GCSE Physics Efficiency Notes
Efficiency tells you what fraction of the energy supplied is transferred usefully. It is always less than 100% because some energy is always wasted.
Key concepts
- Efficiency
- The proportion of the total energy input that is transferred usefully, written as a decimal or percentage.
- Wasted energy
- Energy transferred to a non-useful store — usually the thermal store of the surroundings via friction or resistance.
- Reducing waste
- Lubrication reduces friction; insulation reduces heat loss; streamlining reduces air resistance — all raise efficiency.
Key formulas
Efficiency (energy)
efficiency = useful energy out ÷ total energy in
×100 for a percentage.
Efficiency (power)
efficiency = useful power out ÷ total power in
same idea using watts.
Worked example
A motor is supplied with 500 J and transfers 350 J usefully. Find its efficiency as a percentage.
- 1Write the equation: efficiency = useful ÷ total.
- 2Substitute: efficiency = 350 ÷ 500.
- 3Divide: efficiency = 0.7.
- 4Convert to a percentage: 0.7 × 100 = 70%.
Answer: Efficiency = 70%
Common mistakes
✗ Dividing total by useful instead of useful by total.
✓ Useful energy is always on top — efficiency can never exceed 100%.
✗ Forgetting to multiply by 100 for a percentage.
✓ A decimal answer like 0.7 must become 70% when the question asks for a percentage.
✗ Thinking a machine can be 100% efficient.
✓ Some energy is always dissipated, so real efficiency is always below 100%.
Quick check
Try these, then reveal the answers.
A device wastes 200 J of a 1000 J input. What is its efficiency?Reveal answer
Useful = 800 J, so efficiency = 800 ÷ 1000 = 80%.
Name one way to increase the efficiency of a moving machine.Reveal answer
Lubricate the moving parts to reduce friction.
Can efficiency be greater than 100%?Reveal answer
No — useful output can never exceed total input.
Exam tips
- Check whether the question gives you 'useful' or 'wasted' energy — subtract if you're given the wasted amount.
- Read whether the answer should be a decimal or a percentage.
- Explain improvements in terms of reducing a specific unwanted transfer (friction, heating, air resistance).
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Frequently asked questions
Why is no machine 100% efficient?+
Some energy is always dissipated to the surroundings, usually as heat from friction or electrical resistance, so useful output is always less than total input.
How do you reduce wasted energy?+
Reduce the unwanted transfer directly — lubricate to cut friction, insulate to cut heat loss, and streamline to cut air resistance.
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