GITAM, Department of Engineering Physics

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Force between two long parallel wires

The magnetic force between two long parallel wires each carrying a current I could be calculated directly from the force expression. However, since this case has a particularly simple geometry, it is more convenient to obtain the force from the magnetic field calculated in the previous section. The magnetic field B at wire 2 due to wire 1 is given by

 

This expression is the basis of the definition of the Ampère. We now consider the direction of the force. Now the direction of the force goes as l × B, which points to the left. Thus the force between wires carrying current in the same direction pushes them together. One concludes: like currents attract, unlike currents repel. This is the converse of the rule for charges.

 

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