Along straight wire carries a current $I$. The magnetic field $B$ at a perpendicular distance $r$ from the wire is proportional to which of the following?

Solution

1. For along straight wire carrying a current $I$, the magnetic field $B$ at a perpendicular distance $r$ is given by the formula $B=\frac{{\mu }_{0}I}{2\pi r}$.
2. In this expression, the constants ${\mu }_0$, $I$, and $2\pi$ indicate that the magnetic field $B$ is directly proportional to $1/r$.
3. This relationship shows that the magnetic field strength decreases as the distance from the wire increases.
4. Hence the answer is (C).