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1. A lamp has a power of 60 watts. It gives out light at a rate of 6 watts.
a) The amount of energy supplied to the lamp each second is 60 joules.
b) The amount of energy transferred usefully as light each second is 6 joules.
c) The amount of energy wasted each second
is 54 joules.
d) The efficiency of the lamp is:
Efficiency = (useful energy transferred ÷ total energy input) x 100 %
Efficiency = (6 ÷ 60) x 100 %
Efficiency = 10 %
2. The following diagram is about a radio.
a) The useful energies are light and sound.
b) The wasted energy is heat.
c) The efficiency of the radio is:
Efficiency = (useful energy transferred ÷ total energy input) x 100 %
Efficiency = (350 ÷ 500) x 100 %
Efficiency = 70 %
3. a) In a solar cell, for every 100 J of solar energy shining on it,
only 5 J is transferred to useful
energy (as electricity). The other 95 J is wasted.
b) The efficiency of the solar cell is:
Efficiency = (useful energy transferred ÷ total energy input) x 100%
Efficiency = (5 ÷ 100) x 100%
Efficiency = 5 %
c) Solar cells are not yet in widespread use because they cost a lot to produce and are not
efficient at producing electricity.
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