Choosing energy resources — answer key

1. RQ12 · Practice

Options as numbered on the pupil sheet: 1 — Non-renewable; 2 — Renewable.

Model answer. Coal, natural gas and uranium took millions of years to form or have been in the Earth since it formed: once they are used, they are gone, so they are non-renewable. The wind, the tides, sunlight and the rain that refills a reservoir keep coming, geothermal energy comes from heat that the Earth keeps supplying, and new trees can be grown to replace those made into wood pellets: these are renewable.

A full answer includes

• I matched coal and natural gas to non-renewable.
• I matched uranium to non-renewable.
• I matched wind, the tides, sunlight and water behind a dam to renewable.
• I matched wood pellets and geothermal energy to renewable.
• I did not match uranium to renewable because so little of it is used.

Also accept

• non-renewable: coal, gas, uranium; renewable: wind, wood pellets, tides, sunlight, dam, geothermal

Watch for

• “Uranium is renewable, because a nuclear power station uses so little of it.” — A little fuel goes a long way, but the Earth’s uranium is not replaced: once it is used, it is gone.
• “Biofuels are non-renewable, because they are burnt like coal.” — New trees and crops can be grown to replace those that are burnt, so biofuels are renewable.

2. RQ13 · Practice

Options as numbered on the pupil sheet: 1 — Environmental impact; 2 — Renewability; 3 — Availability; 4 — Reliability; 5 — Scale.

Model answer. Availability asks whether a resource is found where it is needed: a country with no coastline has no waves or tides to use. Reliability asks whether it is there whenever it is needed: the stream is in the right place, but in a long drought it stops. Scale asks whether it gives the right amount of power: one turbine’s 3 MW is far less than a big city’s 2000 MW. Environmental impact asks what harm it does: turbine blades can kill birds. Renewability asks whether a resource will run out: coal will, but willow trees can be grown again.

A full answer includes

• I matched the country with no coastline to availability.
• I matched the stream drying up in a drought to reliability.
• I matched the 3 MW turbine and the 2000 MW city to scale.
• I matched the birds hit by the blades to environmental impact.
• I matched coal gone and willow regrown to renewability.
• I did not match the drought to availability: the stream is in the right place, but not there at every time.

Also accept

• no coastline availability; drought reliability; 3 MW turbine scale; birds environmental impact; coal and willow renewability

Watch for

• “A stream drying up in a drought is a question of availability.” — The stream is in the right place; it just is not there at every time. Being there whenever it is needed is reliability.
• “A country with no coastline is a question of reliability.” — This is about whether the resource is found in that place at all: availability.
• “One small wind turbine for a big city is a question of reliability.” — Even in a steady wind, 3 MW is far too little for 2000 MW. The size of the power is scale.

3. RQ14 · Practice

Model answer. What matters is the power at each hour, not only the total energy. The town’s demand peaks at 14 MW in the evening, when the solar cells give nothing, and on calm days the wind farm gives little too. Then the power from these resources was less than the demand and the town was short, even on Wednesday and Thursday, when the day’s total was more than 220 MWh. At windy, sunny times there was more power than needed, but that spare could not be stored (only the dam stores energy, and its reservoir ran nearly empty), so it was switched off. Wind and sunlight are not reliable: they are not there whenever the town needs them.

A full answer includes

• I chose: on calm evenings, the power was less than the demand.
• I did not choose the statement that energy was destroyed: energy is never destroyed.

Also accept

• on calm evenings the power was less than the demand

Watch for

• “More energy in the week means the town is never short.” — The supply has to be big enough at every hour. Energy from a windy afternoon cannot light a lamp on a calm evening unless it is stored.
• “The turbines and cells destroyed some of the energy.” — Energy is never destroyed. At windy, sunny times the spare power was simply not generated, because nothing could store it.
• “The town needed more energy in week 3 than in week 1.” — The town needs the same 1540 MWh every week. The problem was when the energy came, not how much.
• “Every day, the energy supplied was less than 220 MWh.” — Read the chart: on most days the total was more than 220 MWh, yet Wednesday and Thursday still had hours short.

