AQA · Physics · 8463

GCSE Physics: Efficiency

Energy · AQA GCSE Physics (8463), specification point 4.1.2.2. Cards are generated from the exam board's own specification for "Efficiency", and every card links back to the page it came from. Specification © AQA — used with attribution.

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  1. What is the equation for efficiency in terms of energy?

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    Efficiency = useful output energy transfer ÷ total input energy transfer.

  2. What is the equation for efficiency in terms of power?

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    Efficiency = useful power output ÷ total power input.

  3. Efficiency = ? output energy transfer ÷ total input energy transfer.

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    useful

  4. Efficiency = useful power output ÷ ? power input.

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    total

  5. In what two forms can an efficiency value be expressed?

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    As a decimal (e.g. 0.4) or as a percentage (e.g. 40%).

  6. How do you convert an efficiency decimal to a percentage?

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    Multiply by 100 — e.g. 0.65 becomes 65%.

  7. A motor takes in 500 J and does 200 J of useful work. What is its efficiency?

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    0.4 (or 40%) — 200 J ÷ 500 J.

  8. A lamp has an input power of 60 W and a useful output of 6 W. What is its efficiency?

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    0.1 (or 10%) — 6 W ÷ 60 W.

  9. Why can the efficiency of an energy transfer never exceed 1 (100%)?

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    Useful output can never be greater than total input, because energy is conserved and some is always dissipated (wasted).

  10. Give one way to increase the efficiency of a mechanical energy transfer.

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    Lubricate moving parts to reduce friction, so less energy is wasted as heat.

  11. Give one way to increase the efficiency of heating a building.

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    Add thermal insulation (e.g. loft or cavity wall insulation) to reduce energy dissipated to the surroundings.

  12. What does a low efficiency value tell you about an energy transfer?

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    A large proportion of the input energy is wasted (dissipated) rather than transferred usefully.

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