Single Phase AC Generator

What this figure shows. A single-turn rectangular loop PQRS is drawn fixed and stationary (perpendicular to the plane of the page), while a field magnet with poles N and S is shown able to rotate about a central axis with angular velocity $\omega$; the field direction $\vec B$ and the loop's normal $\hat n$ are marked, with the magnet drawn in its initial (horizontal) position where the field is exactly perpendicular to the plane of the loop PQRS. This starting configuration corresponds to the induced emf being zero at $t=0$ -- the field is entirely normal to the loop's plane at this instant, giving no rate of change of flux at that specific moment despite the flux itself being at its own maximum -- and sets up the reference point …

What this figure shows. Five snapshots of the loop PQRS and the rotating field magnet, at rotation angles $0^{\circ}$, $90^{\circ}$, $180^{\circ}$, $270^{\circ}$ and $360^{\circ}$, are drawn above a sine-wave graph of induced emf against time angle, with the corresponding points labelled O, A, B, C and D on the curve. At O ($0^{\circ}$) the field is perpendicular to the loop and the emf is zero; at A ($90^{\circ}$) the field is parallel to the loop's plane, the flux change is fastest, and the emf reaches its positive maximum $\varepsilon_m$, with current flowing along PQRS; at B ($180^{\circ}$) the field is again perpendicular to the loop and the emf returns to zero; at C ($270^{\circ}$) the field is once more parallel to the loop but with the loop's sides having swapped their relationship to the field, so the emf reaches its NEGATIVE maximum $-\varepsilon_m$, with current now flowing the reverse way along SRQP; and at D ($360^{\circ}$) the emf returns to ze …