Energy resources — final check key

1. RK1 · Final check

Model answer. (a) The energy in the coal’s chemical store is released when it burns in the boiler. It heats water until it boils into high-pressure steam. The steam pushes the blades of the turbine round, and the turbine turns the generator, which generates electricity. (b) Solar cells turn light from the Sun straight into electricity. Solar panels absorb infrared and other radiation from the Sun to heat water; they generate no electricity. (c) Water stored high up behind the dam has energy in its gravitational potential store. When it flows down through pipes it speeds up and turns the turbines, which turn the generators.

A full answer includes

• (a) I wrote that the coal burns in the boiler and boils water into steam.
• (a) I wrote that the steam turns the turbine.
• (a) I wrote that the turbine turns the generator, which generates electricity.
• (a) I did not write that the burning coal turns the generator directly.
• (b) I wrote that solar cells turn light into electricity.
• (b) I wrote that solar panels heat water and make no electricity.
• (c) I wrote that water held high up behind the dam flows down.
• (c) I wrote that the moving water turns turbines, which turn generators.

Also accept

• (a) burn coal in boiler, steam turns turbine, turbine turns generator (b) cells make electricity from light; panels heat water (c) water from behind the dam turns turbines and generators

Watch for

• “In a coal-fired power station the burning coal turns the generator.” — The burning coal boils water. The steam turns the turbine, and the turbine turns the generator.
• “Solar panels and solar cells both make electricity.” — Only solar cells make electricity. Solar panels use the Sun’s radiation to heat water.

2. RK2 · Final check

Model answer. (a) Nuclear fuel: uranium holds energy in the nuclei of its atoms, which were in the Earth when it formed. Coal formed from plants that grew in sunlight; the wind comes from the Sun heating the air unevenly, and the wind makes the waves. (b) In the Sun, energy is released by nuclear fusion: light hydrogen nuclei are squeezed together and join to make helium nuclei. A nuclear power station today does the opposite: large uranium nuclei split apart. (c) Fusion is still being researched. Experimental fusion reactors are being built to find out how energy released by fusion could be used to produce electrical energy on a large scale, but none of them supplies electricity to homes yet.

A full answer includes

• (a) I identified nuclear fuel.
• (a) I did not choose coal: its energy came from plants that grew in sunlight.
• (b) I stated that in the Sun light (hydrogen) nuclei join together.
• (b) I stated that in a nuclear power station large (uranium) nuclei split apart.
• (c) I explained that fusion is still being researched.
• (c) I explained that no fusion power station supplies electricity to homes yet.

Also accept

• (a) nuclear fuel (b) the Sun joins light nuclei (fusion); a nuclear power station splits large nuclei (c) fusion is still being researched; no fusion power station supplies electricity yet

Watch for

• “Wind does not come from the Sun.” — The Sun heats the ground and air unevenly, and this makes the winds: wind energy comes from the Sun’s radiation.
• “The Sun releases its energy by splitting nuclei, like a nuclear power station.” — In the Sun light nuclei join together: that is fusion. Splitting large nuclei is what today’s nuclear power stations do.

3. RK3 · Final check

Model answer. (a) A renewable resource will not run out: nature replaces it as fast as we use it, or it is not used up at all. (b) Renewable: for example wind, sunlight, tides, waves, water behind a dam, geothermal energy, biofuels. Non-renewable: coal, oil, natural gas and nuclear fuel. (c) Wind is renewable and wind turbines release no carbon dioxide while they generate. But they give nothing on calm days, so they are not reliable, and they change how the hills look, make some noise and can harm birds. (d) One nuclear power station gives a very large power from a small amount of fuel, and releases no carbon dioxide while it generates. But uranium is non-renewable, and the radioactive waste has to be stored safely for a very long time.

A full answer includes

• (a) I stated that a renewable resource is replaced by nature as fast as we use it, or is not used up.
• (b) I gave two renewable resources, such as wind and sunlight.
• (b) I gave two non-renewable resources, such as coal and natural gas.
• (b) I did not give nuclear fuel as renewable.
• (c) Advantage: I described that wind is renewable, or that the turbines release no carbon dioxide while they generate.
• (c) Disadvantage: I described that they give nothing on calm days, or that they change the landscape or harm birds.
• (d) Advantage: I described a very large power from a small amount of fuel, or no carbon dioxide while it generates.
• (d) Disadvantage: I described that uranium is non-renewable, or that the radioactive waste must be stored safely for a very long time.

