A rectangular coil of sides l and b carrying a current I is subjected to a uniform magnetic field `vecB` acting perpendicular to its plane. Obtain the expression for the torque on it.

Explanation

We know that a rectangular coil of area A = lb , carrying a current I, when placed in a uniform magnetic field B experiences a torque
`vec(tau) = (I vecA) xx vecB`.
As per question the magnetic field `vecB` acts perpendicular to the plane of the coil. It means that `vecB` is parallel to `vecA`. Hence
Torque `|vec(tau)| = IAB sin theta = IAB sin 0^@ = 0`