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

1. μ₀I²/πd
2. √2μ₀I²/πd
3. μ₀I²/√2πd
4. μ₀I²/2πd

Correct answer: μ₀I²/2πd

Solution

The middle wire experiences forces due to the other two wires. Since the currents are in the same direction, the forces are attractive. The net force per unit length is the vector sum of these forces, which are equal in magnitude but opposite in direction. The result is μ₀I²/2πd.

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