When a wire loop (coil) is rotated in a uniform magnetic field, it functions as an AC generator. The magnetic flux linked with the coil changes periodically, inducing an electromotive force (emf) given by the expression $\varepsilon = \varepsilon_0 \sin(\omega t)$ .

This induced emf is alternating in nature (sinusoidal). In one complete revolution, the coil rotates through $360^\circ$ ($2\pi$ radians). The value of $\sin(\omega t)$ changes from positive to negative (and vice versa) every half-cycle (i.e., every $180^\circ$ or $\pi$ radians).

Therefore, in one full revolution (one complete cycle), the polarity (direction) of the induced emf changes twice .