Figure 6.20 shows a metal rod PQ resting on the smooth rails AB
and positioned between the poles of a permanent magnet. The
rails, the rod, and the magnetic field are in three mutual
perpendicular directions. A galvanometer G connects the rails
through a switch K. Length of the rod = 15 cm, B = 0.50 T,
resistance of the closed loop containing the rod = 9.0
mÃŽÂ©. Assume the field to be uniform.
(a) Suppose K is open and the rod is moved with a speed of
the direction shown. Give the polarity and magnitude of the
(b) Is there an excess charge built up at the ends of the
K is open? What if K is closed?
(c) With K open and the rod moving uniformly, there is
no net force on the electrons in the rod PQ even though
they do experience magnetic force due to the motion of the rod.
(d) What is the retarding force on the rod when K is
(e) How much power is required (by an external agent) to
keep the rod moving at the same speed (=12 cm
when K is closed? How much power is required when K is open?
(f) How much power is dissipated as heat in the closed
What is the source of this power?
(g) What is the induced emf in the moving rod if the
magnetic field is parallel to the rails instead of being
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