Last Updated Sep 26, 2025

Efficiency Simplified Revision Notes for GCSE AQA Physics Combined Science

Revision notes with simplified explanations to understand Efficiency quickly and effectively.

Learn about Energy Changes in a System for your GCSE Physics Combined Science Exam. This Revision Note includes a summary of Energy Changes in a System for easy recall in your Physics Combined Science exam

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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:

$$efficiency =\frac{useful\ energy\ output}{total\ energy\ input} = \frac{useful\ power\ output}{total\ power\ input}$$

• 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:

$$Efficiency = \frac{Useful energy output}{Total energy input}x100$$$$Efficiency = \frac{20J}{100J} X 100$$

2. Do the maths:

$$Efficiency=0.2×100=20$$

Answer:

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The light bulb is 20% efficient.

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