Question

The square loop ABCD, carrying a current i, is placed in uniform magnetic field B, as shown. The loop can rotate about the axis XX'. The plane of the loop makes and angle θ  (θ  < 90°) with the direction of B. Through what angle will the loop rotate by itself before the torque on it becomes zero 

(a) θ 

(b) 90°– θ 

(c) 90° + θ 

(d) 180°– θ 

Answer 1

Correct option (b) 90° + θ

Explanation: 

In the position shown, AB is outside and CD is inside the plane of the paper. The Ampere force on AB acts into the paper. The torque on the loop will be clockwise, as seen from above. The loop must rotate through an angle (90° + θ ) before the plane of the loop becomes normal to the direction of B and the torque becomes zero.