Efficiency of Energy Transfers — AQA GCSE Physics
Efficiency measures the proportion of input energy that is transferred usefully.
What is efficiency?
Efficiency is the fraction (or percentage) of the input energy that is transferred to a useful energy store or output.
$$\text{efficiency} = \frac{\text{useful output energy transfer}}{\text{total input energy transfer}}$$
It can also be calculated using power: $$\text{efficiency} = \frac{\text{useful power output}}{\text{total power input}}$$
Efficiency has no units and is always less than 1 (or less than 100%), because some energy is always dissipated (wasted, usually as heat).
To get a percentage, multiply by 100.
Worked example
A motor is supplied with 200 J of electrical energy and transfers 150 J usefully as kinetic energy.
- Efficiency = 150 ÷ 200 = 0.75 = 75%.
- The other 50 J is dissipated, mostly as heat (from friction).
Improving efficiency
Efficiency can be improved by reducing wasted energy, for example by lubricating moving parts to reduce friction or by insulating to reduce heat loss.
Exam tips
- Learn the efficiency equation (useful ÷ total), for energy or power.
- Efficiency is always less than 100% (some energy is always wasted).
- Multiply by 100 for a percentage.
- Improve efficiency by reducing wasted (dissipated) energy.