Fig. 16.1 shows a coil rotating in a uniform magnetic field to produce an alternating voltage. (a) State the position of the coil when the induced e.m.f. is...

(a) Position of coil at maximum e.m.f. [1]

The induced e.m.f. is maximum when the coil is parallel to the magnetic field (horizontal, with its plane aligned along the field lines).

(b) Why the e.m.f. is maximum at this position [2]

At this position, the sides of the coil are moving perpendicular to the magnetic field lines. This means they are cutting through the field lines at the greatest possible rate. Since the induced e.m.f. depends on the rate of change of magnetic flux (the rate of cutting field lines), this position gives the maximum e.m.f.

(c) Position of coil at zero e.m.f. [1]

The induced e.m.f. is zero when the coil is perpendicular to the magnetic field (vertical, as shown in position P in the diagram). The plane of the coil is at right angles to the field lines.

(d) Why the e.m.f. is zero at this position [1]

At this position, the sides of the coil are moving parallel to the magnetic field lines. They are not cutting any field lines, so there is no change in flux through the coil and no e.m.f. is induced.

(e) Difference between slip rings and split-ring commutator [2]

The split-ring commutator reverses the connection to the external circuit every half-turn, so the output is always in the same direction (rectified). Slip rings maintain continuous connection, preserving the alternating nature of the generated e.m.f.