Fig. 14.1 shows a straight wire placed between the poles of a U-shaped magnet. The wire is connected to a sensitive centre-zero galvanometer. (a) The wire i...

(a) Galvanometer observation when the wire moves upwards

The galvanometer shows a deflection (reading) in one direction. [1]

Moving the wire upwards through the magnetic field causes it to cut across field lines, inducing an e.m.f. that drives a current through the circuit.

(b) Galvanometer reading when the wire moves downwards

The galvanometer deflects in the opposite direction compared to part (a). [1]

Reversing the direction of motion reverses the direction of the induced current.

(c) Law that predicts the direction of the induced current

Lenz's law. [1]

Lenz's law states that the direction of the induced current is such that it opposes the change producing it. This is a consequence of the conservation of energy.

(d) Why no reading when the wire moves from N to S (along the field)

When the wire moves horizontally along the direction of the magnetic field (from N to S), it is moving parallel to the field lines rather than cutting across them. Since no field lines are being cut, there is no change in magnetic flux linkage, no e.m.f. is induced, and the galvanometer shows no reading. [1]

(e) Two changes that would produce a larger deflection

Any two from the following:

1. Move the wire faster through the magnetic field (the rate of cutting field lines increases, producing a larger e.m.f.). [1]
2. Use a stronger magnet (more field lines are available to be cut per unit time). [1]
3. Use a longer piece of wire between the poles (more of the conductor is within the field, so a greater e.m.f. is generated). [1]