Question

$\text{Assertion}:$ 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.

$\text{Reason}:$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, 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 current carrying loop in a magnetic field:

$\therefore U=-\overrightarrow{\mu }.\overrightarrow{B}=-\mu B\cos \theta$

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.

Thus, No work is done to rotate the loop.

Hence, option (a) is the correct alternative.