Question

$PQ$ and $RS$ are long parallel conductors separated by certain distance. $M$ is the mid-point between them (see the figure). The net magnetic field at $M$ is $B$ . Now, the current $2A$ is switched off. The field at $M$ now becomes :-

• A. $2B$
• B. $B$
• C. $\frac{B}{2}$
• D. $3B$

Answer 1

Answer: (B)

Magnetic field due to long straight wire,
$B=\frac{{\mu }_{0}I}{2\pi r}$.
In first case, $B=\frac{{\mu }_0(2)}{2\pi r}-\frac{{\mu }_0(1)}{2\pi r}=\frac{{\mu }_0(1)}{2\pi r}$
In second case, $B_R=\frac{{\mu }_0(1)}{2\pi r}=B$
But the direction will be opposite.