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Homework Help: Drift velocity of electrons

  1. Dec 11, 2014 #1
    1. The problem statement, all variables and given/known data
    There's an engine connected to a battery with a copper wire.
    ρ = 1.69x10^-8
    n = 8.49*10^28 electrons/m^2

    The diameter of the wire is 5mm. The lenght is 1m.
    How long does it take for an electron to go from the battery to the engine if i = 100A?

    2. Relevant equations

    ## v_d = \frac{J}{ne} = \frac{i}{Ane} ##

    3. The attempt at a solution

    I calculated ##v_d## with the equation I wrote above.
    I know that the drift velocity could be considered constant. So I calculated t:

    ## x(t) = 1m = vt => t = 1/v_d ##

    My only question is: why did he give me the resistivity!?
    Is there something unclear to me?

    Thank you
  2. jcsd
  3. Dec 11, 2014 #2


    User Avatar
    Homework Helper

    Your method is correct. The resistivity was not necessary, but one can calculate the mobility from it, and then the drift velocity from the mobility. As the current was given, that was not necessary.
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