Density definition(semiconductor nightmare 2)

[nhrock3]

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 can't understand how the length involved here if its not mentioned
in the first definition

 

[Okefenokee]

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.