Question

A wire carrying current I and other carrying $2I$ in the same direction produce a magnetic field B at the mid-point. What will be the field when $2I$ wire is switched off?

• A. $B/2$
• B. $2B$
• C. $B$
• D. $4B$

Answer 1

Answer: (C)

When two parallel conductors carrying current $I$ and $2I$ in the same direction, then magnetic field at the mid-point is

$B=\frac{{\mu }_{0}2I}{2\pi r}-\frac{{\mu }_{0}I}{2\pi r}=\frac{{\mu }_{0}I}{2\pi r}$
When current $2I$ is switched off then magnetic field due to conductor carrying current $I$ is $B=\frac{{\mu }_{0}I}{2\pi r}.$