Magnetic Field of an Infinite Sheet of Current:
We can consider an instance of a current distribution which does not have cylindrical symmetry, but which is susceptible to Ampere's Law for finding the magnitude of the magnetic field. Consider a sheet of current which is infinitesimally thin but infinitely long and wide. The sheet has a linear current density (i.e. current per unit length).
An infinite sheet of current with current per unit length. We wish to find the magnetic field direction and magnitude at point P a distance h away from the sheet.
Consider a set of wires laid in place of the current sheet. Each wire carries current out of the page. The magnetic field due to wires which are equidistant from the point directly underneath point P can only be in the horizontal direction.
We can replace the sheet with an infinite set of wires arranged so that each wire carries current consistent with an overall current per unit length of as in the case of the current sheet. We see in figure that wires which are equidistant from the line from the set of wires to point P have magnetic fields whose vertical components cancel and whose horizontal components add. Hence, the net magnetic field is in the horizontal direction. This field is uniform since the distribution of wires is infinite, i.e. any position for point P can be considered as the "middle" of an infinitely long current sheet. Note also that the magnetic field direction for the infinite sheet is independent of the distance of P from the sheet and that the same arguments for extending consideration of a finite set of wires to an infinite current sheet state that the magnetic field direction on the other side of the sheet has the field pointing in the opposite direction.
