The rotation of the coil causes the magnetic flux through
it to change, so an emf is induced in the coil The ends of the
coil are connected to an external circuit by means of slip rings
and brushes When the coil is rotated with a constant angular speed w, the angle q
between the magnetic field vector B and the area vector A of the coil at any
instant t is q  = wt (assuming q = 0° at t = 0) As a result, the effective area
of the coil exposed to the magnetic field lines changes with time, and from
Eq