An example for a graph representing an a.c. generator (a rectangular coil of wire rotating in between a magnet) would be a sine graph, because the surface for magnetic flux are the arms of the coil or the conductors AB and CD. However, if the surface has been changed to the area of the rectangular coil, which is the area where the magnetic field of the magnets pass through, then would the graph still be the same?
If the graph is still the same, then my guess would be that when the coil of wire is vertical (the surface for flux is still the area of the coil), the magnetic flux would be at its maximum, and because of it being maximum, the rate of change of magnetic flux would become 0, because at that point, the magnetic flux cannot exceed the maximum over time. And, hence the induced e.m.f will also be 0. But then, it doesn't explain why the induced e.m.f will be positive at first, because there's a negative rate of change of magnetic flux from its vertical to horizontal position from 0 to 1/4 of a revolution
Best Answer
Your graph is a plot of emf vs time. The cosine depends on the angular position of the loop. The amplitude depends on the area.