IGCSE Physics / Energy transfers
Efficiency
Edexcel International GCSE Physics (4PH1), chapter 14: Energy transfers, specification point 4.4. Free lesson.
Read the revision notes · IGCSE Physics past papers
Efficiency is the fraction of an energy or power input that becomes useful output. It helps you compare how well devices transfer energy and calculate useful output from an efficiency.
What efficiency measures
Efficiency tells you what fraction of the input ends up where you want it. The useful output is part of the total output, which includes both useful and wasted energy. Energy is conserved, so total energy output equals total energy input. Efficiency = useful energy output ÷ total energy output × 100%. Use the equation from the equation list. Energy is measured in joules (J).
How to calculate efficiency
Put the useful energy on top of the fraction and the total energy on the bottom. Multiply by 100 to give a percentage. To give efficiency as a decimal, leave out × 100. A decimal efficiency is written without a percent sign. For example, 0.75 is the decimal form of 75%.
How to use wasted energy
Wasted energy is not the useful output, so do not put it on top of the fraction. If a question gives the wasted energy, subtract it from the total first to find the useful energy. For a lamp supplied with 100 J that wastes 90 J, useful energy = 100 − 90 = 10 J. Its efficiency is 10/100 × 100 = 10%.
How to calculate useful power
The same equation works with power, which is the energy transferred each second. Power is measured in watts (W). Rearrange the equation if you are given efficiency and total power and need to find useful power. Efficiency = useful power output ÷ total power output × 100%.
Why efficiency is never 100%
No device is 100% efficient. Moving parts rub together and electrical resistance heats wires, so some energy is always transferred by heating to the thermal store of the surroundings. An efficiency above 100% is wrong and means the fraction has been turned upside down.