A-LevelPhysicsMagnetic fields9702/42 · May/June 2021 · Q99 Marks

State two situations in which a charged particle in a magnetic field does not experience a force. 1. 2.

(a)State two situations in which a charged particle in a magnetic field does not experience a force. 1. 2.[2]
(b)A loosely coiled metal spring is suspended from a fixed point, as shown in [Figure 9.1]. Electrical connections are made to the ends of the spring by means of a flexible lead. The length of the spring is measured before the switch is closed and then again after the switch is closed. When the switch is closed, a magnetic field is set up around each coil of the spring. By reference to these magnetic fields, explain why there is a change in length of the spring. State whether the spring extends or contracts.[4]
(c)With the switch in (b) closed, the small mass on the free end of the spring is now made to oscillate vertically. Use the principles of electromagnetic induction to explain why small fluctuations in the current in the spring are found to occur.[3]

Mark scheme

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

[1] (particle is) stationary/not moving
[1] (particle is) moving parallel to the (magnetic) field
[1] magnetic field around each coil is circular OR each coil is normal to magnetic field due to adjacent coils
[1] current in coil interacts with (magnetic) field to exert force (on coil)
[1] force is normal to both coil and magnetic field OR force parallel to axis (of coil)
[1] forces between coils are attractive so spring contracts
[1] (oscillating) coils cut magnetic flux OR as separation of coils changes, magnetic flux changes
[1] cutting flux causes induced e.m.f. in coils
[1] changing (induced) e.m.f. causes changing current (in coil)

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