RI 2004 Prelims P2 Q6
The diagram below shows an end-on view of a rectangular coil that is free to rotate about an axis between two magnetic poles. Current is passed into the coil via end X and out of the coil via end Y.
(a) Explain the origin of the force acting on end X of the coil, that is, how the force is produced.
(b)
(i) On the figure, draw the instantaneous forces acting on X and Y.
(ii) State how the size of the turning moment on the coil is changing at the instant shown in the figure.
(iii) Suggest one modification that can be made to the coil in the figure in order to obtain larger turning moments in general.
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Answer:
(a) A current carrying coil produces a magnetic field. This magnetic field interacts with the magnetic field from the magnet to produce regions of weaker and stronger magnetic fields. Such different regions of field strength causes resultant force to be produced.
(b)
(i)
(ii) The size of the turning moment decreases since the perpendicular distance from the axle of rotation decresaes when the coil approaches vertical position.
(iii) Wind more turns of wires on the coil or win the coil around an armature (soft iron core)
