Editing Polarization density (section)

===Time-varying polarization density===
When the polarization density changes with time, the time-dependent bound-charge density creates a ''polarization [[current density]]'' of

<math display="block"> \mathbf{J}_p = \frac{\partial \mathbf{P}}{\partial t} </math>

so that the total current density that enters Maxwell's equations is given by

<math display="block"> \mathbf{J} = \mathbf{J}_f + \nabla\times\mathbf{M} + \frac{\partial\mathbf{P}}{\partial t}</math>

where '''J'''<sub>f</sub> is the free-charge current density, and the second term is the [[Magnetization#Magnetization current|magnetization current]] density (also called the ''bound current density''), a contribution from atomic-scale [[Magnet#Two models for magnets: magnetic poles and atomic currents|magnetic dipoles]] (when they are present).