Efficiency
The efficiency of a device is a measure of how much of the input energy appears as useful output energy.
The more energy a device wastes, the less efficient it is.
This is when:
- useful output energy refers to the useful energy in J (Joules) that is transferred by the device;
- input energy refers to the total energy in J supplied to a device.
Efficiency doesn't have a unit
Calculating efficiency
Filament lamp
This is the Sankey diagram for a typical filament lamp:
Efficiency = \(\frac{\text{useful output energy}}{\text{total input energy}}\)
Useful output energy = 10 J
Total input energy = 100 J
efficiency = \(\frac{\text{10}}{\text{100}}\)
efficiency = 0.1
This means that 0.1 (or 10 %) of the electrical energy supplied is transferred as light energy.
The light bulb is not very efficient since most of the energy supplied is not transferred usefully.
Most of the energy will have been dissipated as heat energy.
This is because light bulbs become very hot when they are switched on.
Another way of thinking about this is that for every 拢100 spent on lighting, 拢10 is spent on the light and 拢90 is wasted heating the surroundings.
The efficiency of a device can never be greater than 1 otherwise energy would be created, and the Principle of Conservation of Energy violated.
Occasionally power is shown in W instead of energy in J. The equation for efficiency is similar 鈥 just substitute power for energy: efficiency = \(\frac {useful~output~power}{total~input~power}\)