A-LevelPhysicsMagnetic fields9702/41 · 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]
This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.
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] is such as to oppose the change causing it
- [1] (b) ring cuts (magnetic) flux and causes induced e.m.f. in ring
- [1] (induced) e.m.f. causes (eddy/induced) currents (in ring)
- [1] currents (in ring) cause magnetic field (around ring)
- [1] two fields interact to cause resistive/opposing force
- [1] current (in ring) is in a magnetic field
- [1] which causes resistive force
- [1] currents (in ring) dissipate thermal energy
- [1] (thermal) energy comes from energy of oscillations
- [1] (c) current cannot pass all the way around the ring
- [1] (induced) currents smaller
- [1] smaller resistive force (so more oscillations)
- [1] smaller rate of dissipation of energy (so more oscillations)
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