Energy EfficiencyAQA GCSE Physics: Revision notes

Efficiency compares how much of the energy (or power) put into a system ends up doing something useful, rather than being wasted (usually as heat or sound): efficiency = useful output energy transfer ÷ total input energy transfer.

This can also be written using power output and power input. Efficiency may be given as a decimal (e.g. 0.75) or a percentage (e.g. 75%) - always show your working in the same form the question uses.

Calculations can be used to show, on a common scale, how the total energy supplied to a system is redistributed - some to the useful output, and the rest wasted, usually as thermal energy dissipated to the surroundings. No practical device transfers all of its input energy usefully, so efficiency is always less than 1 (100%).

Unwanted energy transfers (energy 'wasted', often as heat due to friction) can be reduced by:

• Lubrication - reduces friction between moving surfaces, so less energy is transferred to heat
• Thermal insulation - reduces the rate at which thermal energy is transferred out of a system (e.g. a building or a hot water tank)

Reducing unwanted transfers increases the useful proportion of the input energy, which increases efficiency.

At Higher Tier, you should be able to describe ways to increase the efficiency of an intended energy transfer, such as: reducing friction (lubrication), improving insulation to cut unwanted heat loss, and using better-designed components that waste less energy overall.