4. RQ15 · Practice

Model answer. Renewable means the resource will not run out, not that it is harmless. A hydroelectric dam floods a valley and destroys the habitats there; wind turbines change how the hills look and can harm birds; a tidal barrage covers mudflats where birds feed. Biofuels are renewable because new crops and trees can be grown, but burning them releases carbon dioxide, and growing, harvesting and transporting them uses energy, often from fossil fuels. The growing plants take in carbon dioxide, so biofuels add less to the air than fossil fuels, but they are not zero-emission. The power of a wind turbine changes with the wind, from nothing on a calm day to full power in a strong wind.

A full answer includes

• I selected: a renewable resource can still harm the environment.
• I selected: biofuels are renewable, but burning them releases carbon dioxide.
• I selected: a hydroelectric dam floods a valley and destroys habitats.
• I did not select that renewable resources do no harm at all.
• I did not select that wind turbines give a steady power.

Also accept

• renewables can still harm; biofuels renewable but release carbon dioxide; a dam floods a valley

Watch for

• “Renewable resources do no harm, because they will not run out.” — Renewability and environmental impact are different questions. A dam is renewable but floods a valley.
• “Biofuels are zero-emission.” — Burning a biofuel releases carbon dioxide, and farming and transporting it use energy too. It is renewable, not emission-free.
• “Wind turbines give a steady power, whatever the weather.” — The power of a wind turbine changes with the wind: nothing on a calm day, full power in a strong wind.

5. RQ16 · Practice

Model answer. (a) The water in the reservoir is a store of energy, so the dam can be switched on at any hour, for example at the evening peak on a calm, cloudy day, and switched off when the wind and the Sun are enough. It follows the demand; the wind farm generates only when the wind blows. (b) Its turbines give at most 6 MW, less than half the 14 MW peak, so at the peak it cannot keep up on its own. It can also only give out the energy stored in its reservoir: 150 MWh when full, refilled at only 0.5 MW, which is 12 MWh a day. The town needs 1540 MWh in a week; in week 3 the reservoir was almost empty, 1.5 MWh, by the end of Sunday. (c) Building the dam floods the valley behind it, destroying the habitats of the plants and animals that lived there, and sometimes homes and farmland; it also changes the flow of the river below.

A full answer includes

• (a) I wrote that the reservoir stores energy, so the dam can generate whenever it is needed.
• (a) I wrote that the wind farm generates only when the wind blows.
• (b) I wrote that its greatest power, 6 MW, is less than the 14 MW peak.
• (b) I wrote that its stored energy, 150 MWh, is far less than the 1540 MWh needed in a week.
• (b) I did not write that a renewable dam can supply any amount of energy.
• (c) I wrote that it floods the valley behind it, destroying habitats.

Also accept

• (a) it stores energy and can generate whenever needed (b) 6 MW is less than 14 MW, and 150 MWh is far less than 1540 MWh (c) floods a valley, destroying habitats

Watch for

• “A dam is renewable, so it can supply any amount of energy.” — Renewable means the rain keeps refilling it, but only slowly: here 0.5 MW. Its power and its stored energy are both limited.
• “A dam is unreliable, like the wind.” — The dam can be switched on whenever there is water in the reservoir. Its limits are its power and the energy stored, not the weather from hour to hour.

6. RQ17 · Practice

Model answer. (a) The wind farm and the solar cells depend on the weather and the time of day. On calm evenings their power, with the tides, the waves and the dam, was less than the demand. Something reliable must fill those hours. (b) A nuclear power station gives about 1000 MW, far more than the town’s peak demand of 14 MW: its scale is far too big for a backup that runs a few hours a week. It also cannot be switched on and off quickly, so it could not fill short gaps. (c) A geothermal power station needs hot rocks near the surface, and there are none under this town: it is not available here. (d) The gas-fired power station can be switched on at any hour and turned up or down to fill each gap, so the town is never short; with the renewables doing most of the work it burnt 221 MWh of gas instead of 3850 MWh. But burning gas still releases carbon dioxide, 44 t in the week, and gas is non-renewable.

