Question

A conducting circular loop of radius r carries a constant current z. It is placed in a uniform magnetic field $\vec{B}_{0}$ such that $\vec{B}_{0}$ is perpendicular to the plane of the loop. The magnetic force acting on the loop is

• A. $ir\, B_{0}$
• B. $2\pi\, ir\, B_{0}$
• C. zero
• D. $\pi\, ir\, B_{0}$

Answer 1

Answer: (C)

zero

Answer 2

The magnetic field is perpendicular to the plane of the paper. Let us consider two diametrically opposite elements. By Fleming's Left hand rule on element AB the direction of force will be Leftwards and the magnitude will be
$d F=Id/ B sin \, 90? = Id/B$
On element CD, the direction of force will be towards right on the plane of the papper and the magnitude will be dF = Id/B.