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Heat transfer and efficiency

Energy transfer

You should recall from your Key Stage 3 studies how to draw and interpret an energy transfer diagram.

Different types of energy can be transferred from one type to another. Energy transfer diagrams show each type of energy, whether it is stored or not, and the processes taking place as it is transferred. Sankey diagrams also show the relative amounts of each type of energy.

Energy transfer diagrams

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

chemical energy in the fuel is the energy input, the car engine is the process, and energy output is in the form of kinetic energy in the engine and wheels

Process of using chemical energy

This 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.

electrical energy is the input, the electric lamp is the process, and the light energy the energy output

Process of using electrical energy

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

total electrical energy is 100 j, 90 j is transferred as heat energy and 10 j transferred as light energy

Sankey diagram for a filament lamp

Back to Energy and electricity index

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