GCSE Standard Questions: Electricity in the Home
Q3. The diagram shows a mobile phone battery being charged using solar cells. The useful power output of the solar cells is 2.5 W.
(a) The efficiency of the solar cells is 40%. Calculate the total power input to the solar cells.
2.5 W output is 40% of the input power PI which is a factor of 0.4
2.5 = 0.4 x PI
PI = 2.5/0.4 
= 6.25 W
[2 marks]
(b) The mobile phone battery can store 36 kJ of energy and was initially uncharged. Calculate the minimum time, in hours, it would take to fully charge the mobile phone battery using these solar cells.
P = E/t
t = E/P = 36,000/2.5
t = 14400 seconds
t = 4.0 hours
[3 marks]
(c) Explain why the power output of the solar cells may be lower than 2.5 W
There may be lower power input (or light energy input) to the cells because the solar cells are not pointed directly at Sun or there is a large amount of cloud blocking the sunlight or the solar cells may be in the shade or there may be dust on the surface of the solar cell unit.
[2 marks]
(d) Explain why the time taken to recharge this mobile phone battery using these solar cells depends on whether the phone is switched on or off.
When the phone is switched on energy from the solar cell is transferred to the battery and the phone circuitry meaning that only some of the energy from the solar cell charges the battery (so) the battery stores less energy per second
[2 marks]
(Total 9 marks)
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Any talk of 'efficiency' not being 100% or descriptions of energy transfer to the surroundings was ignored by the marker and saying it depended on 'the weather' was insufficient to get you a mark.
