Questions on electromagnetism
1. magnetic flux = BA 1
meanings of B and A, i.e. flux density or field strength and area to it 1
magnetic flux linkage refers to the flux linking/passing through a coil; 1
and equals N × flux where N is the number of turns (of the coil) 1
Faraday’s law: induced e.m.f./voltage is proportional to rate of change of flux
linkage through it /correct mathematical formulation/AW 1
Lenz’s law: the direction of the induced e.m.f./voltage is such as to
oppose the motion/change that produced it 1
relationship of Lenz’s law to conservation of energy or other valid
explanation/discussion/description 2
max 5 marks
quality of written communication 2
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2. (a) (i) F is towards ‘open’ end of tube; using Fleming’s L.H. rule 2
(ii) F = BIw 1
(iii) F = 0.15 × 800 × 0.0025; = 3.0 (N) 2
(b) (i) A voltage is induced across moving metal as it cuts lines of flux/AW; (1)
voltage is proportional to flux change per second/AW; (1)
the flux change per second is Bwv / is proportional to the area of
metal moving through the field per second / is proportional to v (1)
or Faraday’s law fully stated; with reasonable attempt to; (2)
relate flux linkage per second proportionally to speed (1) 3
(ii) flux (linkage) doubles; so using Faraday’s law V doubles/AW 2
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3. sine or cosine wave of regular period and amplitude (1)
V doubles when the speed v of rotation of the coil doubles; (1)
when v doubles the rate of change of flux linking the coil doubles; (1)
the frequency of the a.c. signal doubles/period halves/AW (1)
V doubles when the number n of turns on the coil doubles; (1)
when n doubles there is twice as much flux linking the coil/AW; (1)
the frequency/period of the signal is unchanged; (1)
without iron core flux linking coil is much less/flux would spread in all
directions/flux not channelled through low reluctance path/AW (1)
amplitude of output voltage is smaller (1)
actually is tiny/negligible/mV rather than V max 7
Quality of Written Communication 2
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4. (i) I = V/R = 12/50 (1)
= 0.24 A (1) 2
(ii) Power in primary = power in secondary / IpVp = IsVs (1)
Ip = 0.24 × 12 / 230 = 0.0125 A (1) 2
[4]
5. (a) (i) BA / = 0.05 × 0.05 × 0.026; = 6.5 × 10–5; Wb/T m2 3
(ii) BA sin 45°/BAcos 45° = 4.6 × 10–5 Wb ecf (a)i 1
(iii) 0 1
(b) (i) a point where curve crosses t-axis 1
(ii) voltage is proportional to the rate of change of flux linking the coil; 1
rate of flux change is zero/very small when the flux linking the
coil is a maximum 1
(iii) sinusoidal curve; of double the amplitude; and half the period 3
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6. (a) (i) equally spaced horizontal parallel lines from plate to plate (1)
arrows towards cathode (1) 2
(ii) ½ mv2 = qV; v = √(2eV/m) = √(2 × 1.6 × 10–19 × 7000/9.1 × 10–31) so (1)
v = 4.96 × 107 (m s–1) (1) 2
(b) (i) arrow perpendicular to path towards centre of arc (1) 1
(ii) out of paper/upwards;using Fleming’s LH rule (for conventional
current) (2) 2
(iii) mv2/r; = Bqv; r = mv/Bq = ;= 9.4 × 10–2
(m) 4
(c) change magnitude of current in coils to change field; (1)
change field to change deflection; (1)
reverse field/current to change deflection from up to down (1) max 2 2
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