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Question:

A rigid square loop of side a carrying current I2 is lying on a horizontal surface near a long current I1 carrying wire in the same plane as shown in figure. The net force on the loop due to wire will be:

Attractive and equal to μ0I1I23π

Repulsive and equal to μ0I1I22π

Zero

Repulsive and equal to μ0I1I24π

Solution:

Correct option is B. Repulsive and equal to μ0I1I24π
F3 and F4 each other
Force on PQ will be F1 = I2B1a = I2 μ0I1I23π = μ0I12π
Force on RS will be F2 = I2B2a = I2 μ0I1I22π2aa = μ0I14π
Net force = F1 − F2 = μ0I1I4π repulsion