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Heat transfer and efficiency

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.

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

Energy can be transferred usefully, stored or dissipated. It cannot be created or destroyed. Notice that 100 J of electrical energy is supplied to the lamp. Of this, 10 J is transferred to the surroundings as light energy. The remainder, 90 J (100 J – 10 J) is transferred to the surroundings as heat energy.

The energy transfer to light energy is the useful transfer. The rest is ‘wasted’: it is eventually transferred to the surroundings, making them warmer. This ‘wasted’ energy eventually becomes so spread out that it becomes less useful.

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