Questions on Energy Transitions
Q16. Electricity meters now have a digital readout, but many homes still have old electricity meters in them. These meters have a mechanical dial that turns when electricity in the home is being used to drive an appliance. The turning dial rotates counters on a display to show how much energy has been converted into another form in your house. It displays how many kWh you have used. The dial will turn many times before you use 1kWh. Therefore for short energy use experiments you have to count the turns rather than look at the display.
A group of students used a classroom version of such an electricity meter to measure how much electrical energy was used in two minutes as they performed each of the following household tasks:
a) boiled a kettle of water;
b) swept the carpet with a vacuum cleaner;
c) did the ironing.
Here are their results:
Time/s |
Number of turns of the electricity meter dial |
|
Kettle |
Vacuum Cleaner |
Iron |
0 |
0 |
0 |
0 |
16 |
4 |
1 |
2 |
32 |
8 |
2 |
4 |
43 |
12 |
3 |
4 |
64 |
16 |
4 |
4 |
80 |
20 |
5 |
4 |
96 |
24 |
6 |
6 |
112 |
28 |
7 |
8 |
128 |
32 |
8 |
10 |
(a) For each of the appliances, plot a graph of 'number of turns of the electricity meter dial' against 'time taken'. (10 marks)
(b) How can you tell from your graph which appliance used the most electricity in two minutes? (1 mark)
The graph with the steepest gradient shows the appliance that used the most electricity.
(c) Use your graph to find out how many times did the dial turn for each of the appliances in 24 seconds? (4 marks)
Kettle - 6 turns
Vacuum cleaner - 1.5 turns
Iron - 3 turns
Using the graph
(d) Use your graph to find out how long did it take each of the appliances to make the dial turn round three times? (4 marks)
Kettle - 12s 
Vacuum Cleaner - 48s
Iron 24s
Using the graph
(e) Explain how the function of the iron makes the graph for the iron looks rather different to the graph for the kettle and the hoover. (2 marks)
The horizontal (flat) portion of the graph indicates no energy was being converted during that time. The iron has a thermostatfitted which stops the iron from getting too hot. It switches off when it reaches a specified temperature and then switches on the electricity again when it gets cooler. The kettle and vacuum cleaner take a steady supply of electricity from the source, they are not fitted with thermostats therefore there are no flat portions on their graphs.
(Total 21 marks)
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