Q. A current $i$ flows in a circular coil of radius $r$. If the coil is placed in a uniform magnetic field $B$ with its plane parallel to the field, magnitude of the torque that acts on the coil is
A) Zero
B) $2\pi riB$
C) $\pi r^{2}iB$
D) $2\pi r^{2}iB$
Explanation
(c) As shown in the following figure, the given situation is similar to a bar magnet placed in a uniform magnetic field perpendicularly. Hence torque on it
$\tau = MB\sin{90{^\circ}} = (i\pi r^{2})B$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.