# Density definition(semiconductor nightmare 2)

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

$$j = -nev$$
$$j \frac{charge}{seconds}\frac{1}{cm^2}= -n \frac{electrons}{cm^3}e \frac{charge}{electrons}v \frac{cm}{seconds}$$