A special metal S conducts electricity without any resistance. A closed wire loop, made of S, does not allow any change in flux through itself by inducing a suitable current to generate a compensating flux. The induced current in the loop cannot decay due to its zero resistance. This current gives rise to a magnetic moment which in turn repels the source of magnetic field or flux. Consider such a loop, of radius a, with its centre at the origin. A magnetic dipole of moment m is brought along the axis of this loop from infinity to a point at distance r(>> a) from the centre of the loop with its north pole always facing the loop, as shown in the figure below. The magnitude of magnetic field of a dipole m, at a point on its axis at distance r, is where to is Hom 2 r the permeability of free space. The magnitude of the force between two magnetic dipoles with moments, m and m, separated by a distance r on the common axis, with their north poles facing each other, is km, m, where k is a constant of appropriate dimensions. The direction of this force is along the line 9 r joining the two dipoles. a When the dipole m is placed at a distance r from the center of the loop (as shown in the figure), the current induced in the loop will be proportional to (A) m/r (B) m/r (C) m/r (D) m/r