Q 12-04-143JEE MainJEE Main 2022 (25 Jun, Shift 1)Easy
A long straight wire with a circular cross-section having radius $R$ is carrying a steady current $I$. The current $I$ is uniformly distributed across this cross-section. Then the variation of magnetic field due to current $I$ with distance $r$ $(r < R)$ from its centre will be
Answer: (A) $B\propto r$
By Ampère's law inside the wire: $B\cdot2\pi r = \mu_0 I\dfrac{r^2}{R^2}\Rightarrow B = \dfrac{\mu_0 I r}{2\pi R^2}\propto r$.
Solution by Sreeraj P, M.Sc Physics