Main Question or Discussion Point
drude replaces V=RI with E=rho*j
current density is the amount of charges that crosses a certain area
in an amount of time.
but here they say that if current flows threw a wire of length L etc..
that the current density will be ..
but in the first definition of density there is no length of a wire aspect involved
so i cant understand how the length involved here if its not mentioned
in the first definition