n ˆ

n ˆ b Figure . Side view of current loop The magnitude of torque acting on the arm PQ about AB is τ PQ b IaB =       sin q and it points in the direction of AB. The magnitude of the torque acting on the arm RS about AB is τ RS b IaB =       sin q and points also in the same direction AB as shown in Figure . . The total torque acting on the entire loop about an axis AB is given by τ =       +       b b Ia b PQ RS sin sin ( sin ) τ = IAB sin q along the direction AB . TORQUE ON A CURRENT LOOP The force on a current carrying wire in a magnetic field is responsible for the motor operation. . . Torque on a current loop placed in a magnetic field Consider a rectangular loop PQRS carrying current I is placed in a uniform magnetic field B . Let a and b be the length and breadth of rectangular loop respectively. The unit vector n ˆ normal to the plane of the loop makes an angle θ with the magnetic field as shown in Figure . . F PQ F QR F RS F SP θ n ^ a b Figure . Rectangular coil placed in a magnetic field The magnitude of the magnetic force acting on the current-carrying arm PQ is IaB IaB PQ = sin p . The direction of the force is found using right hand cork screw rule and its direction is upwards. The magnitude of the force on the arm QR is IbB IbB QR =