Physics (AS & A Level) · Oct / Nov 2016 · Paper 42 · Question 9
9702
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A stiff wire is held horizontally between the poles of a magnet, as illustrated in Fig. 9.1. S N stiff wire Fig. 9.1 When a constant current of 6.0 A is passed through the wire, there is an additional downwards force on the magnet of 0.080 N.
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9(a)[3]
(by Newton’s third law) force on wire is up(wards) by (Fleming’s) left-hand rule/right-hand slap rule to give current in direction left to right shown on diagram [3]
9(b)[4]
force ∝ current or F = BIL or B (= 0.080 / 6.0L) = 1 / 75L maximum current = 2.5 × √2 = 3.54 A maximum force in one direction = (3.54 / 6.0) × 0.080 = 0.047 N difference (= 2 × 0.047) = 0.094 N or force varies from 0.047 N upwards to 0.047 N downwards [4]
10 nuclei emitting r.f. (pulse) Larmor frequency/r.f. frequency emitted/detected depends on magnitude of magnetic field nuclei can be located (within a slice) changing field enables position of detection (slice) to be changed [4]
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