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Homework Help: Drift speed and current density

  1. Oct 9, 2008 #1
    1. The problem statement, all variables and given/known data

    A piece of copper wire joins a piece of aluminum wire whose diameter is five times that of the copper. The same current flows in both wires. The density of conduction electrons in copper is 1.1 [tex]\times[/tex] 1029 m-3 in aluminum it is 2.1[tex]\times[/tex]1029 m-3.
    what is the drift speed and current density in each?

    2. Relevant equations

    I = nAqVd
    J = nqVd
    3. The attempt at a solution
    the cross sectional area is unknown and the current is unknown. i think the drift speed of one will be a function of the other. I have an expression but since i am using a web software, i would like to see if someone else gets the same expression as i do.
  2. jcsd
  3. Oct 10, 2008 #2


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    Staff Emeritus
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    Gold Member

    1. You are correct - the values of the drift speeds and current densities can not be determined with the info provided. You can only determine the ratios.

    2. You have to tell us what answers you get (and how you got them), and we will let you know if you've made a mistake (and where you made it). It doesn't work the other way round.
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