## What is Efficiency?

The efficiency of a device is the proportion of input energy that is converted to useful energy. **Efficiency** is a measure of how much work or energy is conserved in an energy transfer, work or energy can be lost, for **example** as wasted heat energy. The **efficiency** is the energy output, divided by the energy input, and can be given as a decimal always less than 1 or a percentage. No machine has an **efficiency** of 100%.

## Efficiency equation

To calculate Efficiency we write the equation like this.

$efficiency = {\text{useful energy output} \over\text{total energy input}}$

## Efficiency demo

In this tutorial you will learn how to calculate how efficient a device is at changing energy from one form to another.

## Chilled practice question

Calculate the efficiency of a light bulb with an input energy of 500 J. The bulb emits 200 J of light energy and 300 J of heat energy.

## Frozen practice question

A tumble drier is 80 % efficient. Its useful energy is 45 KJ what is its input energy in Joules.

## Science in context

The efficiency of a device is the proportion of input energy that is converted to useful energy

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