The energy efficiency for any energy transfer can be calculated using the…

The idea

Efficiency measures how much of the energy supplied to a device is transferred usefully rather than wasted. For any energy transfer, identify the total input energy transfer and the useful output energy transfer, both measured in joules (J). The useful output is the energy transferred to the intended store or pathway, while the rest is dissipated, often by heating the surroundings. Efficiency is usually expressed as a decimal or percentage. For example, a lamp receiving 100 J of electrical energy and transferring 20 J as light has an efficiency of 0.20 or 20%. The equation applies to all energy transfers, including mechanical, electrical and thermal processes, provided the same unit is used for both energies.

What earns the marks

• State that efficiency compares useful output energy transfer with total input energy transfer for any energy transfer.
• Identify the total input energy transfer as the energy supplied to the device or process, measured in joules (J).
• Identify the useful output energy transfer as the energy transferred to the intended useful store or pathway, measured in joules (J).
• Explain that wasted energy is transferred to less useful stores, often by heating the surroundings, and is not included in the useful output.
• Calculate efficiency as a decimal by dividing useful output energy transfer by total input energy transfer, then multiply by 100 to express it as a percentage.
• Recognise that efficiency has no unit because it is a ratio of two energies in the same unit.

Common mistakes and how to put them right

• Using the wasted energy instead of the useful output energy in the numerator.

  Put it right by identifying the useful output energy transfer first and dividing it by the total input energy transfer.

• Forgetting to convert a decimal efficiency to a percentage when asked.

  Put it right by multiplying the decimal by 100 and adding the percentage symbol.

• Mixing units, such as using kilojoules for input and joules for output.

  Put it right by converting both energies to the same unit, usually joules, before dividing.

Exam tips

• Underline the useful output energy transfer and the total input energy transfer in the question before substituting values into the equation.
• Show the substitution clearly, including units, then give the final efficiency as a decimal or percentage as requested.
• Check that the calculated efficiency is less than 1 or less than 100%, since wasted energy means the useful output is always smaller than the total input.