Q 12-04-209JEE MainJEE Main 2019 (8 Apr, Shift 1)Easy
A circular coil having $N$ turns and radius $r$ carries a current $I$. It is held in the XZ plane in a magnetic field $B\hat i$. The torque on the coil due to the magnetic field is
Answer: (A) $B\pi r^2IN$
A coil in the XZ plane has its magnetic moment along $\pm\hat j$, with $|\vec m| = NI\pi r^2$. The field is along $\hat i$, perpendicular to $\vec m$, so
$$\tau = mB\sin90^\circ = B\pi r^2IN$$
Solution by Sreeraj P, M.Sc Physics