A-LevelPhysicsMagnetic fields9702/42 · May/June 2018 · Q99 Marks

State what is meant by the magnetic flux linkage of a coil.

(a)State what is meant by the magnetic flux linkage of a coil.[3]
(b)A coil of wire has 160 turns and diameter 2.4 cm. The coil is situated in a uniform magnetic field of flux density 7.5 mT, as shown in Fig. 9.1. The direction of the magnetic field is along the axis of the coil. The magnetic flux density is reduced to zero in a time of 0.15 s. Show that the average e.m.f. induced in the coil is 3.6 mV.[2]
(c)The magnetic flux density B in the coil in (b) is now varied with time t as shown in Fig. 9.2. Use data in (b) to show, on Fig. 9.3, the variation with time t of the e.m.f. E induced in the coil.[4]

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Mark scheme

Marking points from the official mark scheme (May/June 2018). The mark or mark type is shown in brackets.

[B1] (magnetic) flux density × area
[B1] magnetic flux density normal to area or reference to cross-sectional area or × sin (angle between B and A)
[B1] x number of turns on coil
[C1] e.m.f. = BAN/t or e.m.f = rate of change of flux linkage
[A1] = (7.5 × 10⁻³ × π × {1.2 × 10⁻²}² × 160)/0.15 = 3.6 x 10⁻³ V
[B1] sketch: zero for 0–0.10 s, 0.25–0.35 s, and 0.425–0.55 s, and non-zero outside these ranges
[B1] two horizontal steps, with zero voltage either side
[B1] with same polarity
[B1] correct values (1st step 3.6 mV and 2nd step 7.2 mV)

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