大象传媒

Electrical efficiency

The of an electrical device depends on:

  • how much is wasted - transferred to unwanted stores
  • how much energy is transferred to useful stores

The more energy a device wastes, the less efficient it is.

To calculate efficiency, use the equation:

\(efficiency = \frac{useful~output~energy~transfer}{input~energy~transfer}\)

This is when:

  • energy transfer is measured in joules (J)
  • useful output energy refers to the useful energy that is transferred by the device (eg thermal energy by a heater)
  • input energy refers to the total energy supplied to a device

Learn more about efficiency in this podcast

Example

The energy supplied to a light bulb is 200 J. A total of 28 J of this is usefully transferred. How efficient is the light bulb?

\(efficiency = \frac{useful~output~energy~transfer}{input~energy~transfer}\)

\(efficiency = \frac{28}{200}\)

\(= 0.14\)

Efficiency can be calculated as a percentage. Multiply by 100 to convert this answer into a percentage:

\(percentage~efficiency = 0.14 \times 100\)

\(= 14\%\)

In this example, the light bulb is not very efficient since most of the energy supplied is not transferred usefully. Most of the energy will have been by . This is because light bulbs become very hot when they are switched on, which increases the thermal store of the surroundings.

Improving efficiency - Higher

Efficient devices use less fuel than less efficient device. They can operate at a lower power because less of the supplied energy store is wasted.

Efficiency can be improved by:

  • making devices and machines from materials which reduce unwanted energy transfer
  • improving the technology used, for example modern LEDs are much more efficient than traditional bulbs
  • using insulation to prevent thermal energy losses to the surroundings