they heat water. Infrared and other electromagnetic radiation from the Sun is absorbed by a dark, matt black surface, and water flowing through pipes behind it is heated. The hot water is used in the building. Solar panels in this sense do not generate electricity.
The trap is the names. In everyday speech, people call the electricity-making ones 'solar panels' too. This syllabus keeps them apart: solar cells make electricity from light; solar panels heat water using infrared.

Part 4 of 8

Five questions to ask about any resource

You judge each resource against five criteria. Learn what each word means precisely, because an answer that mixes them up does not answer the question that was asked.
Renewability: will it run out? A renewable resource is replaced naturally as fast as we use it. A non-renewable resource is a finite store that will eventually be used up. Fossil fuels and nuclear fuel are non-renewable. Everything else in this subtopic is renewable, with biofuels renewable only as long as new crops or trees are grown to replace those used.
Availability: where and when can you use it? Hydroelectric power needs high ground, a valley to flood and enough rain. Tidal power needs a suitable estuary. Geothermal needs hot rock near the surface. Wave power needs a coast. Fuels, by contrast, can be transported to a power station almost anywhere.
Reliability: does it produce electricity whenever it is needed? A fuel can be burned on demand. Wind, waves and sunlight cannot be switched on: they depend on the weather and the time of day.
Scale: how much power does one installation produce? A fossil-fuel, nuclear or hydroelectric power station produces a large output from one site. One wind turbine, one wave device or one solar cell produces a small output, so many are needed, which takes up a large area of land or sea.
Environmental impact: what does it do to the air, land, water and living things? This covers gases released, waste produced, habitats destroyed, noise and the look of the landscape.
A strong answer names the resource, the criterion and the consequence together. 'Wind turbines are unreliable because they generate no electricity when the wind is not blowing' does the job. 'Wind is bad' or 'better for the environment' with no reason says nothing a mark can be given for.

Part 5 of 8

The trade-offs, resource by resource

Fossil fuels: non-renewable. Reliable, since fuel can be stored and burned whenever needed, and produce electricity on a large scale. Burning them releases carbon dioxide, a greenhouse gas that contributes to global warming and climate change. Burning coal also releases sulfur dioxide, which causes acid rain.
Biofuels: renewable if replanted, and reliable, since they can be stored and burned on demand. Burning them releases carbon dioxide too, but the plants took carbon dioxide from the air as they grew, so the overall increase can be much smaller than for fossil fuels. Growing fuel crops uses large areas of land that could grow food or remain as natural habitat.
Nuclear fuel: non-renewable. Reliable, and large scale. No carbon dioxide is released while it generates. But it produces radioactive waste, some of which stays dangerous for thousands of years and must be stored safely, and an accident can release radioactive material. Nuclear power stations are expensive to build and to take apart safely at the end of their life.
Geothermal: renewable and reliable, producing energy continuously. Available only in the few places where hot rock is near the surface. It releases very little carbon dioxide compared with burning fossil fuels.
Hydroelectric: renewable, reliable as long as the reservoir holds water, and large scale. It can start generating quickly when demand rises. It releases no carbon dioxide while generating, but building the dam floods a valley, destroying habitats and sometimes homes. It needs a suitable location.
Tidal: renewable. The tides rise and fall at times that can be predicted, so tidal power is more reliable than wind or waves, but the output varies through the day and is lowest when the water is barely moving near high and low tide. Few estuaries are suitable, and a barrage changes the habitat of the estuary for birds and fish and can affect shipping.
Waves: renewable but unreliable, because calm seas give little output. Small scale, and only possible on a coast. The devices can be a hazard to boats and can affect marine habitats.
Wind: renewable but unreliable, because there is no output when the wind does not blow. Each turbine is small scale, so large wind farms are needed. No carbon dioxide is released while generating, but turbines are noisy and change the look of the landscape.
Solar cells: renewable, but no output at night and less in cloudy weather, so unreliable. Small scale per cell, so large solar farms take up a lot of land. No pollution while generating, and useful in remote places with no connection to the grid.
Solar panels for heating water: renewable, but they only work well in sunshine, and each one serves a single building. They reduce the fuel or electricity needed to heat water.

