Energy cannot be created or destroyed, only converted from one form to another. These conversions can be shown in Sankey diagrams. Effeciency is a measure of how much useful energy is converted.

Energy transfers

Different forms of energy can be transferred from one form to another. Energy transfer diagrams show each form of energy - whether it is stored or not - and the processes taking place as energy is transferred.

The energy transfer diagram below shows the useful energy transfer in a car engine. You can see that a car engine transferschemical energy, which is stored in the fuel, into kinetic energy in the engine and wheels.

Process of using chemical energy

The next diagram shows the energy transfer diagram for the useful energy transfer in an electric lamp. You can see that the electric lamp transfers or converts electrical energy into light energy.

Process of using electrical energy

Note that these energy transfer diagrams only show the useful energy transfers. However, car engines are also noisy and hot, and electric lamps also give out heat energy.

Sankey diagrams

Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved.

This Sankey diagram for an electric lamp shows that most of the electrical energy is transferred as heat rather than light.

Sankey diagram for a filament lamp

Energy can be transferred usefully, stored or dissipated. It cannot be created or destroyed. This is called conservation of energy.

In the above Sankey diagram, note that 100 J of electrical energy is supplied to the lamp. Of this, 10 J is transferred to the…