The a.c. generator — Sketch

Sketch a graph of e.m.f. against time for a simple a.c. generator and relate its shape to the position of the rotating coil.

The graph is a sine wave, alternating between positive and negative peaks. The e.m.f. is at a maximum (peak) when the plane of the coil is parallel to the magnetic field, because the coil sides are then cutting field lines at the fastest rate. The e.m.f. is zero when the plane of the coil is perpendicular to the field, because at that instant the coil sides are moving parallel to the field lines and cutting none.