Three infinitely long parallel wires are equally spaced along a straight line. The two outer wires carry equal currents I in the same direction. The middle wire carries a non-zero current. For which direction of the middle current do the forces exerted by the two outer wires cancel?
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Question

Three infinitely long parallel wires are equally spaced along a straight line. The two outer wires carry equal currents I in the same direction. The middle wire carries a non-zero current. For which direction of the middle current do the forces exerted by the two outer wires cancel?

Options

G1 Physics · Accepted Answer

The two outer wires produce equal-magnitude but opposite magnetic fields at the midpoint. Hence the net field there is zero, so the middle wire has zero net force for either current direction.