A rectangular coil with sides of length a and b is placed in a magnetic field of flux density B and free to rotate about an axis perpendicular to the paper. A current of I Amps flows in the coil. The field exerts a force (F) on the sides of the coil of length b of = BNIb

Torque on the coil = BANI sinq |

where A = ab, the area of one face of the coil.

The maximum torque occurs when the plane of the coil is lying along the field lines (q = 90

Maximum torque (C_{o}) = BANI |