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?
Useful output energy = 28 J
Total input energy = 200 J
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 =
efficiency = 0.4
Useful output energy = 600 J
total input energy =
Total input energy = 1500 J
The hairdryer must be supplied with 1500 J of energy.