Two very long, thin, parallel conductors are placed a distance d apart. If each carries the same current i in the same direction, then they will

repel one another with a force of μ0i²/(2πd)
attract one another with a force of μ0i²/(2πd)
repel one another with a force of μ0i²/(2πd²)
attract one another with a force of μ0i²/(2πd²)

Correct answer: attract one another with a force of μ0i²/(2πd)

Solution

Two long parallel wires carrying current in the same direction attract each other, with force per unit length F/L = mu0*i^2/(2*pi*d).

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