AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Efficiency: efficiency = useful power output / total power input
This equation gives efficiency as the ratio of useful power output to total power input.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This equation gives efficiency as the ratio of useful power output to total power input. Power is the rate of energy transfer, measured in watts, so a 60 W lamp that emits 12 W as useful light has efficiency 12 / 60 = 0.20, or 20%. The other 48 W is transferred to the surroundings, often by heating. Because both quantities are powers in watts, the units cancel and efficiency has no unit; it is a number between 0 and 1, or a percentage between 0% and 100%. No real device reaches 100% efficiency because some energy is always dissipated. When using the equation, identify the useful output first, then the total input, substitute, and evaluate. If a percentage is required, multiply the decimal by 100.
What earns the marks
- Write the equation as efficiency = useful power output / total power input.
- Identify the useful power output and the total power input in watts from the question.
- Substitute the values correctly and evaluate the ratio.
- Convert a decimal answer to a percentage by multiplying by 100 when required.
- Interpret the result, for example 0.25 means 25% of the input power is transferred usefully.
Common mistakes and how to put them right
Using the wasted power as the numerator.
Put it right by remembering the numerator is the useful output, while wasted power is total input minus useful output.
Adding a unit such as W or J to the efficiency value.
Put it right by cancelling the power units, since efficiency is a ratio with no unit.
Treating a percentage answer as a decimal, for example writing 0.75% instead of 75%.
Put it right by multiplying the decimal by 100 before adding the % sign.
Exam tips
- Write the equation first, then substitute, so the examiner can see the correct relationship even if the final value is wrong.
- Check that the useful output is smaller than the total input; if it is not, you have probably swapped the values.
- Give efficiency as a decimal or a percentage as requested, and never attach a unit to it.
Try the lesson
The start of the Efficiency: efficiency = useful power output / total power input lesson, made from checked teaching notes for this specification point.
Have a go first
A light bulb transfers 60 J of energy usefully as light every second. The total power input to the bulb is 150 W. Calculate the efficiency of the bulb.
Key words
Say what each word means before you turn the card.
Efficiency
Try answering aloud or in your head before turning the card.
Power
Try answering aloud or in your head before turning the card.
Useful power output
Try answering aloud or in your head before turning the card.
See every definition
- Efficiency
- A ratio of useful power output to total power input, with no unit, given as a number between 0 and 1 or a percentage between 0% and 100%.
- Power
- The rate of energy transfer, measured in watts.
- Useful power output
- The power transferred usefully by a device, which is the numerator in the efficiency equation and is smaller than the total input.
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