A disc of radius $R$ rotates with a constant angular velocity $\omega$ about its own axis. Surface charge density of this disc varies as $a=\alpha r^2$ where $\text{'}r\text{'}$ is the distance from center of the disc. The magnetic field intensity at the center of the disc is

The rectangular coil of area $A$ is in a field $B.$ Find the torque about the $Z$-axis when the coil lies in the position shown and carries a current $I.$

A $100$ turns coil shown in figure carries a current of $2 \mathrm{A}$ in a magnetic field $B=0.2 \mathrm{Wb} {\mathrm{m}}^{-2}.$ The torque acting on the coil is