Question

A circular loop of conducting wire, carrying a current I in anticlockwise sense, lies in x-y plane as shown in figure. In each statement of column-I, a external constant magnetic field is given, which exist in space containing the loop. The term B_o given in column-I is positive constant. With the given magnetic field in each situation of column-I, match the results given in column-II.

Column-I	Column-II
	(p) Resultant magnetic force due to external magnetic field on the loop is zero
	(q) Magnitude of torque due to external magnetic field on loop is non zero
	(r) Magnetic potential energy of current carrying loop in presence of external magnetic field is nonnegative.
	(s) Magnetic force on the loop has tendency to expand the loop

Answer 1

Correct option (A) p,q,r (B) p,q,r, (C) p,s (D) p,q,r

Explanation:

(A), (B) and (D): In all these three cases, uniform and constant magnetic field is in plane of the loop, Hence angle between magnetic field and dipole moment of loop is 90° . Therefore magnetic potential energy is zero, that is, nonnegative. Torque is maximum in magnitude. Force on a current carrying loop in uniform magnetic field is always zero. The magnetic field only causes the the loop to rotate, as it acts at each point on the loop in direction perpendicular to plane of the loop. Hence magnetic force on the loop has no tendency to expand the loop. 

(C) In this option, dipole moment of loop and external magnetic field are in same direction, hence magnetic potential energy is negative and net torque on loop is zero. Magnetic field exert zero net zero force on loop in such a way that, on each point on loop magnetic force is radially outwards leading to tendency of expansion of loop.