Moving Charges and Magnetism — Physics STD 12 Science — Question

The torque on current loop is $\tau=\text{MB}\sin\theta$, where $\theta$ is angle between magnetic moment and magnetic field.

$\Rightarrow\text{M}=\frac{\tau}{\text{B}\sin\theta}$

If B or 1 T, $\sin\theta=1$ or $\theta=90^\circ$ then $\text{M}=\tau$.

That is the magnetic moment of a current loop is defined as the torque acting on the loop when placed in a magnetic field of 1 T such that the loop is oriented with its area vector normal to the magnetic field.

Also, M = IA

i.e., magnetic moment of a current loop is the product of current flowing in the loop and area of loop. Its direction is perpendicular to the plane of the loop and determined by using right hand thumb rule.

Magnetic moment of revolving electron,

$\text{M}=\frac{\text{evr}}{2}$