# Homework Help: Momentum of electromagnetic fields

1. Aug 3, 2006

### golfingboy07

I am having a little trouble with this. To attack this problem (I reckon anyway) is to recognise that the current density vector J(x) can be written it terms of the number density and the velocity. But I'm not sure how to incorporate the Taylor expansion with the cross product. If somebody could give me a few hints just to get me started again that would be great!

Cheers

GM

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2. Aug 4, 2006

### Meir Achuz

You have to:
1. Expand J in a 3D Taylor expansion.
2. Integrate by parts to get from phi to E.
3. Go from \int xj to m by writing xj as a sum of sym and asym parts.