# 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

Maybe you should keep all your questions to one thread since they all seem to be about solid state physics. To answer your question here: if you multiply a density time a velocity you end up with flux (something/(area)).

list all the units just like we did in the other thread and you'll see. (I'll help you one more time with this.)

$$j = -nev$$

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

FYI: Typically, when you work SS problems, you express velocities in (cm/s) not (meters/s). Be careful of unit conversions. Memorize conversions between meters to Angstroms and cm to Angstroms. Trust me on that.