# Force between Current Loops

by klawlor419
Tags: current, force, loops
 P: 117 I am working on a problem involving force between two loops of current. The problem is to prove the for any arbitrary loops of current, Newtons third law holds true. I understand the basics of the approach but I am having trouble seeing why a term goes to zero. The basic setup is to use the Biot-Savart law to predict the field of a segment of the current loop then to use the Lorentz force law to predict the force acting on a segment of the second loop. This has the form, $$d\mathbf{F}_{12}=\frac{\mu_0 I_1 I_2}{4\pi s^3}(d\mathbf{l_1}\times(d\mathbf{l_2}\times \mathbf{s}))$$ Which when simplified by the triple-product and integrated gives the following form, $$\mathbf{F}_{12}=\frac{\mu_0 I_1 I_2}{4\pi s^2}(\oint\oint d\mathbf{l_2}(d\mathbf{l_1}\cdot \hat{s}) -\oint\oint\hat{s}(d\mathbf{l_1}\cdot d\mathbf{l_2}))$$ Its easy enough to see right from here that F12=-F21 just by the fact that you pick up a minus sign by switching the separation vector. So in that sense the problem is solved, at least from what I see right now. However I was looking in the Griffiths EM and apparently the first term on the left cancels out somehow and I can't figure why. (Problem 5.49) Any suggestions?
 P: 185 a couple of things. 1. s can't be pulled out of the integrals. 2. look at this term: $$\mathbf{F}_{12}=\frac{\mu_0 I_1 I_2}{4\pi}\oint\oint d\mathbf{l_2}\left( d\mathbf{l_1}\cdot \frac{\hat{s}}{s^2}\right)$$ then realize that if we do the integral over loop 1 first, we have $$\oint d\mathbf{l_1}\cdot \frac{\hat{s}}{s^2} = \oint d_1\frac{1}{s} = 0.$$ where the subscript on 1 means i'm treating all the terms associated with loop 1 as variables and freezing all terms associated with loop 2. Just remember $\nabla f \cdot d\mathbf{r} = df$ and you're integrating over a loop that starts and end at the same place.
 P: 117 Ah nice, I see now. Thanks for the trick.
 P: 1 Force between Current Loops please could you explain why the integral over loop 1 is zero? I'm struggling to see your method.. :/
P: 117
 Quote by jsholliday7 please could you explain why the integral over loop 1 is zero? I'm struggling to see your method.. :/

Theres an important identity involving one of the terms. Once you use that identity, you can deduce by divergence theorem that the entire term for loop 1 vanishes.
P: 37
 Quote by klawlor419 I am working on a problem involving force between two loops of current. The problem is to prove the for any arbitrary loops of current, Newtons third law holds true. I understand the basics of the approach but I am having trouble seeing why a term goes to zero. The basic setup is to use the Biot-Savart law to predict the field of a segment of the current loop then to use the Lorentz force law to predict the force acting on a segment of the second loop. This has the form, $$d\mathbf{F}_{12}=\frac{\mu_0 I_1 I_2}{4\pi s^3}(d\mathbf{l_1}\times(d\mathbf{l_2}\times \mathbf{s}))$$ Which when simplified by the triple-product and integrated gives the following form, $$\mathbf{F}_{12}=\frac{\mu_0 I_1 I_2}{4\pi s^2}(\oint\oint d\mathbf{l_2}(d\mathbf{l_1}\cdot \hat{s}) -\oint\oint\hat{s}(d\mathbf{l_1}\cdot d\mathbf{l_2}))$$ Its easy enough to see right from here that F12=-F21 just by the fact that you pick up a minus sign by switching the separation vector. So in that sense the problem is solved, at least from what I see right now. However I was looking in the Griffiths EM and apparently the first term on the left cancels out somehow and I can't figure why. (Problem 5.49) Any suggestions?
So if there is a loop current in a start say 10 light years away and I switch on a loop current in the earth now the Newtons third law holds true.

Please, prove it with integrals and divergence theorem.

By the way I did not see any t (time) variable in your formulae.
 Mentor P: 16,942 Alva, the Biot Savart law uses the magnetostatic approximation. It doesn't apply in the far field and there is no time. This is a standard approximation, but it is an approximation.

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