The copper disc of radius r is rotated with
an angular speed, omega. A uniform magnetic
field of magnitude B is acting perpendicular
to the disc. We know from Faraday's law that
the magnitude of the e.m.f. induced is equal
to the rate of change of flux linkage and when
then total flux linkage of the disc doesn't
change as it rotates you can imagine it consisting
of lots of individual wires from the centre
to the edge which are rotating about and these are cutting
through magnetic field lines which means that
there must be an e.m.f. induced across them.
To calculate the e.m.f. calculate the rate
of flux swept out by a wire going from the
centre to the edge of this disc over an entire
rotation.
