Question

Current $I$ is flowing in infinitely long wire. Which of the following graphs represents the variation of $B$ w.r.t. axial distance?

• A.
• B.
• C.
• D.

Answer 1

Answer: (A)

$B=\frac{{\mu }_{0}ir}{2\pi R^2}$ for $r<R$
$\Rightarrow B\propto r$
$B=\frac{{\mu }_{0}i}{2\pi r}$ for $r>R$
$\Rightarrow B\propto \frac{1}{r}$