Efficiency (Cambridge (CIE) IGCSE Combined Science: 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

• This means that:

  • 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

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

  • This means that 70% of the energy transferred from the power station to the National Grid is wasted energy

• 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

Sankey diagram of electricity production

Sankey diagram showing the energy transfers involved in generating electricity in a gas-fired power station

Calculating efficiency

Extended Tier Only

• Efficiency is represented as a percentage and 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:

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