An electric motor is supplied with 800 J of energy. 200 J of this energy is wasted, dissipated as thermal energy. Calculate the efficiency of the motor as a percentage. Give your answer to 2 significant figures.
[3 marks]
Total for this question: 3
All equation deep-dives
Efficiency
Energy
AQA 8463
On the equation sheet
ORIGINAL
efficiency = useful output energy transfer / total input energy transfer
The top line is the USEFUL output only, and an efficiency can never be more than 1 (100%).
Current GCSE 8463 policy: The full AQA GCSE Physics 8463 equation sheet is provided with both papers in 2026; use fluency practice to learn selection and rearrangement, not to memorise the list.
Board and spec code confirmed against AQA 8463 · registry checked 2026-07-17How this checking works
Know the equation
| Symbol | Quantity | Unit |
|---|---|---|
| efficiency | fraction of the input transferred usefully | no unit (decimal or %) |
| useful output | useful output energy transfer (or useful power output) | J (or W) |
| total input | total input energy transfer (or total power input) | J (or W) |
Rearrangements
- efficiency = useful power output / total power input
- useful output = efficiency x total input
- total input = useful output / efficiency
Apply it — mark your own working
Work each one out on paper first, then reveal the mark scheme and tick the marks you actually earned. That is exactly how you should mark past papers.
Q1
Do the calculation on paper first — then mark it.
Q2
An electric motor has an efficiency of 0.40. Its useful power output is 120 W. Calculate the total power input to the motor. Give your answer to 2 significant figures.
[3 marks]
Total for this question: 3
Do the calculation on paper first — then mark it.
Where the marks get lost
- Putting the wasted energy (200 J) on the top line instead of the useful output energy (600 J).
- Dividing input by output, which gives an 'efficiency' bigger than 1 (more than 100%) — impossible.
- Multiplying a decimal efficiency by 100 when a decimal was asked for, or substituting 40 instead of 0.40.
Exam tip: Write the efficiency as a decimal while you calculate, and convert to a percentage only at the end if the question asks for one. Sanity-check: useful output is always smaller than total input.
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Everything here stays free. If efficiency is where the marks keep going, I teach it one to one from the mark schemes. Free intro call, then a free first lesson.