A full answer includes

• (a) I explained that wind and sunlight are not reliable.
• (a) I wrote that on calm evenings the renewables gave less power than the demand.
• (a) I did not write that the town needs no backup because the week’s total was enough.
• (b) I suggested that a nuclear power station gives far more power than the town’s 14 MW peak: its scale is too big.
• (b) Also accepted: I suggested that it cannot be switched on and off quickly to fill a few short hours.
• (c) I suggested that there are no hot rocks near the surface here: geothermal is not available.
• (d) Advantage: I described that it can be switched on whenever it is needed.
• (d) Disadvantage: I described that burning gas releases carbon dioxide, or that gas is non-renewable.

Also accept

• (a) renewables are not reliable: short on calm evenings (b) scale: far too big; or it cannot be switched on and off quickly (c) availability: no hot rocks (d) can be switched on any time; but releases carbon dioxide, non-renewable

Watch for

• “The renewables made more energy than the town needed, so it needs no backup.” — Over the week, yes, but not at every hour: there were 27 hours when the power was less than the demand.
• “More solar cells would fill the gap.” — Most of the short hours were in the evenings and at night, when solar cells give nothing, however many there are.

7. RQ18 · Practice

Model answer. Energy is never destroyed. Only 60 J of every 200 J is transferred usefully, by the current. The other 140 J is wasted: it heats the engine, the generator and the air around them (some leaves as sound, which also ends up heating the air). It spreads out into the surroundings, where it is no longer useful. It cannot go back into the biogas: once the gas has burnt, that chemical store has been emptied.

A full answer includes

• I chose: it heats the engine, the generator and the air.
• I did not choose “destroyed”: energy cannot be destroyed.

Also accept

• it heats the engine, the generator and the air

Watch for

• “It is destroyed inside the engine as it runs.” — Energy cannot be destroyed. The 140 J is transferred to the surroundings by heating them.
• “It is stored in the generator for use later.” — A generator stores no energy. The 140 J spreads into the engine, the generator and the air, and cannot be collected again.
• “It goes back into the biogas’s chemical store.” — The biogas has burnt: its chemical store has been emptied. The energy has gone into heating the surroundings.

8. RQ19 · Practice

Model answer. Efficiency compares the useful energy output of a device with the total energy put in. The useful output can never be more than the input, because energy cannot be created, so no device is more than 100 % efficient; in practice some energy is always wasted, usually by heating the device and its surroundings. Whether an output is useful depends on the job: heating is useful for a kettle but wasted for a lamp. No device destroys energy, whatever its efficiency; a highly efficient one wastes less.

A full answer includes

• I selected: efficiency compares the useful energy output with the total input.
• I selected: no device can be more than 100 % efficient.
• I selected: in most devices the wasted energy ends up heating the surroundings.
• I selected: whether an output is useful depends on what the device is for.
• I did not select that an efficient device destroys less energy.
• I did not select that a high efficiency means more energy wasted.

Also accept

• useful output compared with total input; never more than 100 %; wasted energy heats the surroundings; useful depends on the job

Watch for

• “An efficient device destroys less energy than an inefficient one.” — No device destroys energy. An efficient device wastes less of it, heating the surroundings.
• “A device with a high efficiency wastes more of its energy.” — It is the other way round: a high efficiency means more of the input is useful, so less is wasted.
• “A very good machine could be more than 100 % efficient.” — That would mean more energy coming out than going in, which would create energy. It is impossible.
• “Heating is always wasted energy.” — For an electric heater or a kettle, heating is the useful output. Whether energy is useful depends on the job.

9. RQ20 · Practice

Model answer. (a) The newer power station: more of the energy put in comes out as useful electrical energy. (b) Each joule of gas gives more electrical energy in the newer station, so it burns less gas to give the town the same electrical energy, and releases less carbon dioxide too. (c) In every power station some of the energy is transferred by heating instead of usefully: the used steam must be cooled, which heats the cooling water, the river and the air, and moving parts are warmed by friction. Energy is not destroyed, so what is wasted is missing from the useful output, which is always less than the energy put in.

