# Different results for Magnetic field using different methods

In calculating the magnetic field created by this current at the center point of the loop using Biot-Savart and using the vector potential will there be a difference? If so what is it and why?

Using Biot-Savart the Magnetic field is simply a superposition of fields created by straight wire and loop. But how does one calculate the vector potential at the center?

• Have you tried it and gotten a difference? If so, it would help if you summarize or show your calculations. Dec 7, 2011 at 19:02

The vector potential at point $x$ can be calculated as the integral over the wire of the vector current divided by the distance to $x$. Take the curl of the resulting expression, move the curl inside the integral sign, do a few manipulations and wala!, you've got the Biot-Savart law! They are exactly the same.

• I haven't been able to set up the integral to calculate the vector potential that doesn't diverge. And what do you mean move the curl inside the integral? Dec 9, 2011 at 7:36
• I see what's bothering you, the 1/r divergence. That disappears when you differentiate under the integral sign. $\partial_{x} \int dy A(x,y) = \int dy \partial_{x} A(x,y)$. A mathematician might take offense. Dec 9, 2011 at 7:58
• I'll give this a shot. Would you recommend do the integration in polar coordinates or just Cartesian? Dec 9, 2011 at 8:13
• A is only a function of x when it comes to the infinite, straight wire. Dec 10, 2011 at 18:10
• I know, I was just trying to show what I mean by differentiating under the integral sign. Why don't you calculate the vector potential for the ring separately (it won't diverge). Dec 10, 2011 at 22:54