Question Checkup 30.2.1: A power cord for an electric appliance consists of two paral......

Physics for Engineers and Scientists Volume 2 [1423312]

A power cord for an electric appliance consists of two parallel straight wires carrying currents in opposite directions. Is the magnetic force between them attractive or repulsive?

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The force on them is repulsive. Recall from Eq. (29.5) that the magnetic force on a positive charge moving parallel to a current in a long wire is negative, or radially inward. For currents moving in antiparallel directions, the force is repulsive. This is also easily verified from the vector form $F = IL \times B$ .

$F=-\frac{\mu_0}{2 \pi} \frac{q v I}{r} \quad \text { for } v \text { parallel to current }$ (29.5)

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