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Dec27-12, 01:49 PM
Sci Advisor
PF Gold
P: 2,082
1) You are correct that the force does not depend on the absolute position of the two-loop system. Listing x is just a formality--he specifies that one small loop is at x and the other is at x'. The important part is that the equation contains only the difference x-x', with no dependence on x itself.

2) I'm not following your question. Eq. (1.15) is not about force, it just shows that div B =0. This really doesn't depend on the current, following instead from the vector calculus relation that div(curl v)=0 for any vector v