The square loop ABCD, carrying a current i, is placed in a uniform magnetic field B, as shown. The loop can rotate about the axis XX^'. The plane of the loop makes an 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
 

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 

  before the plane of the loop becomes normal to the direction of B and the torque becomes zero.