Hello all. As far as I know, the equations to obtain current density is: J=∑(qi ni vi) , where qi is the charge of each class of charge carrier in the material, ni is the volumen density of that class of charge carrier, and vi is the average drift speed of those charge carriers. Current density is a vector quantity that may depend on time and point coordinates (position). My question is, where does this position and time dependence come from? Can volume density of charge carriers depend both on the position and time considered (ni=ni(r, t))? Why? Can drift speed depend both on position and time (vi=vi(r, t))? Why? Thank you so much.