OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Recall and apply: efficiency = useful output energy transfer (J) / input…
Full specification wording
Power and efficiency: recall and apply: efficiency = useful output energy transfer (J) / input energy transfer (J)
Efficiency compares the useful energy a device delivers with the total energy supplied to it.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 1 of 8
- 4 key terms
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. The equation is efficiency = useful output energy transfer (J) ÷ input energy transfer (J). Both quantities are energies in joules, so the division gives a number with no unit; multiply by 100 to express it as a percentage. For example, a lamp supplied with 500 J of electrical energy that transfers 100 J as light has efficiency 100 ÷ 500 = 0.20, or 20%. The remaining 400 J is dissipated to the surroundings, usually by heating. Efficiency can never exceed 1 (100%) because a device cannot output more useful energy than it receives. When applying the equation, identify which value is the input and which is the useful output, keep both in joules, and check that the answer is a sensible fraction below 1.
What earns the marks
- Selects the relationship efficiency = useful output energy transfer ÷ input energy transfer.
- Substitutes energies in joules for both the useful output and the input.
- Calculates a value below 1 and, where required, converts it to a percentage by multiplying by 100.
- States that efficiency has no unit because joules divide by joules.
Common mistakes and how to put them right
Divides input energy by useful output energy.
Put it right: Keep the useful output on top and the input on the bottom.
Adds a unit such as joules to the efficiency value.
Put it right: Treat efficiency as a dimensionless fraction or percentage.
Uses the wasted energy as the useful output.
Put it right: Subtract wasted energy from input to find the useful output first.
Exam tips
- Write the equation down before substituting numbers so the numerator and denominator stay in the right order.
- Convert any answer to a percentage only when the question asks for one, and label it clearly.
- Sanity-check the result: an efficiency above 100% signals the values have been swapped.
Try the lesson
The start of the Power and efficiency: recall and apply: efficiency = useful output energy transfer (J) / input energy transfer (J) lesson, made from checked teaching notes for this specification point.
Have a go first
A kettle is supplied with 500 J of energy and transfers 400 J usefully to the water. What is the efficiency of the kettle?
Listen
Efficiency compares the useful energy a device delivers with the total energy supplied to it. The equation is efficiency = useful output energy transfer (J) ÷ input energy transfer (J). Both quantities are energies in joules, so the division gives a number with no unit; multiply by 100 to express it as a percentage. For example, a lamp supplied with 500 J of electrical energy that transfers 100 J as light has efficiency 100 ÷ 500 = 0.20, or 20%. The remaining 400 J is dissipated to the surroundings, usually by heating. Efficiency can never exceed 1 (100%) because a device cannot output more useful energy than it receives. When applying the equation, identify which value is the input and which is the useful output, keep both in joules, and check that the answer is a sensible fraction below 1. A strong answer earns marks for these points. Selects the relationship efficiency = useful output energy transfer ÷ input energy transfer. Substitutes energies in joules for both the useful output and the input. Calculates a value below 1 and, where required, converts it to a percentage by multiplying by 100. States that efficiency has no unit because joules divide by joules. Exam tip: Write the equation down before substituting numbers so the numerator and denominator stay in the right order.
See it
Key words
Say what each word means before you turn the card.
Efficiency
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Useful versus wasted energy
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Ratio and percentage calculation
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Dissipation of energy to the surroundings
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See every definition
- Efficiency
- Compares the useful energy a device delivers with the total energy supplied to it, calculated as useful output energy transfer divided by input energy transfer.
- Useful versus wasted energy
- The remaining energy that is not useful output is dissipated to the surroundings, usually by heating.
- Ratio and percentage calculation
- The division gives a number with no unit; multiply by 100 to express it as a percentage.
- Dissipation of energy to the surroundings
- Energy that is not useful output is dissipated to the surroundings, usually by heating.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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