AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
Students may be required to calculate or use efficiency values as a…
Full specification wording
Efficiency: Students may be required to calculate or use efficiency values as a decimal or as a percentage.
Efficiency can be expressed as a decimal between 0 and 1 or as a percentage between 0% and 100%.
Checked against the AQA 8463 specification (opens in a new tab) on .
- Point 6 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Efficiency can be expressed as a decimal between 0 and 1 or as a percentage between 0% and 100%. The two forms carry the same information: 0.25 and 25% both mean that one quarter of the input is transferred usefully. To change a decimal to a percentage, multiply by 100; to change a percentage to a decimal, divide by 100. Questions may give efficiency in one form and expect an answer in the other, or may require you to use the value to find a missing energy or power. For example, a lamp with efficiency 0.20 supplied with 300 J transfers 0.20 × 300 J = 60 J usefully, leaving 240 J wasted. A device that is 45% efficient and receives 800 W delivers 0.45 × 800 W = 360 W usefully.
What earns the marks
- Converts a decimal efficiency to a percentage by multiplying by 100, and a percentage to a decimal by dividing by 100.
- Uses an efficiency value to find useful output by multiplying efficiency by total input.
- Uses an efficiency value to find total input by dividing useful output by efficiency.
- Recognises that efficiency cannot exceed 1 or 100% for a real device and interprets values accordingly.
- Calculates wasted energy or power as the difference between total input and useful output.
- Presents the final value in the form requested, with the percentage symbol where appropriate.
Common mistakes and how to put them right
Treating a percentage as a decimal without converting, for example multiplying 800 W by 45 instead of by 0.45.
Put it right by dividing the percentage by 100 first.
Forgetting to multiply by 100 when a percentage is required, leaving an answer such as 0.45 when 45% is expected.
Put it right by checking the requested form.
Assuming efficiency can be greater than 1 or 100%.
Put it right by remembering that useful output can never exceed total input for a real device.
Exam tips
- Underline the form requested, decimal or percentage, before you calculate, and convert at the end if needed.
- When finding a missing input, rearrange the equation first and then substitute, which reduces arithmetic errors.
- Sanity-check your answer: a useful output should be smaller than the total input, and an efficiency should lie between 0 and 1 or 0% and 100%.
Try the lesson
The start of the Efficiency: Students may be required to calculate or use efficiency values as a decimal or as a percentage. lesson, made from checked teaching notes for this specification point.
Have a go first
An electric heater transfers 80 J of useful energy from a total input of 100 J. What is the efficiency of the heater as a percentage?
Listen
Efficiency can be expressed as a decimal between 0 and 1 or as a percentage between 0% and 100%. The two forms carry the same information: 0.25 and 25% both mean that one quarter of the input is transferred usefully. To change a decimal to a percentage, multiply by 100; to change a percentage to a decimal, divide by 100. Questions may give efficiency in one form and expect an answer in the other, or may require you to use the value to find a missing energy or power. For example, a lamp with efficiency 0.20 supplied with 300 J transfers 0.20 × 300 J = 60 J usefully, leaving 240 J wasted. A device that is 45% efficient and receives 800 W delivers 0.45 × 800 W = 360 W usefully. A strong answer earns marks for these points. Converts a decimal efficiency to a percentage by multiplying by 100, and a percentage to a decimal by dividing by 100. Uses an efficiency value to find useful output by multiplying efficiency by total input. Uses an efficiency value to find total input by dividing useful output by efficiency. Recognises that efficiency cannot exceed 1 or 100% for a real device and interprets values accordingly. A common slip: Treating a percentage as a decimal without converting, for example multiplying 800 W by 45 instead of by 0.45. Instead, dividing the percentage by 100 first. Exam tip: Underline the form requested, decimal or percentage, before you calculate, and convert at the end if needed.
Key words
Say what each word means before you turn the card.
Efficiency
Try answering aloud or in your head before turning the card.
Decimal to percentage conversion
Try answering aloud or in your head before turning the card.
Useful output
Try answering aloud or in your head before turning the card.
Wasted energy or power
Try answering aloud or in your head before turning the card.
See every definition
- Efficiency
- A value expressed as a decimal between 0 and 1 or as a percentage between 0% and 100%, where both forms carry the same information.
- Decimal to percentage conversion
- To change a decimal to a percentage, multiply by 100; to change a percentage to a decimal, divide by 100.
- Useful output
- Found by multiplying efficiency by total input, and it can never exceed total input for a real device.
- Wasted energy or power
- Calculated as the difference between total input and useful output.
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 may be required to calculate or use efficiency values as a decimal or as a percentage.
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