Part 6 of 8

Where the energy came from in the first place: the Sun, and fusion

Trace each resource backwards and you reach the Sun's radiation for almost all of them. That is the fact you need: radiation from the Sun is the main source of energy for all our energy resources except geothermal, nuclear and tidal.
Fossil fuels formed from the remains of living things that died millions of years ago. Those living things got their energy, directly or through food chains, from plants that used sunlight in photosynthesis. Biofuels are the same story, but with plants that grew recently.
Hydroelectric: the Sun heats seas and lakes, water evaporates, the water vapour forms clouds, and rain falls on high ground and fills the reservoir. The Sun lifted that water. Wind comes from uneven heating by the Sun, and waves are made by wind, so both trace back to the Sun as well. Solar cells and solar panels use the radiation directly.
Now the three exceptions, and why each one is an exception. Geothermal energy comes from the heat inside the Earth. Nuclear energy is stored in the nuclei of uranium, which has been in the Earth since it formed. Tides are caused by the gravitational pull of the Moon, and to a smaller extent the Sun, on the oceans. That is gravity, not radiation.
So where does the Sun's own energy come from? In the Sun's core, hydrogen nuclei join together to form helium nuclei. Joining light nuclei to make a heavier one is nuclear fusion, and it releases a huge amount of energy, which leaves the Sun as electromagnetic radiation.
Do not mix up the two nuclear processes. Fusion is light nuclei joining; it happens in the Sun. Fission is a heavy nucleus splitting; it happens in today's nuclear power stations.
Scientists are carrying out research into how the energy released by nuclear fusion could be used to produce electrical energy on a large scale. It is not yet done in power stations. One reason is that fusion needs extremely high temperatures: nuclei are positively charged and repel each other, so they must be moving very fast to get close enough to join. Producing and containing such temperatures on Earth is very difficult.

Part 7 of 8

Efficiency: how much of the energy ends up useful

Energy is never created or destroyed, so the total energy coming out of a device equals the total energy put in. But not all of it ends up where you want it.
The useful output is the energy that goes to the store you wanted: electrical energy from a power station, kinetic energy from a motor, light from a lamp. The rest is wasted.
Wasted energy is not destroyed. It is dissipated: spread out into the surroundings, mostly heating them, and sometimes as sound. Once spread out that thinly, it cannot easily be used again.
In a power station, energy is wasted at every stage: hot gases leave the chimney, the condenser warms its cooling water or the air, and friction heats the moving parts of the turbine and generator. Current in the cables heats them too.
Efficiency tells you what fraction of the energy put in is transferred usefully. A more efficient device wastes a smaller fraction. A more efficient power station needs less fuel to produce the same electrical output.
Because energy is conserved, the useful output can never be more than the total input. Machines with moving parts always waste some energy through friction, so their efficiency is always less than 100%.

Part 8 of 8

Efficiency as an equation you must recall

There is no formula sheet in Paper 4, so you must be able to write this from memory. Efficiency in terms of energy:
$efficiency = \frac{useful energy output}{total energy input} (\times 100 %)$
Leave out the × 100% and you get a decimal between 0 and 1. Include it and you get a percentage. Both are acceptable, but say which one you mean: write 0.38 or 38%, never 38 on its own and never 0.38%.
Efficiency in terms of power:
$efficiency = \frac{useful power output}{total power input} (\times 100 %)$
Power is the energy transferred per second, measured in watts: $1$ W $= 1$ J/s. You meet it properly in the next subtopic. The power form gives the same answer as the energy form, because dividing both energies by the same time leaves their ratio unchanged.
Efficiency has no unit. Joules divide by joules, or watts by watts, and the units cancel. That only works if both are in the same unit, so convert kJ to J, or both to MW, before dividing.
Useful output is always on top. If your answer comes out bigger than 1, or bigger than 100%, you have divided the wrong way up. Use that as a check every time.
To find an output or an input, rearrange. Convert the percentage to a decimal first: 40% becomes $0.40$ .
$useful output = efficiency \times total input total input = \frac{useful output}{efficiency}$
The wasted energy, or wasted power, is whatever is left over, because energy is conserved: wasted = total input − useful output.

