Exam-style Questions: Forces in Magnetic Fields

  1. Fig. 7.1 shows the magnetic field between the two pole pieces of a large U-shaped magnet, with the north pole vertically above the south pole. When the strength of a magnetic field is measured along the lineAB, it is found to vary as shown in Fig. 7.2.

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    Fig. 7.1

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    Fig. 7.2.

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    Fig. 7.3

    a) Describe in words how the magnetic flux linkage in a coil changes as the coil in Fig. 7.1 moves from A to B.

    (3 marks)

    b) State Faraday's law of electromagnetic induction.

    (2 marks)

    c) Draw, on the axes provide in Fig. 7.3, a graph to show how the e.m.f .induced in the coil varies as the coil moves from A to B.

    (4 marks)

    (Marks available: 9)

  2. A diagram of a loudspeaker is shown in Fig. 6.1.

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    The thin copper wire is wound onto a paper tube that surrounds the south pole of the circular magnet. The copper coil has 250 turns and has a mean radius of 1.5 cm. The magnet provides a field of magnetic flux density 3.6 x 10-2 T at right angles to the wire.

    a) Show that the length of the copper wire in the magnetic field is about 24 m.

    (2 marks)

    b) Calculate the magnitude of the force acting on the copper wire due to the magnetic field when carrying a constant current of 48 mQ.

    force = ..................... N

    (3 marks)

    (Marks available: 5)

  3. a) Fig. 6.1 shows the magnetic field pattern for a current-carrying conductor placed between the poles of a permanent magnet.

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    i) State the direction of the current in the conductor.

    (1 mark)

    ii) On Fig. 6.1, mark with a cross (X) a point between the poles of the magnet where the magnetic field is weakest.

    (1 mark)

    b) Like the Earth, the planet Jupiter has its own magnetic field.

    A small Spacecraft orbuting Jupiter records a tiny force of 3.0 x 10-6 N experienced on a 2.7 m long conductor. The conductor carries a current of 200 mA and is at right angles to the magnetic field.

    Determine the magnitude flux density B at the position of the spacecraft.

    B = ..................... unit .................

    (3 marks)

    (Marks available: 5)

Answer

Answer outline and marking scheme for question:

  1. a) increases to a constant value then decreases

    (3 marks)

    b) induced e.m.f proportional to rate of change of flux linkage

    (2 marks)

    c) pulses of reasonable shape in opposite directions

    (4 marks)

    (Marks available: 9)

  2. a) length = 2pr (x N)

    length = 2p x 0.015 x 250

    length = 23.57 (m) ˜ 24 (m) (9.4 x 10-2 (m) scores 1/2)

    (2 marks)

    b) F = BIL

    F = 3.6 x 10-2 x 48 x 10-3 x 24 (-1 for 10n error)

    F = 4.147.. x 10-2˜ 4.1 x 10-2 (N) (F ˜ 4.1 x 10-2 (N) if length 23.57 m is used)

    (Allow ecf from a)(i) but -1 mark for not using given value of 24 m)

    (3 marks)

    (Marks available: 5)

  3. a) i) Into (plane of) paper.

    (1 mark)

    ii) Correct region to the left of the conductor

    (1 mark)

    b) F = BIL (Allow any subject)

    B = 3.0 x 10-6 / (0.2 x 0.027)

    B = 5.56 x 10-4˜ 5.6 x 10-4

    unit: tesla / T / NA-1m-1 / Wb m-2

    (3 marks)

    (Marks available: 5)