Electromagnetic inductionEdexcel International A Level Physics: Subtopic test

1. 1
   A bar magnet is pushed into a coil of 400 turns that is connected to a sensitive voltmeter. A student observes the voltmeter reading while moving the magnet.

   (a)

   Which change would increase the peak e.m.f. induced as the magnet is pushed into the coil?

   [1 mark]

   • AUsing a magnet with a weaker field
   • BPushing the magnet in faster
   • CUsing a coil with fewer turns
   • DPushing in the opposite pole first at the same speed

   (b)

   The magnet is held stationary inside the coil. The induced e.m.f. is

   [1 mark]

   • Aat its maximum, because the flux linkage is greatest
   • Bconstant and non-zero, because the flux linkage is constant
   • Cequal to NΦ, the flux linkage
   • Dzero, because the flux linkage is not changing

   (c)

   Explain why an e.m.f. is induced across the coil only while the magnet is moving.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Two coils are wound on the same soft iron core. The primary coil is connected to a battery through a switch. The secondary coil is connected to a sensitive voltmeter.

   (a)

   The switch is closed. The voltmeter connected to the secondary coil shows

   [1 mark]

   • Aa brief e.m.f. that falls back to zero
   • Ba steady e.m.f. for as long as the switch is closed
   • Cno e.m.f., because the coils are not electrically connected
   • Dan e.m.f. only when the switch is opened

   (b)

   Which change would increase the peak e.m.f. induced in the secondary coil when the switch is closed?

   [1 mark]

   • AReplacing the soft iron core with a plastic core
   • BUsing a battery of lower e.m.f. in the primary circuit
   • CUsing a secondary coil with more turns
   • DWinding the secondary with thicker wire but the same number of turns

   (c)

   The battery is replaced by a low-frequency alternating supply. Explain why a continuous alternating e.m.f. is now induced in the secondary coil.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A flat coil of 250 turns has an area of 4.0 × 10⁻³ m². The plane of the coil is perpendicular to a uniform magnetic field. The flux density of the field is increased steadily from zero to 0.60 T in a time of 0.20 s.

   (a)

   Calculate the magnitude of the e.m.f. induced in the coil while the flux density is increasing.

   [3 marks]

   (b)

   The flux density is now reduced from 0.60 T to zero in 0.050 s. Calculate the magnitude of the new e.m.f. and use Lenz's law to explain the direction of the induced current compared with the first case.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A strong cylindrical magnet is released from rest above a vertical coil of 500 turns and falls along the axis of the coil, passing completely through it. The ends of the coil are connected to a data logger of very high resistance that records the induced e.m.f. against time.

   (a)

   Describe and explain how the induced e.m.f. varies with time as the magnet falls from above the coil to below it.

   [6 marks]

   (b)

   The data logger is replaced by a short length of wire so that the coil forms a closed circuit. Use Lenz's law and the principle of conservation of energy to explain why the magnet now falls with an acceleration smaller than g while it is in the coil.

   [6 marks]

   Total for question 4: 12 marks

End of questions