Worked examples

Example 1. An electric motor is supplied with $2400$ J of electrical energy. It lifts a load and gives it $1800$ J of gravitational potential energy. Calculate the efficiency of the motor, and state how much energy is wasted.

Show the working

Identify the two energies. The useful output is the energy that went where it was wanted, into the load: $1800$ J. The total input is everything supplied: $2400$ J.
Write the equation from memory, then substitute. Both values are in J, so they can be divided directly.
$efficiency = \frac{1800}{2400} = 0.75$
Multiply by 100% if you want a percentage: $0.75 \times 100 % = 75 %$ . Check: the answer is below 1, so it is the right way up.
The wasted energy is what is left over, because energy is conserved.
$2400 - 1800 = 600 J$
Say where it went: it is dissipated to the surroundings, mostly heating them, through friction in the motor and current in its wires.

Answer: Efficiency $= 0.75$ (75%), with no unit. Energy wasted $= 600$ J, dissipated to the surroundings.

Example 2. A power station has an efficiency of 40%. Its useful electrical power output is $660$ MW. Calculate the total power input from the fuel, and the power wasted.

Show the working

Here you know the output and the efficiency, and want the input, so the equation must be rearranged. The input must be bigger than the output; keep that in mind as a check.
Convert the efficiency to a decimal: 40% $= 0.40$ .
Total input = useful output ÷ efficiency. Leave the power in MW: the answer will then come out in MW too.
$total power input = \frac{660}{0.40} = 1650 MW$
Check: $1650$ MW is bigger than $660$ MW, as it must be.
The power wasted is the difference.
$1650 - 660 = 990 MW$
Notice that more power is wasted than is delivered usefully. With an efficiency below 50%, that is always the case.

Answer: Total power input $= 1650$ MW; power wasted $= 990$ MW.

Example 3. In a hydroelectric power station, $1.5 \times 10^{5}$ kg of water falls through a vertical height of $80$ m every second. The generators produce $1.0 \times 10^{8}$ J of electrical energy every second. Calculate the efficiency of the power station. Take $g = 9.8$ N/kg.

Show the working

The energy input is the gravitational potential energy the water gives up as it falls. That store is what the turbines tap. Use $Δ E_{p} = m g Δ h$ for one second's worth of water.
$Δ E_{p} = 1.5 \times 10^{5} \times 9.8 \times 80 = 1.176 \times 10^{8} J$
The useful output in the same second is the electrical energy: $1.0 \times 10^{8}$ J. Both values are per second, so they can be compared directly.
Substitute into the efficiency equation, useful output on top.
$efficiency = \frac{1.0 \times 10^{8}}{1.176 \times 10^{8}} = 0.850$
Round to two significant figures, matching the data, and check it is below 1. It is.

Answer: Efficiency $= 0.85$ (85%).

Example 4. A country must choose between building a coal-fired power station and a wind farm with the same maximum power output. Compare the two in terms of reliability and environmental impact.

Show the working

Compare means say how they are alike or different, so every point you make should mention both resources. Two criteria are named, so plan at least one point for each.
Reliability. The coal station can generate whenever electricity is needed, as long as coal is delivered. The wind farm generates nothing when the wind does not blow. So the coal station is more reliable.
Environmental impact, the atmosphere. Burning coal releases carbon dioxide, a greenhouse gas that contributes to global warming, and sulfur dioxide, which causes acid rain. The wind farm releases no carbon dioxide while generating. On this point the wind farm has less impact.
Environmental impact, the landscape. The wind farm needs many turbines spread over a large area. They are noisy and change the look of the landscape. Giving an impact for each side shows the comparison is balanced.
Stop there. Renewability and cost were not asked for, so points about them earn nothing here, however true they are.

