A rectangular current carrying loop PQRS is kept in a uniform magnetic field as shown in the figure.

(a) What is the direction of magnetic moment of the current loop?
(b) When is the torque acting on the loop
(i) maximum, (ii) zero?

Explanation

(a) Magnetic moment of the current loop, `vecM=IvecA`. Its direction is normal to the loop inwards the plane of coil.
(b) Torque on the loop, `tau=MB sin alpha`
(i) Torque is maximum if `alpha=90^@`, i.e., the plane of coil is parallel to the direction of magnetic field.
(ii) Torque is zero if `alpha=0^@`, i.e., when plane of the loop is perpendicular to the magnetic field.