GCSE Standard Questions: Electricity in the Home
Q4. The table below shows information about different light bulbs.
The bulbs all have the same brightness.
Type of bulb |
Input power in watts |
Efficiency |
Halogen |
40 |
0.15 |
Compact fluorescent (CFL) |
14 |
0.42 |
LED |
7 |
0.85 |
(a)
(i) Calculate the useful power output of the CFL bulb in watts. [2 marks]
Efficiency = output/input
output power = efficiency x input power 
output power = 0.42 x 14 = 5.88
output power = 5.9 W
(ii) Use your answer to calculate the waste energy in joules produced each second by a CFL bulb. [1 mark]
Wasted power = Input power - Output power
= 14 - 5.88 = 8.12W
1W = 1 J/s
So the wasted energy in one second is 8.12 J
8.1 J
(b)
(i) A growth cabinet is used to investigate the effect of light on the rate of growth of plants.
In the cabinet the factors that affect growth can be controlled. A cooler unit is used to keep the temperature in the cabinet constant. The cooler unit is programmed to operate when the temperature rises above 20 °C.
The growth cabinet is lit using 50 halogen bulbs. Changing from using halogen bulbs to LED bulbs would reduce the cost of running the growth cabinet. Explain why. [4 marks]
If you used LED bulbs the input power/energy would be much less (7 J/s instead of 40 J/s for each bulb) this would significantly reduce cost of running the unit.
Also it would produce less wasted energy/power as not only would a smaller amount of energy be used each second but the LEDs are more efficient than the halogen bulbs.
It is the 'wasted energy' that takes the form of heat rather than light and increases the temperature of the cabinet, so less 'wasted energy' means the cooler would turn on for less frequently - again reducing the running cost of the unit,
(ii) A scientist measured the rate of growth of plants for different intensities of light. Explain what type of graph should be drawn to present the results? [1 mark]
A line graph would be needed as both variables are continuous.
(c) The table below gives further information about both a halogen bulb and an LED bulb.
Type of bulb |
Cost to buy |
Lifetime in hours |
Operating cost over the lifetime of one bulb |
Halogen |
£1.50 |
2 000 |
£16.00 |
LED |
£30.00 |
48 000 |
£67.20 |
A householder needs to replace a broken halogen light bulb. Compare the cost efficiency of buying and using halogen bulbs rather than an LED bulb over a time span of 48 000 hours of use.
Your comparison must include calculations. [4 marks]
Halogen bulbs
Over 48,000 hours you would need to use 48,000/2,000 = 24 halogen bulbs
Cost of running the bulbs = 24 x £16.00 = £384.00
Cost of buying them = 24 x £1.50 = £36.00
Total cost = £384.00 + £36.00 = £420
LED bulbs
Over 48,000 hours you would need 1 LED bulb
Cost to run = £67.20
Cost of purchase = £30.00
Total cost = £67.20 + £30.00 = £97.20
It is therefore £420 - £97.20 = £322.80 cheaper to use an LED bulb rather than halogen bulbs.
(Total 12 marks)
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