Exam-style Questions: Lenz's Law

  1. A bar magnet is fixed on the end of a string to make a simple pendulum, as shown in the diagram.

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    a) If a flat sheet of copper is placed just below the swining magnet, what happens inside the copper?

    (1 Mark)

    b) What effect does this have on the swinging magnet and why does it have this effect?

    (2 Marks)

    c) What happens if a glass plate is used instead of copper?

    (2 Marks)

    (Marks available: 5)

  2. A magnet is dropped through a wire coil. As it goes through a current is generated in the coil which produces a magnetic field near the coil.

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    a) Which magnetic pole (N or S) is produced at X as the magnet goes into the top of the coil?

    (1 Mark)

    b) Explain your answer to a)?

    (2 Marks)

    c) Which magnetic pole (N or S) is produced at Y as the magnet leaves the bottom of the coil? Why?

    (2 Marks)

    (Marks available: 5)

Answer

Answer outline and marking scheme for question:

  1. a) An electric current is induced in the copper by the changing magnet field from the swinging magnet.

    (1 Mark)

    b) The magnet (or pendulum) decreases its swing, because the induced current is produced in the direction which opposes the change producing it. (1 Mark) Lenz's Law) (1 Mark)

    (2 Marks)

    c) Glass is an insulator (1 Mark) , so no (or negligible) current is produced in it and the pendulum is not affected by the presence of the glass. (1 Mark)

    (2 Marks)

    (Marks available: 5)

  2. a) North Pole

    (1 Mark)

    b) By Lenz's Law the induced current must oppose the change which causes it (1 Mark) - so it must repel the magnet.(1 Mark)

    (2 Marks)

    c) North pole (N) (1 Mark) - to attract the South pole of the magnet as it moves away (1 Mark).

    (2 Marks)

    (Marks available: 5)