- #1
skrat
- 748
- 8
Homework Statement
Calculate the vector potential of a loop with current ##I##, raidus ##a##. Calculate it for anywhere in space and use approximation where ##r>>a##.
Homework Equations
##\vec A=\frac{\mu _0I}{4\pi }\oint\frac{d\vec l}{|\vec r-\vec{r(l)}|}##
The Attempt at a Solution
Ok, let's put the origin of my coordinate system in the centre of the loop and let the loop be in xy plane.
than $$d\vec l=dl(-\sin \varphi , \cos \varphi , 0)$$ $$\vec r=r(\sin \vartheta \cos \varphi, \sin \vartheta \sin \varphi , \cos \vartheta)$$ and $$\vec{r(l)}=a(\cos \varphi , \sin \varphi,0)$$.
BUT this is all a complete nonsense because:
$$|\vec r - \vec{r(l)}|=\sqrt{r^2+a^2-2ar\sin \vartheta}$$ and therefore the integral $$\vec A=\frac{\mu _0I}{4\pi }\oint \frac{dl(-\sin \varphi , \cos \varphi , 0)}{\sqrt{r^2+a^2-2ar\sin \vartheta}}$$ considering only cases when ##r>>a## gives me $$\vec A=\frac{\mu _0I}{4\pi r}\oint dl(-\sin \varphi , \cos \varphi , 0)(1+\frac a r \sin \vartheta)$$ but for ##dl=ad\varphi## both integrals are ##0##.
Ammm, what am I doing wrong here? :/