Fig. 12.1 shows two coils of wire, coil P and coil Q, placed close together on a bench. Coil P is connected to a battery through a switch S. Coil Q is conne...

Marking Scheme and Explanation

(a) The changing magnetic field from coil P induces an e.m.f. in coil Q. [1]

When the switch is closed, current begins to flow in coil P, building up a magnetic field. The field lines from P pass through Q (mutual induction). While the field is changing, an e.m.f. is induced in Q, and the galvanometer deflects.

(b) The deflection is only brief because the galvanometer deflects only while the current in P is changing (while the magnetic field is building up). [1]

Once the current reaches its steady value, the magnetic field through Q becomes constant. A constant field produces no change in flux, so no e.m.f. is induced and the galvanometer returns to zero. [1]

This is a fundamental principle of electromagnetic induction: it is the rate of change of flux that matters, not the flux itself. A steady current in P produces a steady field, which produces zero induced e.m.f. in Q.

(c) When the switch is opened, the current in P drops from its steady value to zero. [1]

The magnetic field through Q decreases. This is a change in the opposite direction to the increase that occurred when the switch was closed. The induced e.m.f. (and therefore the deflection) is in the opposite direction. [1]

By Lenz's law, when the switch was closed, the induced current in Q opposed the increasing flux (tried to prevent the build-up). When the switch is opened, the induced current opposes the decreasing flux (tries to maintain the field), requiring current in the opposite direction.

(d) Any one of: place a soft iron core through both coils / increase the number of turns on either coil / move the coils closer together / open and close the switch more rapidly. [1]

A soft iron core channels the magnetic flux from P through Q more efficiently, increasing the flux linkage and the induced e.m.f. More turns on P creates a stronger field; more turns on Q generates more e.m.f. from the same flux change. Closer coils also increase the fraction of P's flux that links Q.