1.6 Efficiency
9 exam questions worth 18 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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A kettle takes in 1000 J of electrical energy and transfers 900 J to the water. Calculate its efficiency.
Mark scheme
efficiency = 900 / 1000 (1); 0.90 / 90% (1)
Model answer
efficiency = useful output ÷ total input = 900 ÷ 1000 = 0.90 (90%).
A motor has a useful power output of 36 W and a total power input of 48 W. Calculate its efficiency as a percentage.
Mark scheme
36 / 48 (1); 75 % (1)
Model answer
36 ÷ 48 = 0.75, which is 75%.
Which value could be the efficiency of a real device?
Choose one answer.
Correct Not quite: the answer is 0.85
Mark scheme
C: 0.85
Model answer
0.85 (C). Efficiency is always less than 1 for a real device.
A machine is 40% efficient. It takes in 250 J of energy. Calculate the useful energy output.
Mark scheme
0.40 × 250 (1); 100 J (1)
Model answer
useful output = 0.40 × 250 = 100 J.
A filament lamp has an efficiency of 0.10. State what happens to the other 90% of the energy supplied.
Mark scheme
transferred / wasted as thermal energy / heat to the surroundings
Model answer
It is wasted as thermal energy, which is dissipated to the surroundings.
Describe two ways to increase the efficiency of a machine with moving parts.
Mark scheme
lubricate / oil the moving parts (1); reduce friction / use thermal insulation / streamline to reduce air resistance (1)
Model answer
Lubricate the moving parts to reduce friction, and use streamlined shapes to reduce air resistance.
A coal-fired power station has a power input of 1200 MW and delivers 420 MW of electrical power. Calculate the efficiency of the power station, as a percentage, and the power that is wasted.
Mark scheme
efficiency = 420 ÷ 1200 (1); 35% (1); wasted power = 1200 − 420 = 780 MW (1)
Model answer
Efficiency = 420 / 1200 = 0.35 (35%). Wasted power = 1200 − 420 = 780 MW.
An LED lamp has an input power of 8.0 W and an efficiency of 25%. A filament lamp, with an efficiency of 10%, gives the same useful power output. Calculate the input power of the filament lamp.
Mark scheme
useful power of the LED = 0.25 × 8.0 = 2.0 W (1); filament lamp: input = useful ÷ efficiency = 2.0 ÷ 0.10 (1); 20 W (1)
Model answer
Useful power = 0.25 × 8.0 = 2.0 W. The filament lamp needs input power = 2.0 / 0.10 = 20 W.
Explain why no real device can be 100% efficient.
Mark scheme
some energy is always transferred to the thermal store of the surroundings (e.g. by friction / resistance) (1); this wasted energy is dissipated so cannot be recovered (1)
Model answer
Some of the energy is always transferred to the thermal store of the surroundings by friction or the resistance of wires. This energy is dissipated, so the useful output is always less than the input.
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