Answer: The coal-fired station is more reliable: it can generate on demand, while the wind farm generates nothing when the wind does not blow. The wind farm has less impact on the atmosphere: it releases no carbon dioxide while generating, while burning coal releases carbon dioxide (global warming) and sulfur dioxide (acid rain). However, the wind farm spreads over a large area and causes noise and visual impact on the landscape.

Where people go wrong

Mistake: Writing that the turbine produces the electricity.

Instead: The turbine spins; the generator, turned by the turbine, produces the electricity.

Each part has one job: the boiler makes steam, the turbine turns, the generator transfers the energy electrically. A description that skips the generator leaves out the step where electricity is actually produced.

Mistake: Treating solar cells and solar panels as the same thing.

Instead: Solar cells transfer energy from light into electrical energy. Solar panels absorb infrared to heat water.

The syllabus lists them as two separate methods using two different parts of the Sun's radiation, and only one of them generates electricity.

Mistake: Using 'renewable' to mean 'clean' or 'reliable'.

Instead: Renewable means only that the resource is replaced naturally and will not run out. Reliability and environmental impact are separate criteria.

Wind is renewable but unreliable. Biofuels are renewable but release carbon dioxide when burned. Mixing the criteria up answers a different question from the one asked.

Mistake: Saying the wasted energy is 'used up', 'lost' or 'destroyed'.

Instead: Say it is dissipated: transferred to the surroundings, mostly heating them.

Energy is conserved. It cannot be destroyed, only spread out so thinly that it is no longer useful.

Mistake: Saying the Sun releases energy by nuclear fission, or that today's nuclear power stations use fusion.

Instead: The Sun uses fusion: light hydrogen nuclei join to form helium. Today's power stations use fission: heavy uranium nuclei split. Fusion power stations are still being researched.

The words differ by a few letters, but they are opposite processes. Writing the wrong one gives a wrong answer, not a near miss.

Check yourself

In a gas-fired power station, state what the boiler, the turbine and the generator each do.
Answer
The boiler: burning gas heats water in it to make high-pressure steam. The turbine: the steam makes it spin. The generator: turned by the turbine, it produces the electricity.
What is the difference between a solar cell and a solar panel? Which of them works without a turbine or generator?
Answer
A solar cell transfers energy from light directly into electrical energy. A solar panel absorbs infrared from the Sun to heat water. The solar cell generates electricity with no turbine and no generator; the solar panel does not generate electricity at all.
Give one reason why tidal power is more reliable than wave power.
Answer
The tides rise and fall at times that can be predicted, whereas waves depend on the wind and weather, so a calm sea gives little wave power.
Fossil fuels and nuclear fuel are both non-renewable. Give one environmental disadvantage of each that the other does not have.
Answer
Fossil fuels release carbon dioxide when burned, contributing to global warming (nuclear fuel releases none while generating). Nuclear fuel produces radioactive waste that must be stored safely for a very long time (fossil fuels do not).
Name the three energy resources whose energy does not come from the Sun's radiation, and give the source of each.
Answer
Geothermal: heat from inside the Earth. Nuclear: energy stored in the nuclei of uranium. Tidal: the gravitational pull of the Moon (and, less, the Sun) on the oceans.
Name the process that releases energy in the Sun. How is it different from the process in today's nuclear power stations, and why is it not yet used to generate electricity?
Answer
Nuclear fusion: light hydrogen nuclei join to form helium. Power stations use fission, where heavy nuclei split. Fusion needs extremely high temperatures that are very difficult to produce and contain on Earth, so it is still being researched as a way to generate electricity on a large scale.
A lamp is supplied with $60$ J of electrical energy and gives out $9.0$ J of light. Calculate its efficiency. Why could a correct answer never be more than 100%?
Answer
Efficiency $= 9.0 / 60 = 0.15$ , or 15%. The useful output can never exceed the total input, because energy cannot be created.
A motor has an efficiency of 80%. Its total power input is $500$ W. Calculate its useful power output and the power wasted, and say where the wasted energy goes.
Answer
Useful output $= 0.80 \times 500 = 400$ W. Power wasted $= 500 - 400 = 100$ W, dissipated to the surroundings, mostly as heating through friction and current in the wires.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A gas-fired power station burns natural gas to generate electricity. The total power input from the fuel is $1500$ MW. The useful electrical power output is $570$ MW.