A full answer includes

• (a) I stated: the newer power station.
• (b) I explained that it turns more of each joule of gas into electrical energy.
• (b) I explained that it therefore needs less gas for the same electrical energy.
• (b) I did not write that both make the same electrical energy because energy is conserved.
• (c) I explained that some energy is always wasted, heating the cooling water and the air.
• (c) I explained that energy is not destroyed, so the useful output is always less than the input.

Also accept

• (a) the newer one (b) more electrical energy from each joule of gas, so less gas (c) energy is always wasted heating the surroundings

Watch for

• “Both power stations make the same electrical energy from the same gas, because energy is conserved.” — The total energy is conserved, but the newer station turns more of it into electrical energy and wastes less.

10. RQ21 · Practice

Model answer. The useful energy output is the energy gained by the water, 700 kJ; the total energy input is the energy released by the burning gas, 2000 kJ. Efficiency = useful energy output ÷ total energy input × 100 % = 700 ÷ 2000 × 100 % = 35 %. The units cancel, so efficiency is a number or a percentage. The other 1300 kJ heated the pan, the stove and the air around them.

Checked automatically

• Efficiency: 35 % (within ±0.5 %)
  • Common wrong answer 0.35 : That is the efficiency as a fraction. Multiply by 100 % to give a percentage.
  • Common wrong answer 286 : You divided the input by the output. Divide the useful output by the total input; the answer can never be more than 100 %.
  • Common wrong answer 65 : That is the percentage wasted, heating the pan, the stove and the air.

A full answer includes

• I wrote efficiency = useful energy output ÷ total energy input × 100 %.
• I found 700 kJ ÷ 2000 kJ × 100 % = 35 %.
• I did not divide the input by the output: the answer can never be more than 100 %.

Also accept

• 35 %

Watch for

• “Efficiency = input ÷ output.” — That gives 286 %, more than 100 %, which is impossible. Divide the useful output by the total input.

11. RQ22 · Practice

Model answer. The useful power output is the power given to the blades, 320 W, and the total power input is the electrical power, 500 W. Efficiency = useful power output ÷ total power input × 100 % = 320 ÷ 500 × 100 % = 64 %. The other 180 W heats the motor and the air, and some leaves as sound.

Checked automatically

• Efficiency: 64 % (within ±0.5 %)
  • Common wrong answer 0.64 : That is the efficiency as a fraction. Multiply by 100 % to give a percentage.
  • Common wrong answer 156 : You divided the input by the output. Divide the useful output, 320 W, by the input, 500 W.
  • Common wrong answer 36 : That is the percentage wasted. The useful output is the 320 W given to the blades.

A full answer includes

• I wrote efficiency = useful power output ÷ total power input × 100 %.
• I found 320 W ÷ 500 W × 100 % = 64 %.
• I did not give the 36 % that is wasted.

Also accept

• 64 %

Watch for

• “Efficiency can only be worked out from energies, not powers.” — Power is energy per second, so the same equation works with powers: useful power output ÷ total power input × 100 %.

12. RQ23 · Practice

Model answer. Efficiency = useful energy output ÷ total energy input, so useful energy output = efficiency × total energy input = 0.90 × 400 kJ = 360 kJ every second. The other 40 kJ heats the turbines, the generators and the water. The useful output must be less than the input, which is a quick check.

Checked automatically

• Useful electrical energy output each second: 360 kJ (within ±1%)
  • Common wrong answer 36000 : Write 90 % as 0.90 before multiplying.
  • Common wrong answer 444 : You divided by 0.90. The useful output is less than the input, so multiply.
  • Common wrong answer 40 : That is the energy wasted each second.

A full answer includes

• I rearranged: useful energy output = efficiency × total energy input.
• I found 0.90 × 400 kJ = 360 kJ.
• I did not divide by 0.90: the output must be less than the input.

Also accept

• 360 kJ

Watch for

• “Useful energy output = total energy input ÷ efficiency.” — That gives 444 kJ, more than the 400 kJ put in, which is impossible. Multiply the input by the efficiency.

