# Current geometry and Ampere's law

Under the right circumstances, Ampere's law $\oint \vec H\cdot d\vec \ell=I_{encl}$ can be used to deduce the field $\vec H$ at a point from the current enclosed by the circuit which produces $\vec H$. This can be done when one can find a current-enclosing contour on which the field is constant in magnitude, something that can occur only in highly symmetrical situations: the symmetries of the current distribution are reflected in the symmetries of $\vec H$, meaning that the geometry of the Amperian loop enclosing the current is usually closely related to the symmetry of the source current distribution.

All textbook examples use cylindrical or planar current distributions (or modifications thereof, such as the infinite solenoid or the toroid, or even semi-infinite cylinders), resulting in circular or rectangular loops.

Can people provide examples of other non-trivial current distributions, coordinate systems and contours for which one can put Ampere's law to good use to find the field $\vec H$?

• You can use the differential form $\vec\nabla\times{}\vec{B}=\mu_0\vec{J}$ in any geometry, and you probably do every time you use a FEM method to solve a magnetics problem. Mar 12 '17 at 16:44
• @ThePhoton I can't see your suggestion being correct. For an infinitely long wire carrying current $I$ the $\vec B$ field at distance $\rho$ is just $\vec B=\frac{mu_0I}{2\pi \rho}\hat \phi$ and one easily verifies that $\nabla\times \vec B=0$ near that point, consistent with $\vec J=0$ near that point. Of course this is expected since $\vec\nabla\times B$ is local whereas the integral form of Ampere's law is global. Mar 12 '17 at 17:45
• The two forms are mathematically equivalent according to Stokes' theorem. Mar 13 '17 at 15:39
• The differential form works at every point not on contours and surfaces. So if you are using the differential form, you will have to work out the curl of B at every point. Mar 16 '17 at 11:57
• Ampere's law is used to derive the continuity conditions for H. Hence Ampere's law is used extensively whenever there is an interface between different media. Nov 25 '18 at 23:03

Ampere's Law is useful only for finding the magnetic intensity or magnetic magnetic field in those electrical distributions where the current is steady and there is a high degree of symmetry. Elsewhere, this law is valid but the mathematical equations become extremely complicated and hence it is not useful for finding the magnetic intensity or magnetic magnetic field in non-symmetrical electrical distributions.

• In what sense do you think your answer adds anything informative to whatever @ZeroTheHero was asking? Unless I have missed it, you are repeating what he wrote. Mar 12 '17 at 15:48
• @hyportnex Presumably there ought to be situations where elliptical or hyperbolic coordinates would be appropriate, but I've never seen this done anywhere. I will take anything above and beyond cylindrical or planar. Mar 12 '17 at 15:54
• @ZeroTheHero neither have I, and I think the reason for that is (warning: here comes the vigorous handwaving) that cylindrical symmetry has really one free variable (the radial distance) and in the integral formulation you have one current against which on the other side you have a scalar product of two vectors, and now try to find the 3 components from one equation. Mar 12 '17 at 15:59