JEE Physics 2023 Question with Solution

Given: A circular current loop lies in the $xy$ plane and carries current $I$ in the anticlockwise direction when seen from the $+z$ direction. We need the variation of $B_y$ on the $yz$ plane at a fixed distance $a$ from the center, with $a$ less than the loop radius.

Find: The correct graph of $B_y$ versus $z$.

Using symmetry of the circular loop and the right-hand rule:

At the plane of the loop, the $\hat{j}$ component cancels by symmetry. Also, the direction of $B_y$ reverses when $z$ changes from positive to negative.

So, $B_y$ is an odd function of $z$. Therefore the required graph must pass through the origin and have opposite signs on the two sides of $z=0$.

The solution explicitly states that the correct option is A. This disagrees with the answer key, which marks option (3). Since the solution is the primary source, the answer is taken as A.

Therefore, the correct option is A.