The time dependent form of the Nernst–Planck equation is a conservation of mass equation used to describe the motion of a charged chemical species in a fluid medium. It extends Fick's law of diffusion for the case where the diffusing particles are also moved with respect to the fluid by electrostatic forces: It is named after Walther Nernst and Max Planck.
Where J is the diffusion flux density, t is time, D is the diffusivity of the chemical species, c is the concentration of the species, z is the valence of ionic species, e is the elementary charge, kB is the Boltzmann constant, T is the temperature, is velocity of fluid, is the electric potential, is the magnetic vector potential.
Setting time derivatives to zero, and the fluid velocity to zero (only the ion species moves),
In the static electromagnetic conditions, one obtains the steady state Nernst–Planck equation
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