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Q.
The magnetic field $B$ at a distance $r$ from a long straight wire carrying current varies with distance $r$ as shown in the figure.
Moving Charges and Magnetism
Solution:
The magnetic field $B$ at a distance $r$ from a long straight wire carrying current $I$ is given by
$B=\frac{\mu_{0} 2 I}{4 \pi r}=\frac{\mu_{0} I}{2 \pi r}$
$ \therefore B \propto \frac{1}{r}$