A straight copper wire that is 1 milimeter in diameter carries a current of 20 miliamps. Whas the magnitude of the largest magnetic field created by this wire in Tesla?
Isn't this Amperes law? Assume all current is at the center, so distance for B is one radius.
Another approximation is uniform current density, you integrate current density,
another approximation is that all the current is one the skin, you have to integrate dB. But having done this, I have found the maximum B is the same.