Efficiency

\eta=\dfrac{W_u}{W_d}\cdot 100\%

Result unit: % (dimensionless)

What it describes

The ratio of useful work to the work supplied — it tells you what fraction of the energy was put to use.

\eta

efficiency

[%]

W_u

useful work (output)

[J]

W_d

work supplied (input)

[J]

No device converts all the supplied energy into useful output — some escapes (friction, heat). That is why efficiency is always less than 100%.

Assessing machines, engines and devices. It can also be computed as the ratio of useful power to power drawn.

A device draws 500 J and delivers 400 J of useful output. Find its efficiency.

Substitution

W_u=400

W_d=500

Result

\eta=\dfrac{400}{500}\cdot100\%=80\%

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