Energy resources — return questions key

1. RR1 · Return

Model answer. (a) Wood pellets are a biofuel: a fuel made from living things that grew recently, unlike fossil fuels, which formed millions of years ago. (b) It works in the same way as a coal-fired power station. The pellets burn in the boiler, and the energy released from their chemical store boils water into high-pressure steam; the steam turns the turbine, and the turbine turns the generator. Only the fuel is different: the wood grew in the last few years, while coal formed from plants that died millions of years ago. (c) Energy is released in the Sun by nuclear fusion and reaches the Earth as radiation. The trees absorb the light in their leaves and use it to make the substances they grow from (photosynthesis), so the energy is stored in the chemical store of the wood until it is burnt.

A full answer includes

• (a) I wrote: a biofuel.
• (b) I wrote that it generates electricity in the same way as a coal-fired power station.
• (b) I wrote that the pellets burn in a boiler, and the steam turns a turbine and generator.
• (b) I wrote that only the fuel is different: it grew recently, not millions of years ago.
• (b) I did not write that a biofuel power station needs no boiler.
• (c) Supplement: I started with nuclear fusion in the Sun.
• (c) I wrote that radiation from the Sun reached the trees, which used it to grow.
• (c) I wrote that the energy was stored in the chemical store of the wood.

Also accept

• (a) biofuel (b) no difference in the way it works: boiler, turbine, generator; only the fuel differs (c) fusion in the Sun, radiation, trees grow, chemical store of the wood

Watch for

• “Wood pellets are a fossil fuel.” — Fossil fuels formed from living things that died millions of years ago. Wood pellets come from trees grown in the last few years: they are a biofuel.
• “A biofuel power station needs no boiler, because biofuel is renewable.” — Being renewable says where the fuel comes from, not how it is used. The pellets are burnt to boil water, like coal.

2. RR2 · Return

Model answer. (a) Sunlight will not run out, so it is renewable, and the solar cells release no carbon dioxide or other gases while they generate, unlike the coal-fired power station. (b) The cells generate only when light falls on them: nothing at night and little on cloudy days, so they are not reliable. (c) A hospital’s machines keep patients alive and must never stop. At night, on dull days or if the supply from the power station fails, the hospital needs a source it can switch on whenever it is needed, such as the diesel generator. (d) The scale is too small: the roof only has room for cells giving 50 kW at most, an eighth of the 400 kW the hospital can need, so the cells can supply only part of its electricity.

A full answer includes

• (a) I described that sunlight is renewable, or that the cells release no carbon dioxide while they generate.
• (b) I described that they give no power at night, and little on cloudy days.
• (b) I did not write that they are unreliable because sunlight will run out.
• (c) I explained that the hospital’s machines must never stop.
• (c) I explained that a backup can be switched on whenever it is needed, such as at night or if the supply fails.
• (d) I described that 50 kW is much less than the 400 kW needed.
• (d) I used the word scale: the cells can supply only part of what the hospital needs.

Also accept

• (a) renewable, no carbon dioxide while generating (b) nothing at night, little when cloudy (c) power must never stop; a backup can be switched on any time (d) scale: 50 kW is much less than 400 kW

Watch for

• “Solar cells are unreliable because sunlight will run out.” — Sunlight will not run out (that is renewability). They are unreliable because it is not there at every hour.
• “With enough solar cells the hospital needs no backup.” — However many cells there are, they give nothing at night.

3. RR3 · Return

Model answer. (a) The useful output is the energy transferred to the phone’s battery, where it is kept in the battery’s chemical store. (b) Some of the electrical energy put in is wasted: it heats the charger, and the charger heats the air around it. So the useful output is less than the energy put in, and the charger is less than 100 % efficient. (c) Energy cannot be destroyed. The warmth shows where the wasted energy has gone: into the internal store of the charger and the air, spread out where it is no longer useful. (d) The new charger gives the same useful output while wasting less energy as heating, so less energy is put in for the same result: it is more efficient.

A full answer includes

• (a) I stated: the energy transferred to the phone’s battery (its chemical store).
• (b) I explained that some of the energy put in is wasted, heating the charger.
• (b) I explained that the charger is therefore less than 100 % efficient.
• (c) I explained that energy cannot be destroyed.
• (c) I explained that the wasted energy heats the charger and the air, where it is no longer useful.
• (c) I did not agree that the warmth shows energy being destroyed or made.
• (d) I suggested that the new charger is more efficient: it wastes less energy as heating.

Also accept

• (a) energy to the battery (b) some energy is wasted heating it: less than 100 % efficient (c) not destroyed: it heats the surroundings (d) more efficient, wastes less

Watch for

• “A warm charger is making extra energy.” — Energy cannot be created. The warmth is part of the energy put in, wasted as heating.

4. RR4 · Return

Model answer. (a) The generator takes in 470 kJ and gives out 440 kJ, so its efficiency = useful energy output ÷ total energy input × 100 % = 440 kJ ÷ 470 kJ × 100 % = 93.6 %, about 94 %. The 500 kJ is the input to the whole turbine, not to the generator. (b) The two powers must be in the same unit: 12 kW = 12 000 W. Efficiency = useful power output ÷ total power input × 100 % = 7800 W ÷ 12 000 W × 100 % = 65 %, to 2 significant figures. The other 4200 W heats the motor and the air, and some leaves as sound.

Checked automatically

• (a) Efficiency of the generator: 93.6 % (within ±0.5 %)
  • Common wrong answer 88 : You divided by 500 kJ, the energy given to the blades. The generator’s input is 470 kJ.
  • Common wrong answer 107 : You divided the input by the output. Divide 440 kJ by 470 kJ.
  • Common wrong answer 0.936 : That is the fraction. Multiply by 100 % to give a percentage.
• (b) Efficiency of the motor: 65 % (2 significant figures)
  • Common wrong answer 65000 : Change 12 kW to 12 000 W first: both powers must be in the same unit.
  • Common wrong answer 650 : Both powers must be in the same unit: 12 kW = 12 000 W.
  • Common wrong answer 0.65 : That is the fraction. Multiply by 100 % to give a percentage.
  • Common wrong answer 154 : You divided the input by the output. Divide 7800 W by 12 000 W.
  • Common wrong answer 35 : That is the percentage wasted.

A full answer includes

• (a) I used the generator’s own input, 470 kJ, and its output, 440 kJ.
• (a) I found 440 kJ ÷ 470 kJ × 100 % = 94 % (93.6 %).
• (a) I did not divide by 500 kJ, the energy given to the blades.
• (b) I changed 12 kW to 12 000 W (or 7800 W to 7.8 kW).
• (b) I found 7800 W ÷ 12 000 W × 100 % = 65 %, to 2 significant figures.
• (b) I did not divide 7800 by 12 without changing the units.

Also accept

• (a) 94 % (b) 65 %

Watch for

• “Efficiency = total input ÷ useful output.” — That gives more than 100 %, which is impossible. Divide the useful output by the total input.
• “The efficiency of one stage uses the energy put into the whole machine.” — For the generator alone, use the energy that goes into the generator, 470 kJ.
• “You can divide powers in different units, such as W by kW.” — Change both to the same unit first: 12 kW = 12 000 W. Otherwise the answer is 1000 times too big.