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Q & A: unbalanced electromagnetic forces?

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Most recent answer: 08/13/2009
Q:
Now my friend gave me a question today in which two current carrying wire are placed perpendicular to each other and I found that they do not experience equal and opposite forces that is it appears to be a form of violation of Newton's 3rd law. Is it? And how.......
- dipendra (age 18)
kanpur,india
A:
I'm trying to picture these two perpendicular wires and their fields, and not getting unbalanced forces. Maybe I'm just missing something.

However, it certainly is possible to have currents flowing in a pair of wires and have unequal forces as the wires approach. That would seem to violate conservation of momentum (the general form of Newton's Third Law) but it turns out not to. The reason is that the electromagnetic fields (electric E and magnetic B) also carry momentum. The Poynting vector ExB is proportional to not only the flow of energy but also the momentum. So if two circuits are brought together so that the integral of ExB changes, the field momentum will have changed. That means the net momentum of the objects will have changed by an opposite amount, so the forces on the objects couldn't have been balanced.

Mike W.

(published on 08/13/2009)

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