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Q.
The correct plot of the magnitude of magnetic field $\vec{B}$ $vs$ distance $r$ from centre of the wire is, if the radius of wire is $R$
Moving Charges and Magnetism
Solution:
The magnetic field from the centre of wire of radius $R$ is given by
$B=\left(\frac{\mu_{0}I}{2R^{2}}\right) r \left(r <\,R\right) $
$\Rightarrow B \propto r$
and $B=\frac{\mu_{0}I}{2\pi r} \left(r >\,R\right)$
$\Rightarrow B \propto \frac{1}{r}$
From this descriptions, we can say that the graph $\left(b\right)$ is acorrect representation