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Two very long parallel wires are a distance d apart and carry currents I1 and I2 in the same direction, producing a mutual force of magnitude F. If the current in one wire is changed to twice its original value and its direction is reversed, while the separation is increased to 3d, what is the new force between the wires?

1. -2F/3
2. F/3
3. -2F
4. -F/3

Correct answer: -2F/3

Solution

F = mu0*I1*I2/(2*pi*d). Doubling one current and tripling the separation scales the magnitude by 2/3, and reversing that current flips attraction to repulsion (sign change). New force = -2F/3.

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