GCSE Combined Science revision · Physics: energy, electricity, particles and atoms

Energy transfers and efficiency

The idea

Energy cannot be created or destroyed, only transferred usefully, stored or dissipated. Wasted energy is usually dissipated as heat to the surroundings. Efficiency = useful output energy ÷ total input energy (or useful power out ÷ total power in). Unwanted energy transfers can be reduced by lubrication and thermal insulation; materials with lower thermal conductivity transfer heat more slowly.

Find the useful output and the total input.
Divide useful by total.
Multiply by 100 for a percentage.

Watch out Efficiency can never be more than 100% (or 1).

See it worked

Worked example

A light bulb uses 60 J of electrical energy and gives out 12 J of light. Calculate its efficiency.

Efficiency = 12 ÷ 60 = 0.2.
= 20%.

Answer 20%

Your turn

Question 1Calculator allowed2 marks

A motor has an input power of 500 W and a useful output power of 400 W. What is its efficiency as a percentage?

Question 2Calculator allowed1 mark

How can unwanted energy transfer in a machine be reduced?

Question 3Calculator allowed2 marks

A kettle is 90% efficient. It is supplied with 200 000 J. How much energy, in joules, is usefully transferred?

In past papers

57 official questions in 40 papers since 2018. The latest:

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