Efficiency (OCR GCSE Physics A, Combined (Gateway Science Suite)): Revision Notes

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1.1.14 Efficiency

Efficiency

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Efficiency is the ratio of the useful work done by a machine, engine, device, etc, to the energy supplied to it, often expressed as a percentage.

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Formula:

efficiency=useful energy outputtotal energy input=useful power outputtotal power inputefficiency =\frac{useful\ energy\ output}{total\ energy\ input} = \frac{useful\ power\ output}{total\ power\ input}
  • The efficiency of a system can be increased by: o Reducing waste output (lubrication, thermal insulation, etc.) o Recycling waste output (e.g. absorbing thermal waste and recycling as input energy)
lightbulbExample

Example Question: A light bulb uses 100 joules of electrical energy but only gives off 20 joules of light energy. How efficient is the light bulb?

Solution:

  1. What do we know?
  • The light bulb uses 100 joules of energy.
  • It only gives off 20 joules of light energy.
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2. Use the efficiency formula:

Efficiency=UsefulenergyoutputTotalenergyinputx100Efficiency = \frac{Useful energy output}{Total energy input}x100Efficiency=20J100JX100Efficiency = \frac{20J}{100J} X 100
  1. Do the maths:
Efficiency=0.2×100=20Efficiency=0.2×100=20%

Answer:

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The light bulb is 20% efficient.

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