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Analogously, it is possible to have any form of "current density", meaning the flow of a quantity per unit time per unit area. see
This article uses the
where ρ is the
Qualitatively, the change in charge density (charge per unit volume) is due to the contracted volume of charge due to
Charges (free or as a distribution) at rest will appear to remain at the same spatial position for some interval of time (as long as they're stationary). When they do move, this corresponds to changes in position, therefore the charges have velocity, and the motion of charge constitutes an electric current. This means that charge density is related to time, while current density is related to space.
The four-current unifies charge density (related to electricity) and current density (related to magnetism) in one electromagnetic entity.