JEE Main · 2021 · 18 Mar 2021 · Shift 1 · Magnetic Effects of Current

The torque $\vec{\tau }=\vec{\mu }\times \vec{B}$ on a current loop turns it until its magnetic moment $\vec{\mu }$ points along $\vec{B}$, and $\vec{\mu }$ is perpendicular to the plane of the loop, so the plane settles normal to the field. In that orientation the force $Id\vec{\ell }\times \vec{B}$ on every element of the wire points radially outward, so the flexible wire is pushed out until it encloses the largest area its fixed perimeter allows. That shape is a circle. The loop therefore takes a circular shape with its plane normal to the field.