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 3 – Conservation of energy: 3.11 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 transfers with the total energy supplied to it.
Checked against the Pearson Edexcel 1PH0 specification (opens in a new tab) on .
- Point 9 of 10
Study notes
From the checked teaching notes for this specification point.
The idea
Efficiency compares the useful energy a device transfers with the total energy supplied to it. Write efficiency = useful energy transferred ÷ total energy supplied. Both quantities are energies in joules (J), so efficiency is a ratio with no unit; multiply by 100 to give a percentage. Example: a lamp supplied with 200 J transfers 40 J as useful light, so efficiency = 40 J ÷ 200 J = 0.20, or 20%. The remaining 160 J is transferred to the surroundings, often by heating, and is wasted. Because useful energy can never exceed the total supplied, efficiency is always between 0 and 1 (0% to 100%). To use the equation, identify the useful output and the total input from the question, substitute in joules, then divide.
What earns the marks
- Recalls the equation efficiency = useful energy transferred ÷ total energy supplied.
- Substitutes the useful energy and total energy values in joules into the equation correctly.
- Calculates efficiency as a decimal or converts it to a percentage by multiplying by 100.
- States that efficiency has no unit and lies between 0 and 1 (0% and 100%).
Common mistakes and how to put them right
Divides total energy by useful energy.
Put it right: Keep useful energy on top and total energy underneath.
Adds a unit such as joules to the efficiency.
Put it right: State that efficiency is a ratio with no unit.
Forgets to multiply by 100 when a percentage is asked for; convert the decimal to a percentage.
Uses the wasted energy as the useful value.
Put it right: Identify the useful output from the question first.
Exam tips
- Write the equation first, then substitute the values, so the working is clear and easy to mark.
- Check the units of both energies match (both in joules) before dividing.
- Decide whether the answer should be a decimal or a percentage and present it accordingly.
- Sanity-check the result: an efficiency above 1 (or 100%) signals the values were swapped.
Try the lesson
The start of the Topic 3 – Conservation of energy: 3.11 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 kettle is supplied with 1000 J of energy and transfers 800 J usefully to the water. What is the efficiency of the kettle?
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