13. RQ24 · Practice

Model answer. Efficiency = useful power output ÷ total power input, so total power input = useful power output ÷ efficiency = 300 W ÷ 0.20 = 1500 W. Check: 300 W is 20 % of 1500 W, and the input is bigger than the useful output, as it must be.

Checked automatically

• Power of the light falling on the panel: 1500 W (within ±1%)
  • Common wrong answer 60 : You multiplied by 0.20. The input must be bigger than the useful output, so divide.
  • Common wrong answer 6000 : Divide by 0.20; do not multiply by 20.
  • Common wrong answer 15 : Write 20 % as 0.20: 300 W ÷ 0.20 = 1500 W.

A full answer includes

• I rearranged: total power input = useful power output ÷ efficiency.
• I found 300 W ÷ 0.20 = 1500 W.
• I did not multiply by 0.20: the input must be bigger than the output.

Also accept

• 1500 W

Watch for

• “Total power input = efficiency × useful power output.” — That gives 60 W, less than the 300 W coming out, which is impossible. Divide by the efficiency.

14. RQ25 · Practice

Model answer. (a) Energy is conserved in the boiler: 1000 kJ goes in and 880 kJ goes on to the steam, so 1000 kJ − 880 kJ = 120 kJ is wasted, heating the boiler, the chimney gases and the air. (b) The turbine stage wastes 880 kJ − 400 kJ = 480 kJ, most of it carried away when the used steam is cooled, and the generator wastes 400 kJ − 380 kJ = 20 kJ. The total wasted is 120 + 480 + 20 = 620 kJ. Quicker: of the 1000 kJ put in, only 380 kJ leaves as electrical energy, so 1000 kJ − 380 kJ = 620 kJ is wasted. (c) For the whole station the useful output is the 380 kJ of electrical energy and the total input is the 1000 kJ in the pellets: efficiency = 380 kJ ÷ 1000 kJ × 100 % = 38 %.

Checked automatically

• (a) Energy wasted in the boiler: 120 kJ (exact value)
  • Common wrong answer 880 : That is the energy passed on to the steam. Subtract it from the 1000 kJ put in.
• (b) Total energy wasted: 620 kJ (exact value)
  • Common wrong answer 20 : That is only the generator’s waste. Every stage wastes some energy.
  • Common wrong answer 480 : That is only the turbine stage’s waste. Add the boiler’s and the generator’s too.
  • Common wrong answer 380 : That is the useful electrical energy. Subtract it from the 1000 kJ put in.
• (c) Efficiency of the whole power station: 38 % (within ±0.5 %)
  • Common wrong answer 95 : That is the generator alone: 380 ÷ 400. Use the 1000 kJ put in at the start.
  • Common wrong answer 43 : You divided by 880 kJ. Use the 1000 kJ put in at the start.
  • Common wrong answer 40 : That is the share passed on by the turbine. The useful output of the whole station is the 380 kJ that reaches the cables.
  • Common wrong answer 0.38 : That is the fraction. Multiply by 100 % to give a percentage.

A full answer includes

• (a) I found 1000 kJ − 880 kJ = 120 kJ.
• (b) I found 1000 kJ − 380 kJ = 620 kJ (or 120 kJ + 480 kJ + 20 kJ).
• (b) I did not give the waste of one stage only.
• (c) I wrote efficiency = useful energy output ÷ total energy input × 100 %.
• (c) I found 380 kJ ÷ 1000 kJ × 100 % = 38 %.
• (c) I did not divide by 400 kJ, the input to the generator alone.

Also accept

• (a) 120 kJ (b) 620 kJ (c) 38 %

Watch for

• “The energy wasted in the boiler is the 880 kJ passed to the steam.” — The 880 kJ is passed on usefully. The wasted energy is what does not go on: 1000 kJ − 880 kJ.
• “The total wasted is what the last stage wastes.” — Every stage wastes some energy. Add the energy wasted in the boiler, the turbine stage and the generator, or take the 380 kJ output from the 1000 kJ input.
• “The efficiency of the whole station is the generator’s efficiency.” — The generator alone is 95 % efficient. For the whole station, divide the final useful output, 380 kJ, by the energy put in at the start, 1000 kJ.

