A conducting ring of mass 2 kg and radius 0.5 m is placed‘on a smooth horizontal plane. The ring carries a current of I =4A. A horizontal magnetic field B = 10 T is switched on at time t = 0 as shown in figure. The initial angular acceleration of the ring will be

A non-conducting non-magnetic rod having circular cross section of radius R is suspended from a rigid support as shown in the figure. A light and small coil of 300 turns is wrapped tightly at the left end of the rod where uniform magnetic field B exists in vertically downward direction.Air of density $\rho$ hits the half of the right part of the rod with velocity V as shown in the figure. What should be current in clockwise direction (as seen from O) in the coil so that rod remains horizontal? Give answer in m A. Given

$\frac{2}{Lv}\sqrt{\frac{\pi RB}{\rho }}=\frac{1}{\sqrt{5}}{ \mathrm{A}}^{-1/2}$