An arrangement of three parallel straight wires placed perpendicular to plane of paper carrying same current I along the same direction is shown in Figure. Magnitude of force per unit length on the middle wire B is given by

Step-by-step solution

Force per unit length between two parallel long current carrying wires is = μ 0 I 1 I 2 2πd Here, force between BC and AB will be same in magnitude. F B C = F B A = μ 0 I 2 2 π d F r e s = F B C 2 + F B A 2 ⟹ F r e s = 2 μ 0 I 2 2 π d = μ 0 I 2 2 π d