University of Edinburgh

BSL Physics Glossary - efficiency - definition

The efficiency of a system is related to the energy input and the useful energy output. In the system there is a change from one sort of energy to another. If we take the example of a car. The input energy is the fuel, petrol, which is chemical energy. The engine changes the energy to kinetic energy and the pistons move up and down. But the amount of energy which comes out of the system is much less than the input. Some of the energy comes out in less useful ways, not in movement. For example as heat energy from the exhaust. The actual kinetic energy which is the useful output is only about 20% of the original input energy. So the efficiency of the petrol engine car is rather poor. Some systems are much more efficient, but we never get 100% of the input as output.

This is the formula for calculating the efficiency of a system:

Efficiency = Useful Energy Output/Energy Input x 100%