(a) Describe how the power station generates electrical energy from the gas. Refer to the boiler, the turbine and the generator in your answer. 3 marks No calculator

Mark scheme

B1 gas is burned and the thermal energy heats water in the boiler to make steam
B1 the steam turns / spins the turbine
B1 the turbine turns the generator, which produces the electricity; do not credit 'the turbine produces electricity' for this mark

(b) Calculate the efficiency of the power station. 2 marks

Mark scheme

C1 efficiency = useful power output / total power input, or 570 / 1500 seen
A1 0.38 or 38% or 19/50; 38 with no % sign is not accepted

(c) State what happens to the energy that is not transferred to the electrical output. 1 mark No calculator

Mark scheme

B1 it is dissipated / transferred to the surroundings, heating them (as thermal energy); 'it is lost' or 'it is wasted' alone is insufficient

Question 2. A small island is not connected to any mainland electricity supply. Its electricity is generated by a diesel generator. The islanders are considering using wind turbines and solar cells instead.

(a) Give two advantages of using wind turbines instead of the diesel generator. 2 marks No calculator

Mark scheme

B1 any one from: wind is renewable / will not run out (diesel will); no carbon dioxide / greenhouse gas released while generating; no other polluting gases or smoke (e.g. sulfur dioxide) released while generating; no fuel has to be bought or shipped to the island / no fuel costs; the supply does not depend on fuel deliveries owtte
B1 a second, different point from the list; 'better for the environment' with no reason is insufficient

(b) Explain why solar cells alone could not supply the island with electricity at all times. 2 marks No calculator

Mark scheme

B1 solar cells produce no electricity at night / much less when it is cloudy
B1 but electricity is still needed at those times, so the supply is unreliable / not available when it is needed owtte; accept: so another source or a store of energy would be needed

(c) Suggest one reason why a hydroelectric power station may not be possible on the island. 1 mark No calculator

Mark scheme

B1 any one from: no high ground / hills to hold water at a height; no suitable river or valley to dam; not enough rainfall to keep a reservoir filled; flooding a valley would use too much of the small island's land / destroy homes or habitats owtte

Question 3. Most of the energy resources used on Earth obtain their energy from radiation emitted by the Sun.

(a) State the process that releases energy in the Sun. 1 mark No calculator

Mark scheme

B1 (nuclear) fusion; accept: (hydrogen) nuclei joining (to form helium); 'fission' scores 0, and an answer giving both fusion and fission is CON

(b) State two energy resources whose energy does not come from the Sun's radiation. 2 marks No calculator

Mark scheme

B1 any one from: geothermal; nuclear (fuel); tidal
B1 a second, different resource from the list

(c) Explain how the chemical energy stored in coal came from the Sun. 2 marks No calculator

Mark scheme

B1 coal formed from the remains of plants that lived (millions of years) ago
B1 the plants absorbed light from the Sun to grow / for photosynthesis, storing the energy as chemical energy

(d) Scientists are researching how energy released by nuclear fusion could be used to generate electricity on a large scale. Suggest one reason why fusion is not yet used in power stations. 1 mark No calculator

Mark scheme

B1 any one sensible reason, e.g.: fusion needs extremely high temperatures (and/or pressures), which are very difficult to produce / contain / keep going; nuclei repel because they are positively charged, so they must move very fast to fuse; the very hot fuel (plasma) is hard to contain; so far experiments use more energy than they release; the technology is not yet developed / is very expensive owtte

Question 4. An electric motor in a crane has an efficiency of 60%. While lifting a load, the motor gives the load $4.5 \times 10^{4}$ J of gravitational potential energy.

