Q. The square loop $ABCD$, carrying a current $i$, is placed in uniform magnetic field $B$, as shown. The loop can rotate about the axis $XX'$. The plane of the loop makes an angle $\theta\ (\theta < 90{^\circ})$ with the direction of $B$. Through what angle will the loop rotate by itself before the torque on it becomes zero
A) $\theta$
B) $90{^\circ} - \theta$
C) $90{^\circ} + \theta$
D) $180{^\circ} - \theta$
Explanation
(c) In the position shown. $AB$ is outside and $CD$ is inside the plane of the paper. The Ampere force on $AB$ acts into the paper. The torque on the loop will be clockwise, as seen from above. The loop must rotate through an angle $(90{^\circ} + \theta)$ before the plane of the loop becomes normal to the direction of the direction of $B$ and the torque becomes zero
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.