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From the principle of conservation of energy, we know that energy is not used up but simply changes from one form to another.

The total amount of energy stays the same.

However, not all of the energy used in a particular process or device is useful.

Some is always wasted, for example, as heat, sound, and light.

The efficiency of a device is a measure of how much of the input energy appears as useful output energy.

The more energy a device wastes, the less efficient it is.

This is when:

• useful output energy is measured in joules, J;
• input energy is the total energy in J supplied to a device.

Calculating efficiency

Example: filament lamp

A filament lamp converts 100 J of electrical energy into 90 J of heat energy and 10 J of light energy.

Calculate the efficiency of the lamp.

Answer

Useful output energy = 10 J

Total input energy = 100 J

efficiency = 0.1

This means that 0.1 (or 10 %) of the electrical energy supplied is converted to light energy.

The light bulb is not very efficient since most of the energy supplied is not converted to useful light energy.

Most of the energy will have been converted to wasted heat energy.

This is why light bulbs become very hot when they are switched on.

Another way of thinking about this is that for every £100 spent on lighting, £10 is spent on the light and £90 is wasted heating the surroundings.

The efficiency of a device can never be greater than 1 otherwise energy would be created, and the principle of conservation of energy violated.