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This question, on the forces which act on a current-carrying coil in a magnetic field, had a facility
of 63%, which was a substantial improvement on the pre-examination facility. It was not a
particularly effective discriminator.
This question contained a synoptic element, because two of the unit combinations quoted were
mechanical. The facility of the question was 44%.
Overlooking the word not in the stem of the question presumably caused 34% of the candidates
to choose distractor A, where F = B I l ought to have shown that A T m is a correct unit of force.
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This question was a test of F = B // that, with a facility of 86%, proved to be the least demanding
question in this Section A. When the question was pre-tested just over half of the students
selected the correct response.Fleming’s left hand rule, as applied to a beam of positive ions
Most candidates were able to use Fleming’s left hand rule in order to give the correct force
direction in part (a) (i). Sometimes a candidate’s answer was contradictory and went unrewarded,
for example ‘downwards towards the S pole’. Most answers to part (a) (ii) were reasonably good
when explaining why the wire would vibrate, but rarely explained why these vibrations are
vertical. An explanation by reference to the mutually perpendicular field, current and force
directions was required in a complete answer. The reversal of force direction with change of
current direction was well understood.
Fewer candidates made reference to the continuous current reversals brought about by ac
causing the process to repeat, or to the fact that the size of the current affects the magnitude of
the magnetic force.
It was evident that a large number of candidates had made a second, more enlightened, attempt
at part (b) once they had realised that direct substitution of I = 2.4 A into F = BIL did not lead to
the value of force (about 40 mN) they had been asked to show. Once they realised that the
maximum force is caused by the peak current, it became a straightforward matter to secure three
marks.
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