Also accept

• (a) will not run out (b) e.g. wind, solar / coal, gas (c) renewable, no carbon dioxide / unreliable, landscape or birds (d) huge scale, no carbon dioxide / non-renewable, radioactive waste

Watch for

• “Nuclear fuel is renewable, because so little is needed.” — Uranium is not replaced once it is used, so it is non-renewable.

4. RK4 · Final check

Model answer. (a) Efficiency describes how much of the total energy put into a device is transferred as useful energy; the rest is wasted. (b) Kettle X is more efficient. Both give the same useful output: the same water gains the same energy as it is heated to boiling. Kettle Y also heats its case and the air around it and makes sound, so more of its energy is wasted, and it must take in more electrical energy for the same useful output. (c) For example, insulate kettle Y’s case, or fit a lid that closes well, so that less energy is transferred from the hot water to the case and the air. Less energy is wasted, so less is needed for the same useful output, and the efficiency goes up.

A full answer includes

• (a) I stated that efficiency is how much of the total energy put in is transferred as useful energy.
• (b) I stated: kettle X.
• (b) I explained that both kettles give the water the same useful energy.
• (b) I explained that kettle Y wastes more energy heating its case and the air, and as sound, so it needs more energy put in.
• (b) I did not choose kettle Y because it takes in more energy.
• (c) I suggested a change such as insulating the case or fitting a lid that closes well.
• (c) I explained that less energy is then wasted, so less is put in for the same useful output.

Also accept

• (a) useful energy output compared with total energy input (b) X: same useful output, Y wastes more as heating and sound (c) insulate the case: less wasted

Watch for

• “Kettle Y is more efficient because it takes in more energy.” — Efficiency compares the useful output with the input. Y gives the same useful output from more input, so it is less efficient.
• “A hot kettle case is useful, because a kettle is for heating.” — The kettle is for heating the water. Energy that heats the case and the air is wasted.

5. RK5 · Final check

Model answer. (a) 1.6 kW = 1600 W. Efficiency = useful power output ÷ total power input × 100 % = 1200 W ÷ 1600 W × 100 % = 75 %. (b) Efficiency = useful power output ÷ total power input, so total power input = useful power output ÷ efficiency = 72 kW ÷ 0.30 = 240 kW. Check: 30 % of 240 kW is 72 kW. (c) The motor wastes 50 kJ − 40 kJ = 10 kJ and the gearbox 40 kJ − 34 kJ = 6 kJ, so 16 kJ in all; or, more quickly, 50 kJ put in − 34 kJ useful = 16 kJ wasted.

Checked automatically

• (a) Efficiency of the motor: 75 % (within ±0.5 %)
  • Common wrong answer 750 : Change 1.6 kW to 1600 W first: both powers must be in the same unit.
  • Common wrong answer 0.75 : That is the fraction. Multiply by 100 % to give a percentage.
  • Common wrong answer 133 : You divided the input by the output. Divide 1200 W by 1600 W.
  • Common wrong answer 25 : That is the percentage wasted.
• (b) Total power input: 240 kW (within ±1%)
  • Common wrong answer 21.6 : You multiplied by 0.30. The input must be bigger than the output, so divide.
  • Common wrong answer 2.4 : Write 30 % as 0.30, not 30.
  • Common wrong answer 2160 : Divide by 0.30; do not multiply by 30.
• (c) Total energy wasted: 16 kJ (exact value)
  • Common wrong answer 10 : That is only the motor’s waste. The gearbox wastes some too.
  • Common wrong answer 6 : That is only the gearbox’s waste. The motor wastes some too.
  • Common wrong answer 34 : That is the useful energy that lifts the load. Subtract it from the 50 kJ put in.

A full answer includes

• (a) I changed 1.6 kW to 1600 W.
• (a) I found 1200 W ÷ 1600 W × 100 % = 75 %.
• (b) I rearranged: total power input = useful power output ÷ efficiency.
• (b) I found 72 kW ÷ 0.30 = 240 kW.
• (c) I found 50 kJ − 34 kJ = 16 kJ (or 10 kJ + 6 kJ).
• (c) I did not give the waste of one stage only.

Also accept

• (a) 75 % (b) 240 kW (c) 16 kJ

Watch for

• “Total power input = efficiency × useful power output.” — That gives 21.6 kW, less than the 72 kW coming out, which is impossible. Divide by the efficiency.
• “You can divide powers in different units, such as W by kW.” — Change both to the same unit first: 1.6 kW = 1600 W.
• “The energy wasted is what one stage wastes.” — Each stage wastes some energy. The total wasted is the 50 kJ put in minus the 34 kJ that lifts the load.