Q 12-04-128JEE MainJEE Main 2023 (6 Apr, Shift 1)Easy
A long straight wire of circular cross-section (radius $a$) is carrying steady current $I$. The current $I$ is uniformly distributed across this cross-section. The magnetic field is
Answer: (B) directly proportional to $r$ in the region $r<a$ and inversely proportional to $r$ in the region $r>a$
Ampère's law: inside, the enclosed current is $I\dfrac{r^2}{a^2}$, giving $B=\dfrac{\mu_0Ir}{2\pi a^2}\propto r$. Outside, $B=\dfrac{\mu_0I}{2\pi r}\propto\dfrac1r$.
Solution by Sreeraj P, M.Sc Physics