O-LevelPhysicsMagnetism5054/22 · Oct/Nov 2019 · Q77 Marks

[Figure 7.1] shows a horizontal, rectangular coil ABCD placed between a magnetic N-pole and an S-pole. There is a current in the coil. An upward force F of size 9.6 × 10⁻³ N acts on side AB of the coil.

(a)(i)Explain why there is a force on side AB.[1]
(a)(ii)Determine the direction of the current in AB and state how the direction is deduced.[2]
(b)Both side AB and side CD of the coil are 2.5 cm from the axis. Determine the total moment acting on the coil.[2]
(c)(i)The coil in Fig. 7.1 is part of a direct current (d.c.) motor. State the name of the device that connects the coil of a d.c. motor to the electricity supply.[1]
(c)(ii)State one change to the arrangement in Fig. 7.1 that produces a greater turning effect in coil ABCD.[1]

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 (Oct/Nov 2019). The mark or mark type is shown in brackets.

[B1] current and magnetic field / magnetic flux (experiences a force) or the two magnetic fields interact
[B1] B to A
[B1] left-hand rule mentioned / described
[C1] (Γ=) Fx or 2 x 9.6 x 10⁻³ x 0.025 or 9.6 x 10⁻³ x 0.025 or 2.4 × 10⁻⁴ (Nm) or 0.024 (N cm)
[A1] 4.8 x 10⁻⁴ Nm or 0.048 N cm
[B1] split-ring commutator or (carbon) brushes
[B1] (soft-)iron core or stronger magnet(s) or larger current or more turns or smaller resistance or greater electromotive force (e.m.f.)

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