Efficiency
The efficiencyThe fraction of the energy supplied to a device which is transferred in a useful form. of an electrical device depends on:
- how much energyThe capacity for doing work. 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
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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 dissipatedThe spreading out and transfer of energy stores into less useful forms, such as thermal energy causing the surroundings to heat up. Dissipated energy is often referred to as 'wasted' energy, since it is not transferred to a useful output. by infrared radiationElectromagnetic radiation emitted from a hot object.. 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 filamentA thin, high resistance wire that gets hot and glows when a current flows through it causing it to emit heat and light. Filaments are used in some types of bulb and electrical heaters. bulbs
- using insulation to prevent thermal energy losses to the surroundings