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  4. The magnetic flux density B at a distance r from a long straight wire carrying a steady current varies with r as shown in figure

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Q. The magnetic flux density $B$ at a distance $r$ from a long straight wire carrying a steady current varies with $r$ as shown in figure

AMUAMU 2001

Solution:

image
The magnetic field due to a long wire carrying current $I$, at a distance $r$ is given by
$B=\frac{\mu_{0}}{2 \pi} \frac{I}{r}$
$\Rightarrow B \propto \frac{1}{r}$
Since $B$ is inversely proportional to $r$,