Efficiency (Cambridge (CIE) O Level Physics): Revision Note

Efficiency of Energy Transfer

• The efficiency of a system is a measure of the amount of 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

• 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 a Power Station

Sankey diagram showing the efficiency of a gas-fired power station

Calculating Efficiency

• Efficiency is represented as a percentage, and can be calculated using the equation:

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

• The efficiency equation can also be written in terms of power:

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

• Where power is defined as the energy transferred per unit of time

  $P=\frac{∆E}{t}$