A thin wire of length ' $L$ ' made of an insulating material is bent to form a circular loop and a positive charge ' $\mathrm{q}$ ' is given so that it is distributed uniformly around the circumference of the loop. The loop is then rotated with an angular speed ' $\omega$ ' about an axis passing through its centre. If a uniform magnetic field B directed parallel to the plane of the loop is applied then the magnitude of the magnetic torque on the loop is