Efficiency — GCSE Physics Revision
Revise Efficiency for GCSE Physics. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP.
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Go to National & Global Energy ResourcesWhat is Efficiency?
Efficiency is a measure of how much useful energy you get out of a device compared to the total energy you put in. It is usually expressed as a percentage. No device is 100% efficient; some energy is always wasted, usually as heat.
Board notes: Covered by all major boards (AQA, Edexcel, OCR) at both Foundation and Higher tiers.
Step-by-step explanationWorked example
A light bulb is supplied with 100J of electrical energy and it produces 20J of light energy. Calculate its efficiency. Solution: Efficiency = (Useful Energy Out / Total Energy In) x 100%. Efficiency = (20J / 100J) x 100% = 20%.
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Common mistakes
- 1Stating that efficiency can be greater than 100%. This is impossible as it would violate the principle of conservation of energy.
- 2Confusing useful energy with wasted energy. Efficiency is about the useful output, not the total input or the wasted part.
- 3Leaving efficiency as a decimal instead of a percentage. The question will usually specify the required format.
Efficiency exam questions
Exam-style questions for Efficiency with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP specifications.
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Efficiency
Core concept
Efficiency is a measure of how much useful energy you get out of a device compared to the total energy you put in. It is usually expressed as a percentage. No device is 100% efficient; some energy is …
Frequently asked questions
How can you improve the efficiency of a device?
Efficiency can be improved by reducing the amount of wasted energy. For example, by lubricating moving parts to reduce friction, or by using insulation to reduce heat loss.
Why can't a machine be 100% efficient?
In any real-world machine, some energy is always converted into non-useful forms, such as heat due to friction or air resistance. This wasted energy means that the useful output energy is always less than the input energy.