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
JEE MAIN 2023, Medium
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$B = \begin{cases}\frac{\mu_0 Ir }{\pi a ^2} & r \leq a \\ \frac{\mu_0 I }{\pi r ^2} & r \geq a \end{cases}$
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