Efficiency (Cambridge (CIE) IGCSE Physics): Revision Note

Efficiency of energy transfer

• The efficiency of a system is a measure of the amount of useful and wasted energy in an energy transfer

• Efficiency is defined as:

The ratio of the useful power or energy output from a system to its total power or energy input

• If a system has high efficiency, this means most of the energy transferred is useful

• If a system has low efficiency, this means most of the energy transferred is wasted

  • If a lamp is 25% efficient, this means that 25% of the energy supplied to it is transferred usefully as light, and the other 75% is wasted.

• No system reaches 100% efficiency

  • Energy is always wasted into other non-useful forms, such as sound and thermal energy which is lost to the surroundings (often due to friction)

Calculating efficiency

Extended Tier Only

• Efficiency can be calculated using two equations

• Efficiency in terms of energy:

$\mathrm{efficiency}=\frac{\mathrm{useful}\mathrm{energy}\mathrm{output}}{\mathrm{total}\mathrm{energy}\mathrm{input}}\left(\times 100\%\right)$

• Efficiency in terms of power:

$\mathrm{efficiency}=\frac{\mathrm{useful}\mathrm{power}\mathrm{output}}{\mathrm{total}\mathrm{power}\mathrm{input}}\left(\times 100\%\right)$

• Efficiency can be given as a decimal between 0 and 1, or as a percentage between 0 and 100%.

• In the production of electricity:

  • Energy is used to heat water to produce steam

  • The steam turns a turbine

  • The turbine turns a generator

  • The generator produces electricity 

    • At each stage of this process, energy is dissipated to the surroundings

• The overall efficiency of a typical thermal power station is approximately 30%

  • This means that about 70% of the energy from the fuel is wasted inside the power station, mostly as thermal energy transferred to the surroundings, and only about 30% is transferred usefully as electrical energy