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)

B

Answer 2

When currents in both the wries are in same direction, then magnetic field at the centre $B =B_{2}-B_{1}$ $=\frac{\mu_{0}(2 I)}{2 \pi(r / 2)}-\frac{\mu_{0} I}{2 \pi(r / 2)}$ $=\frac{\mu_{0} I}{\pi r}$ When $2 I$ wire is switched off, then magnetic field at the centre $B'=\frac{\mu_{0} I}{2 \pi(r / 2)}$ $=\frac{\mu_{0} I}{\pi r}=B$