A-LevelPhysicsMagnetic fields9702/43 · May/June 2021 · Q109 Marks

State Lenz's law.

(a)State Lenz's law.[2]
(b)A metal ring is suspended from a fixed point P by means of a thread, as shown in [Figure 10.1]. The ring is displaced a distance d and then released. The ring completes many oscillations before coming to rest. The poles of a magnet are now placed near to the ring so that the ring hangs midway between the poles of the magnet, as shown in [Figure 10.2]. The ring is again displaced a distance d and then released. Explain why the ring completes fewer oscillations before coming to rest.[4]
(c)The ring in (b) is now cut so that it has the shape shown in [Figure 10.3]. Explain why, when the procedure in (b) is repeated, the cut ring completes more oscillations than the complete ring when oscillating between the poles of the magnet.[3]

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Mark scheme

Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.

[1] (a) direction of (induced) e.m.f.
[1] (a) is such as to oppose the change causing it
[1] (b) ring cuts (magnetic) flux and causes induced e.m.f. in ring
[1] (b) (induced) e.m.f. causes (eddy/induced) currents (in ring)
[1] (b) currents (in ring) cause magnetic field (around ring)
[1] (b) two fields interact to cause resistive/opposing force OR current (in ring) is in a magnetic field which causes resistive force OR currents (in ring) dissipate thermal energy (thermal) energy comes from energy of oscillations
[1] (c) current cannot pass all the way around the ring
[1] (c) (induced) currents smaller
[1] (c) smaller resistive force (so more oscillations) OR smaller rate of dissipation of energy (so more oscillations)

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