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GCSE Standard Questions: The Generator

(Electromagnetic Induction)

Q2. The diagram shows some parts of a torch which works without batteries. The coil is part of a complete circuit with the LED (light-emitting diode). You have to shake the torch for a short time and then it is ready to use.

(a) Arrange the letters, A, B, C, D and E, in the correct order to explain how shaking the torch produces an electric current.

A
 An electric current is induced in the circuit.
B
 The magnetic field cuts through the coil.
C
The magnet moves in and out of the coil.
D
 A potential difference (p.d.) is induced across the ends of the coil.
E
The torch is shaken to and fro

E → C → B → D → A (all five in order ; two or more in order )

(2 marks)

(b) Give two changes which you would make to the design of the torch to increase the size of the induced potential difference.

Any two from:

  • more powerful / stronger magnet (NOT bigger - bigger does not necessarily mean stronger!)
  • smaller gap between coil and magnet
  • coil with more turns / longer coil (accept more coils - do not accept just 'bigger coil' or just 'more wire')
  • coil with bigger area

- do not accept 'shake faster'

- do not accept 'shake for longer'

(They are NOT design changes are they? They are simply 'usage' changes!)

(2 marks)

(c) A few minutes after shaking, the LED gets dimmer and then stops giving out light. A student tests the torch. She shakes it for a period of time. Then she switches it on and times how long the light lasts. These are her results.

Period of time torch is shaken measured in seconds
How long the light lasts measured in seconds
30
168
60
312
90
420
120
546
150
654

(i) What conclusion can the student come to on the basis of these results?

The longer the torch is shaken, the longer the light lasts.

Accept it is a (strong) positive (non-linear) relationship but do not accept '.... are (directly) proportional ....'

If you plotted these results they would not give you a straight line - so they are not proportional.

(1 mark)

(ii) The student's friend says that the results are not reliable. Her friend is correct. Give two reasons why.

If this investigation was repeated the results would probably not be the same (therefore the results are not reliable). She didn't try repeating them - if she had she would be able to judge better whether they were.

Reasons why they are not reliable:

  • rate / amplitude / angle of shaking could vary
  • it required personal judgement when the LED / light had 'gone out' rather than taking a measurement on an instrument.
  • results / data / measurements have not been repeated / average has not been found and range of radings not determined.

 

(2 marks)

(Total 7 marks)