Pearson Edexcel · GCSE · Physics
Course version: 1PH0 · Version from Sept 2016
Recall and use the equation: efficiency = (useful energy transferred by…
Full specification wording
Topic 8 – Energy – forces doing work: 8.15 Recall and use the equation: efficiency = (useful energy transferred by the device) / (total energy supplied to the device)
Efficiency compares the useful energy a device delivers with the total energy supplied to it, so efficiency = useful energy transferred ÷ total energy supplied.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 15 of 15
Study notes
From the checked teaching notes for this specification point.
The idea
Efficiency compares the useful energy a device delivers with the total energy supplied to it, so efficiency = useful energy transferred ÷ total energy supplied. Because both quantities are energies in joules, the ratio has no unit and is often given as a decimal or multiplied by 100 for a percentage. A motor supplied with 500 J that transfers 350 J usefully has efficiency 350 J ÷ 500 J = 0.70, or 70%. The remaining 150 J is transferred to the surroundings, often by heating, and is not useful. To use the equation, identify the useful output and the total input, keep both in the same unit, divide, and check the result lies between 0 and 1 (or 0% and 100%). A device cannot be more than 100% efficient because energy is conserved.
What earns the marks
- States the equation efficiency = useful energy transferred ÷ total energy supplied.
- Substitutes the useful energy and total energy in the same unit before dividing.
- Expresses the result as a decimal or converts it to a percentage by multiplying by 100.
- Explains that the wasted energy is transferred to the surroundings, often by heating.
Common mistakes and how to put them right
Divides total energy by useful energy.
Put it right: Divide useful energy by total energy instead.
Mixes joules and kilojoules in one calculation.
Put it right: Convert both to the same unit first.
Reports an efficiency above 100%.
Put it right: Check that useful energy is less than total energy.
Exam tips
- Write the equation, then label which value is useful and which is total before substituting.
- Convert both energies to joules so the units cancel and leave a pure ratio.
- Multiply by 100 only when a percentage is asked for, and state the unit as %.
Try the lesson
The start of the Topic 8 – Energy – forces doing work: 8.15 Recall and use the equation: efficiency = (useful energy transferred by the device) / (total energy supplied to the device) lesson, made from checked teaching notes for this specification point.
Have a go first
A fan is supplied with 500 J of energy and transfers 350 J as useful kinetic energy to the air. What is the efficiency of the fan?
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