AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Efficiency: efficiency = useful output energy transfer / total input energy transfer
This equation defines efficiency as the ratio of useful output energy transfer to total input energy transfer.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This equation defines efficiency as the ratio of useful output energy transfer to total input energy transfer. To use it, identify the energy supplied to the device and the energy transferred usefully, then divide. For example, an electric motor is supplied with 500 J and transfers 350 J usefully as kinetic energy, so efficiency = 350 J ÷ 500 J = 0.70 or 70%. The remaining 150 J is wasted, usually heating the motor and surroundings. Efficiency is a dimensionless number between 0 and 1, or a percentage between 0% and 100%. The same unit must be used for both energies. A higher efficiency means a greater proportion of input energy is transferred usefully, reducing wasted energy and running costs.
What earns the marks
- Quote the equation in the form efficiency = useful output energy transfer ÷ total input energy transfer.
- Distinguish useful output energy from total input energy in a given device or process.
- Substitute numerical values into the equation, converting to a common unit such as joules where necessary.
- Calculate the efficiency as a decimal and, if required, convert it to a percentage by multiplying by 100.
- Interpret the result, for example stating that 70% of the input energy is transferred usefully and 30% is dissipated.
- Compare efficiencies of two devices by calculating both values and identifying the device that wastes less energy.
Common mistakes and how to put them right
Forgetting to convert a percentage answer back to a decimal when the question asks for efficiency as a number.
Put it right by dividing the percentage by 100.
Using the wasted energy instead of the total input energy as the denominator.
Put it right by ensuring the denominator is the total input energy, which can be found by adding the useful and wasted energies if not given directly.
Leaving the answer as a fraction without simplifying or converting.
Put it right by giving a decimal or percentage as requested.
Exam tips
- Underline the useful output and total input values in the question before substituting them into the equation.
- Show the substitution line clearly, including units, because method marks are often available even when the final value is wrong.
- Round sensibly and state the unit or symbol: efficiency has no unit, but a percentage answer needs the % sign.
Try the lesson
The start of the Efficiency: efficiency = useful output energy transfer / total input energy transfer lesson, made from checked teaching notes for this specification point.
Have a go first
A lamp transfers 100 J of energy from the mains supply. It transfers 25 J of energy as useful light energy. Using efficiency = useful output energy transfer / total input energy transfer, what is the efficiency of the lamp?
Key words
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Efficiency
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Useful output energy transfer
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Total input energy transfer
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Wasted energy
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See every definition
- Efficiency
- The ratio of useful output energy transfer to total input energy transfer, a dimensionless number between 0 and 1 or a percentage between 0% and 100%.
- Useful output energy transfer
- The energy transferred usefully by a device, which is divided by the total input energy transfer to give efficiency.
- Total input energy transfer
- The energy supplied to a device, which is the denominator in the efficiency equation and can be found by adding the useful and wasted energies.
- Wasted energy
- Energy not transferred usefully by a device, usually heating the device and surroundings, and reduced by a higher efficiency.
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