An arrangement of three parallel straight wires placed perpendicular to plane of paper carrying same current $'I'$ along the same direction as shown in figure. Magnitude of force per unit length on the middle wire $'B'$ is given by
NEET 2017, Medium
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Force between wires $A$ and $B$ $=$ force between wires $B$ and $C$
$\therefore \quad F_{B C}=F_{A B}=\frac{\mu_{0} I^{2} l}{2 \pi d}$
As, $\vec{F}_{A B} \perp \vec{F}_{B C}$ net force on wire $B$
$F_{\text {net }}=\sqrt{2} F_{B C}=\frac{\sqrt{2} \mu_{o} I^{2} l}{2 \pi d}$
$F_{\text {net }}=\frac{\mu_{o} I^{2} l}{\sqrt{2} \pi d}$ or $\frac{F_{\text {net }}}{l}=\frac{\mu_{o} I^{2}}{\sqrt{2} \pi d}$
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