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PhysicsGCSEAQA & EdexcelYears 10–11

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.

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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.

  1. 1Write the equation: efficiency = useful ÷ total.
  2. 2Substitute: efficiency = 350 ÷ 500.
  3. 3Divide: efficiency = 0.7.
  4. 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.

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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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Related topics

EnergyEnergy Stores & Transfers

Key terms explained

Short, exam-ready definitions with a formula, a worked example and an examiner tip.

What is acceleration?What is conservation of energy?What is electric current?What is frequency?What is kinetic energy?What is moment of a force?Full glossary

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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