Question

Assertion :A rectangular current loop is in an arbitrary orientation in an external uniform magnetic field. No work is required to rotate the loop about an axis perpendicular to its plane. Reason: All positions represent the same level of energy.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Assertion is incorrect but Reason is correct

Answer 1

The correct option is A Both Assertion and Reason are correct and Reason is the correct explanation for Assertion

Potential energy of a magnetic dipole in a magnetic field: $U=-\overrightarrow{\mu }.\overrightarrow{B}=\mu B\cos \theta$
A current loop behaves like a magnetic dipole.
$\therefore U=-\overrightarrow{\mu }.\overrightarrow{B}=-\mu B\cos \theta =-\mu B\cos \theta$
If the loop is rotated about an axis perpendicular to the plane of the loop, $\theta$ does not change. Hence, $\overrightarrow{\mu }$ remains constant.
$\therefore$ U does not change.
$\therefore$ No work is done to rotate the loop.