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Density definition(semiconductor nightmare 2)

  1. Mar 20, 2010 #1
    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
  2. jcsd
  3. Mar 20, 2010 #2
    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.)

    [tex]j = -nev[/tex]

    [tex]j \frac{charge}{seconds}\frac{1}{cm^2}= -n \frac{electrons}{cm^3}e \frac{charge}{electrons}v \frac{cm}{seconds}[/tex]

    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.
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