the membrane to the other. The electric field is assumed to be constant across the membrane, so that it can be set equal to Em/L, where L is the thickness of the membrane. For a given ion denoted A with valence nA, its flux jA—in other words, the number of ions crossing per time and per area of the membrane—is given by the formula The first term corresponds to Fick's law of diffusion, which gives the flux due to diffusion down the concentration gradient, i.e., from high to low concentration. The constant DA is the diffusion constant of the ion A. The second term reflects the flux due to the electric field, which increases linearly with the electric field; Formally, it is [A] multiplied by the drift velocity of the ions, with the drift velocity expressed using the Stokes–Einstein relation applied to electrophoretic mobility. The constants here are the charge valence nA of the ion A (e.g., +1 for K+, +2 for Ca2+ and −1 for Cl−), the temperature T (in kelvins), the molar gas constant R, and the faraday F, which is the total charge of a mole of electrons. This is a first-order ODE of the form y' = ay + b, with y = [A] and y''' = d[A]/dz; integrating both sides from z=0 to z=L with the boundary conditions [A](0) = [A]in and [A](L) = [A]out, one gets the solution where μ is a dimensionless number and PA is the ionic permeability, defined here as The electric current density JA equals the charge qA of the ion multiplied by the flux jA Current density has units of (Amperes/m2). Molar flux has units of (mol/(s m2)). Thus, to get current density from molar flux one needs to multiply by Faraday's constant F (Coulombs/mol). F will then cancel from the equation below. Since the valence has already been accounted for above, the charge qA of each ion in the equation above, therefore, should be interpreted as +1 or -1 depending on the polarity of the ion. There is such a current associated with every type of ion that can cross the membrane; this is because each type of ion would require a distinct membrane potential to balance diffusion, but there can only be one membrane potential. By assumption, at the Goldman voltage Em, the total current density is zero (Although the current for each ion type considered here