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{\text{useful output energy}}{\text{total input energy}}

Useful output energy = 28 J

Total input energy = 200 J

efficiency = \frac{\text{28}}{\text{200}}

efficiency = 0.14

This means that 0.14 (or 14 per cent) of the electrical energy supplied is transferred as light energy.


A hairdryer has an efficiency of 0.4. How much electrical energy has to be supplied for the hairdryer to ouput 600 J of useful energy?

Total input energy = \frac{\text{useful output energy}}{\text{efficiency}}

efficiency = 0.4

Useful output energy = 600 J

total input energy = \frac{\text{600 J}}{\text{0.4}}

Total input energy = 1500 J

The hairdryer must be supplied with 1500 J of energy.