AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Efficiency: Students should be able to recall and apply both equations.
Efficiency compares the useful energy or power transferred by a device with the total energy or power supplied to it.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 5 of 7
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Efficiency compares the useful energy or power transferred by a device with the total energy or power supplied to it. The two equations are efficiency = useful output energy transfer ÷ total input energy transfer and efficiency = useful power output ÷ total power input. Because efficiency is a ratio of two quantities measured in the same unit, the units cancel, so efficiency has no unit. Recall means writing either equation from memory; apply means substituting values correctly, including converting a percentage to a decimal or the reverse.
For example: A motor supplied with 500 J that transfers 350 J usefully has efficiency 350 J ÷ 500 J = 0.70, which is 70%.
What earns the marks
- States the energy equation as efficiency = useful output energy transfer ÷ total input energy transfer.
- States the power equation as efficiency = useful power output ÷ total power input.
- Explains that both quantities in a calculation must use the same unit so the ratio is dimensionless.
- Substitutes values correctly into either equation and evaluates the quotient accurately.
- Converts between decimal and percentage form by multiplying or dividing by 100.
- Interprets the result, for example recognising that an efficiency of 0.70 means 70% of the input is transferred usefully.
Common mistakes and how to put them right
Dividing total input by useful output instead of useful output by total input.
Put it right by checking that the answer is less than 1 for a real device.
Mixing units, such as dividing energy in joules by power in watts.
Put it right by converting both quantities to the same type and unit first.
Writing a percentage value such as 70 without converting when a decimal is required, or writing 0.70 when a percentage is required.
Put it right by multiplying or dividing by 100 as the question demands.
Exam tips
- Write the equation you will use before substituting numbers, so the examiner can see your method even if the arithmetic slips.
- Check whether the question asks for a decimal or a percentage, and give the answer in that form with the correct symbol where needed.
- Remember that efficiency is a dimensionless ratio; if you mix energy and power values, your calculation will be invalid.
Try the lesson
The start of the Efficiency: Students should be able to recall and apply both equations. lesson, made from checked teaching notes for this specification point.
Have a go first
A motor is supplied with 200 J of energy and transfers 150 J usefully. Which equation gives the efficiency of the motor?
Listen
Efficiency compares the useful energy or power transferred by a device with the total energy or power supplied to it. The two equations are efficiency = useful output energy transfer ÷ total input energy transfer and efficiency = useful power output ÷ total power input. Because efficiency is a ratio of two quantities measured in the same unit, the units cancel, so efficiency has no unit. Recall means writing either equation from memory; apply means substituting values correctly, including converting a percentage to a decimal or the reverse. For example, a motor supplied with 500 J that transfers 350 J usefully has efficiency 350 J ÷ 500 J = 0.70, which is 70%. A strong answer earns marks for these points. States the energy equation as efficiency = useful output energy transfer ÷ total input energy transfer. States the power equation as efficiency = useful power output ÷ total power input. Explains that both quantities in a calculation must use the same unit so the ratio is dimensionless. Substitutes values correctly into either equation and evaluates the quotient accurately. A common slip: Dividing total input by useful output instead of useful output by total input. Instead, checking that the answer is less than 1 for a real device. Exam tip: Write the equation you will use before substituting numbers, so the examiner can see your method even if the arithmetic slips.
See it
Key words
Say what each word means before you turn the card.
Efficiency
Try answering aloud or in your head before turning the card.
Recall
Try answering aloud or in your head before turning the card.
Apply
Try answering aloud or in your head before turning the card.
See every definition
- Efficiency
- Compares the useful energy or power transferred by a device with the total energy or power supplied to it, and has no unit.
- Recall
- Writing either efficiency equation from memory.
- Apply
- Substituting values correctly into either equation, including converting a percentage to a decimal or the reverse.
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 Efficiency: Students should be able to recall and apply both equations.
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