I am not able to understand reaction of magnetic force and how it is equal to qvb ?
When you start looking at fields you have to generalize Newton's third law. Basically Newtons third law expresses the conservation of momentum. While it may not make a lot of sense to talk about the force on a field, you can certainly talk about its momentum. Any force that the field exerts on matter causes an equal and opposite change in the momentum of the field.
What is the momentum of the B field? I haven't heard of this concept yet. Any suggestions on some good reading in this subject?
Sure, I like this site
Duke also has a pretty good site on EM.
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