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

Question

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-

G1 Physics · Accepted Answer

Torque is zero when magnetic moment μ⃗ is parallel or antiparallel to B⃗. Initially the plane makes angle θ with B, so μ makes 90°-θ with B. It rotates through 90°-θ to reach stable alignment. Hence B.