Efficiency

In this lesson you will learn how to calculate efficiency, interpret Sankey diagrams, and suggest ways to improve the efficiency of devices. This is part of AQA GCSE Combined Science Trilogy (8464), Section 6.1.

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What Is Efficiency?

Efficiency is a measure of how much of the total input energy (or power) is usefully transferred. No device is 100% efficient because some energy is always dissipated.

The Equations

$\text{Efficiency} = \frac{\text{useful output energy transfer}}{\text{total input energy transfer}}$Efficiency=total input energy transferuseful output energy transfer​

$\text{Efficiency} = \frac{\text{useful output power}}{\text{total input power}}$Efficiency=total input poweruseful output power​

Efficiency is a ratio with no units. It can be expressed as a decimal (0 to 1) or as a percentage (0% to 100%).

To convert a decimal to a percentage, multiply by 100.

Exam Tip: Read the question carefully to see whether it asks for a decimal or a percentage. AQA accepts both but you must be consistent.

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Worked Examples

Worked Example 1

A motor uses 800 J of electrical energy and does 600 J of useful mechanical work. Calculate its efficiency.

$\text{Efficiency} = \frac{600}{800} = 0.75 = 75\%$Efficiency=800600​=0.75=75%

Worked Example 2