Torque on a magnetic dipole in a magnetic field:

The forces on the two poles are equal and opposite. They form a couple, which is given by

τ = force * distance

= mB * sin θ

Special cases:

1. When the magnet lies along the direction of magnetic field. θ = 0⁰, sin 0 = 0, τ = 0, Thus torque is minimum.
2. When the magnet lies perpendicular to the direction of magnetic field. θ = 90°, sin 90 = 1, τ = mB, thus torque is maximum.