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Asked in RS Academy GUJCET booklet · Force between parallel currents
Given: two long straight parallel wires carrying equal currents in opposite directions.
Idea: each wire lies in the field of the other and feels ⃗F=Iℓ⃗×⃗B.
Wire 1's field at wire 2 is perpendicular to the plane of the two wires, of magnitude (μ₀ I)/(2π d).
With wire 2's current running the other way, Iℓ⃗×⃗B points away from wire 1.
So the wires push each other apart: antiparallel currents repel, with a force per unit length (μ₀ I²)/(2π d).
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