Question

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

• A. B/2
• B. 2B
• C. B
• D. 4B

Answer 1

Answer: (C)

B

Answer 2

Magnetic field
$B_{1}=\frac{\mu_{0}}{4 \pi} \cdot \frac{2 i}{R}$
and $B_{2}=\frac{\mu_{0}}{4 \pi} \cdot \frac{4 i}{R}$
$B_{2}-B_{1}=\frac{\mu_{0}}{4 \pi} \cdot \frac{2 i}{R}=B$
$B_{2}=2 B_{1}$
$2 B_{1}-B_{1}=B$
$B_{1}=B$