Along straight wire carries a current $I$. The magnetic field $B$ at a distance $r$ from the wire is proportional to:

Solution

1. The magnetic field $B$ produced by an infinitely long straight wire carrying current $I$ is defined by the formula $B=\frac{{\mu }_{0}I}{2\pi r}$.
2. In this expression, ${\mu }_0$, $I$, and $2\pi$ are constants.
3. Therefore, the magnetic field $B$ is inversely proportional to the distance $r$, expressed as $B\propto \frac{1}{r}$.
4. Hence the answer is (C).