15. RT2 · Apply

Model answer. (a) The tides rise and fall twice a day at times that can be worked out years ahead, so the islanders know when the barrage will generate. The wind can drop at any time, often without much warning. (b) The barrage changes how high and how often the water covers the bay, so mudflats where birds feed and fish breed are lost or changed; it can also block boats. (c) Cooking oil is made from crops such as sunflowers, which can be grown again each year, so it is renewable. But burning it in the generator still releases carbon dioxide, and the farming, collecting and shipping of the oil use energy, often from fossil fuels: it is not emission-free. (d) Neither the barrage nor the wind turbines can generate at every hour: the barrage only as the tide goes out, the turbines only when it is windy. A generator can be switched on whenever there is a gap, so it is a reliable backup.

A full answer includes

• (a) I suggested that the tide times are known years ahead, so the barrage’s output is predictable.
• (a) I suggested that the wind can drop at any time.
• (b) I described that it changes the water levels in the bay, covering or changing the mudflats.
• (b) I described harm to the birds or other wildlife that feed there.
• (c) I explained that the oil comes from crops that can be grown again, so it is renewable.
• (c) I explained that burning it still releases carbon dioxide.
• (c) I did not write that a renewable fuel releases no carbon dioxide.
• (d) I explained that at some hours neither the barrage nor the wind gives enough power.
• (d) I explained that a generator can be switched on whenever it is needed.

Also accept

• (a) tides are predictable, wind is not (b) harms the wildlife of the bay’s mudflats (c) crops regrow, but burning releases carbon dioxide (d) neither gives power at every hour; the generator can be switched on any time

Watch for

• “Tidal power is unreliable, like the wind.” — The barrage does not generate all the time, but the times are known years ahead, so it is predictable.
• “A biofuel is renewable, so it releases no carbon dioxide.” — Renewable and emission-free are different things: burning any fuel releases carbon dioxide.

16. RT3 · Apply

Model answer. (a) Energy is conserved, so 100 kJ − 25 kJ = 75 kJ heats the engine, the exhaust gases and the air. (b) Efficiency = useful energy output ÷ total energy input × 100 % = 25 kJ ÷ 100 kJ × 100 % = 25 %. (c) The burning petrol heats the engine and the exhaust gases, and friction between the moving parts heats them too, so some energy is always transferred by heating instead of usefully. Energy cannot be created, and some of it is always wasted, so the useful output is always less than the input and the efficiency is less than 100 %.

Checked automatically

• (a) Energy wasted: 75 kJ (exact value)
  • Common wrong answer 125 : Subtract the useful 25 kJ from the 100 kJ put in.
  • Common wrong answer 25 : That is the useful energy, in the kinetic store of the car.
• (b) Supplement: Efficiency: 25 % (within ±0.5 %)
  • Common wrong answer 0.25 : That is the fraction. Multiply by 100 % to give a percentage.
  • Common wrong answer 75 : That is the percentage wasted. Use the useful output, 25 kJ.
  • Common wrong answer 400 : You divided the input by the output. Divide the useful output by the total input.

A full answer includes

• (a) I found 100 kJ − 25 kJ = 75 kJ.
• (b) Supplement: I wrote efficiency = useful energy output ÷ total energy input × 100 %.
• (b) I found 25 kJ ÷ 100 kJ × 100 % = 25 %.
• (b) I did not give the 75 % that is wasted.
• (c) I explained that the burning fuel and the friction between moving parts always heat the engine and its surroundings.
• (c) I explained that the useful output is therefore always less than the energy put in.

Also accept

• (a) 75 kJ (b) 25 % (c) heating by the burning fuel and friction always wastes some

Watch for

• “The wasted energy is used up.” — Energy is never used up. The wasted energy heats the engine, the exhaust gases and the air.
• “A better engine could be more than 100 % efficient.” — Its useful output can never be more than the energy put in; some is always wasted, so it is always less than 100 %.