A 100-turn coil of radius 10 cm carrying 3.2 A (moment 10 A m²) sits in a 2 T field, free to rotate. The torque when its plane is parallel to the field (m perpendicular to B) is:

1. Torque magnitude: $\tau = mB\sin\theta$. 2. When the plane of the coil is along the field, the moment m is perpendicular to B, so $\theta = 90^\circ$. 3. Then $\sin\theta = 1$, giving $\tau = mB$. 4. Substitute $m = 10$ A m², $B = 2$ T: $\tau = 10\times2 = 20$ N m. 5. The torque would be zero only when m is parallel to B (θ = 0). _Source: NCERT Class 12 Physics Ch 4 "Moving Charges and Magnetism", p.157_