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 (6