Question:
A square loop of side $2 \mathrm{a}$, and carrying current $\mathrm{I}$, is kept in $\mathrm{XZ}$ plane with its centre at origin. A long wire carrying the same current I is placed parallel to the z-axis and passing through the point $(0, b, 0),(b>>a)$. The magnitude of the torque on the loop about zaxis is given by:
Correct Option: 1
Solution:
$\vec{\tau}=\overrightarrow{\mathrm{M}} \times \overrightarrow{\mathrm{B}}$
$=4 \mathrm{a}^{2} \mathrm{I} \times \frac{\mu_{0} \mathrm{I}}{2 \pi \mathrm{b}}$