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# Figure shows a metal rod PQ resting on the smooth rail AB and positioned between the poles of a permanent magnet. The rail, 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 = $9.0 m \Omega$. Assume the field to be uniform. 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. Explain.

(a) The force experienced by electric field is balanced by the magnetic force on the electrons.
(b) The force experienced by electric field is more than the magnetic force on the electrons.
(c) The force experienced by the magnetic field on the electrons is more than the electric field.
(d) none of the above

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A)
When the key is open, there is no net force on the electrons, because the presence of excess charge at P and Q sets up an electric field and magnetic force on the electrons is balanced by force on them due to force by the electric field. Hence there is no net force on the rod.