OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Power and efficiency: calculate energy efficiency for any energy transfer
Efficiency compares the useful energy delivered by a transfer with the total energy supplied to it.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 5 of 8
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Efficiency compares the useful energy delivered by a transfer with the total energy supplied to it. For any transfer, efficiency = useful energy output ÷ total energy input, and multiplying by 100 gives a percentage. A lamp supplied with 500 J of energy that transfers 100 J as light has efficiency 100 ÷ 500 = 0.20, or 20%; the remaining 400 J is dissipated, mainly by heating the surroundings. The same ratio applies to power, so efficiency = useful power output ÷ total power input. Always identify which quantity is useful and which is total before dividing, keep the units consistent, and check that the answer is below 1 (or below 100%).
What earns the marks
- Selects the useful energy output and the total energy input for the stated transfer before dividing.
- Divides useful energy output by total energy input to give a value between 0 and 1.
- Multiplies the ratio by 100 to express efficiency as a percentage.
- Applies the same ratio to power, dividing useful power output by total power input.
- Keeps units consistent and recognises that efficiency has no unit.
Common mistakes and how to put them right
Divides total energy input by useful energy output.
Put it right: Divide the useful output by the total input instead.
Adds the wasted energy to the useful energy to find the total.
Put it right: Treat the supplied energy as the total input.
Reports efficiency as a value greater than 1 or 100%.
Put it right: Check that the useful output is smaller than the total input.
Gives efficiency a unit such as joules.
Put it right: State it as a fraction or percentage with no unit.
Exam tips
- Write the equation first, then substitute the two values so the useful and total quantities cannot be swapped.
- Convert both energies to the same unit before dividing.
- Sanity-check the answer: a percentage efficiency must lie between 0% and 100%.
Try the lesson
The start of the Power and efficiency: calculate energy efficiency for any energy transfer lesson, made from checked teaching notes for this specification point.
Have a go first
A lamp is supplied with 200 J of energy and transfers 40 J usefully as light. Calculate the efficiency of the lamp.
Listen
Efficiency compares the useful energy delivered by a transfer with the total energy supplied to it. For any transfer, efficiency = useful energy output ÷ total energy input, and multiplying by 100 gives a percentage. A lamp supplied with 500 J of energy that transfers 100 J as light has efficiency 100 ÷ 500 = 0.20, or 20%; the remaining 400 J is dissipated, mainly by heating the surroundings. The same ratio applies to power, so efficiency = useful power output ÷ total power input. Always identify which quantity is useful and which is total before dividing, keep the units consistent, and check that the answer is below 1 (or below 100%). A strong answer earns marks for these points. Selects the useful energy output and the total energy input for the stated transfer before dividing. Divides useful energy output by total energy input to give a value between 0 and 1. Multiplies the ratio by 100 to express efficiency as a percentage. Applies the same ratio to power, dividing useful power output by total power input. Exam tip: Write the equation first, then substitute the two values so the useful and total quantities cannot be swapped.
See it
Key words
Say what each word means before you turn the card.
Useful energy output
Try answering aloud or in your head before turning the card.
Total energy input
Try answering aloud or in your head before turning the card.
Efficiency as a ratio and percentage
Try answering aloud or in your head before turning the card.
Energy dissipation in transfers
Try answering aloud or in your head before turning the card.
See every definition
- Useful energy output
- The energy delivered by a transfer that is wanted, selected before dividing by the total energy input.
- Total energy input
- The energy supplied to a transfer, which is treated as the total input rather than adding wasted energy to the useful energy.
- Efficiency as a ratio and percentage
- Useful energy output divided by total energy input, giving a value between 0 and 1, and multiplying by 100 gives a percentage.
- Energy dissipation in transfers
- The remaining energy that is not useful output, dissipated mainly by heating the surroundings.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Power and efficiency: calculate energy efficiency for any energy transfer
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge