It works on the principle of electromagnetic induction, i.e. when a coil continuously rotates in a magnetic field, the magnetic flux associated with it keeps on changing; thus an emf is induced in it.
(b) When the coil rotates in a magnetic field, its effective area, i.e. (i.e. area normal to the magnetic field) keeps on changing. Hence magnetic flux keeps on changing.
(c) Let the coil be rotating with angular velocity
at any instant 't' when the normal lo the plane of the coil makes an angle with the magnetic field. Hence magnetic flux,