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Q & A: conservation of momentum and electromagnetism

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Most recent answer: 10/30/2014
Q:
The action -reaction law of Newton does follow for the moving charge,however, the law of the conservation of momentum is still safe.
- Rahul singh (age 23)
Bhopal, India
A:

I think that there might be a typo in the question. Perhaps what you're asking about is how it can be that sometimes when you sum the forces on a collection of moving charges, you don't get zero, contrary to what the simplest versions of Newton's Third Law would say. You're correct that momentum is conserved anyway. The reason is that there's an additional form of momentum, in the electromagnetic field itself. There's a momentum density proportional to the Poynting vector E×B. The change in that field momentum just makes up for the change in the particle momentum.

Mike W.

posted without vetting until Lee returns from the Serengeti 


(published on 10/30/2014)

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