(a) Calculate the total energy input to the motor. 2 marks

Mark scheme

C1 total energy input = useful energy output / efficiency, or 4.5 × 10^4 / 0.60 seen
A1 7.5 × 10^4 J or 75 kJ or 75 000 J; unit required

(b) Deduce how much energy is wasted by the motor. 1 mark

Mark scheme

B1 3.0 × 10^4 J or 30 kJ (ecf: answer to (a) minus 4.5 × 10^4 J); unit required

How this is examined

Judging a resource against a named criterion: renewability, availability, reliability, scale or environmental impact: Give, Explain or Compare parts worth one to four marks, set around a named place or a choice between two resources Each point names the resource, the criterion and the consequence: 'wind turbines generate no electricity when the wind does not blow, so the supply is unreliable'. In a Compare, each point says something about both resources: 'the coal station is more reliable than the wind farm because…'. 'Better for the environment' or 'eco-friendly' with no reason is insufficient.
The boiler → turbine → generator chain for a named resource: a three-mark Describe that names a fuel or resource and asks you to refer to the boiler, turbine and generator where they are used One mark per link: the fuel is burned (or, for nuclear, fission releases energy) and heats water in the boiler to make steam, while for geothermal hot rock underground heats the water and there is no boiler; the steam turns the turbine; the turbine turns the generator, which produces the electricity. For wind, hydroelectric, tidal and wave power there is no boiler: moving air or water turns the turbine. Solar cells use none of the three.
Calculating efficiency, and rearranging it to find an input or an output: a two- or three-mark Calculate, with energies in J or kJ or powers in W or MW The equation written from memory, useful output over total input; both values in the same unit; an answer between 0 and 1 (or 0 and 100%) with no unit. When rearranged: the percentage turned into a decimal first, and the answer carrying J or W.
Tracing a resource back to the Sun, and naming the three that are not: a one- or two-mark State or Explain 'Geothermal, nuclear and tidal' for the exceptions. For an Explain, the chain of steps, e.g. for coal: formed from plants that lived long ago; the plants used sunlight in photosynthesis and stored it as chemical energy.
Saying where the wasted energy goes: a one- or two-mark State or Explain alongside an efficiency calculation 'Transferred to the surroundings, heating them' or 'dissipated as thermal energy'. 'It is lost' or 'used up' on its own does not say where the energy went.
Fusion in the Sun, and fusion power as research rather than a working power station: a one-mark State, or a one-mark Suggest about why fusion is not yet used to generate electricity The word 'fusion': 'fission' scores nothing. For the research point: fusion needs extremely high temperatures, which are very difficult to produce and contain on Earth.

Words to know

Renewable resource: An energy resource that is replaced naturally as fast as it is used, so it will not run out.
Non-renewable resource: A finite energy resource that will eventually be used up, such as fossil fuels and nuclear fuel.
Fossil fuel: Coal, oil or natural gas: a fuel formed from the remains of living things that died millions of years ago, with energy in a chemical store.
Biofuel: A fuel made from living things that grew recently, such as wood, ethanol or biogas.
Boiler: The part of a thermal power station where energy from the fuel heats water to make high-pressure steam.
Turbine: A set of blades on a shaft, made to spin by steam, water or wind.
Generator: The device, turned by the turbine, that produces the electric current.
Solar cell: A device that transfers energy from light directly into electrical energy.
Solar panel: A device that absorbs infrared and other radiation from the Sun to heat water.
Geothermal energy: Energy from hot rocks underground, used to make steam or hot water.
Nuclear fission: The splitting of a heavy nucleus, such as uranium, releasing energy. Used in today's nuclear power stations.
Nuclear fusion: The joining of light nuclei, such as hydrogen, to form a heavier nucleus, releasing energy. The energy source of the Sun.
Reliability: Whether a resource can produce electricity whenever it is needed.
Dissipated: Spread out into the surroundings, mostly as heating, so that the energy is no longer useful.
Efficiency: Useful energy output ÷ total energy input, or useful power output ÷ total power input; × 100% for a percentage. It has no unit.

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