When a rectangular coil is rotated in a uniform magnetic field about an axis passing through its centre and perpendicular to field, then magnetic flux associated with coil is given as φ=BA‌cos‌θ=BA‌cos‌ω‌t....(i) ∴ Induced emf, e=−‌
dφ
dt
=−‌
d
dt
BA‌cos‌ω‌t [From Eq. (i)] =+BAω‌sin‌ω‌t ⇒‌‌e=e0‌sin‌ω‌t....(ii) where, e0 is the peak value of induced emf. i.e. ‌‌e0=BAω From Eq. (ii) it is clear that, induced emf varies sinusoidally.