Question 78 of 107

A wire cd of length l and mass m is sliding without friction on conducting rails ax and by as shown. The vertical rails are connected to each other with a resistance R between a and b. A uniform magnetic field B is applied perpendicular to the plane abcd such that cd moves with a constant velocity of

(1) mgRBl

(2) mgRB2l2

(3) mgRB3l3

(4) mgRB2l