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Homework Help: Drift velocity

  1. Mar 21, 2012 #1
    1. The problem statement, all variables and given/known data
    a wire carrying a current of 3.2A has a radius of 1.2mm. the material of the wire has 2.5x10^25 free electrons per cubic metre. the elementary charge is 1.6x10^-19

    2. Relevant equations
    cross section = pi*r^2

    3. The attempt at a solution
    pi*0.0012^2 = 4.5x10^6

    3.2/4.5x10^6*2.5x10^25*1.16x10^-19 = 0.1777 recurring
    I am sure this is not correct as it seems to fast, I have seen some examples and they are more like 2.8x10^-4 m/s
    Am i supposed to make 2.5x10^25 into 2.5x10^19 for mm^3 instead of m^3?
    Any help please?
  2. jcsd
  3. Mar 21, 2012 #2


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    2.5 x10^25 electrons per cubic metre is very low value for a metal wire. It should be in the range 10^28-10^29 electron/m^3.

  4. Mar 21, 2012 #3
    you have the cross sec area as 4.5x10^6 ....it should be 4.5x10^-6.... this is just a typing error and does not affect your answer.
    For the figures you have given I also get 0.177
    I agree with ehild regarding electron concentration for a metal. 10^28 to 10^29
  5. Mar 21, 2012 #4
    Yeah it was a typo sorry. OK so the electron drift is 0.177 m/s. What happens if the cross section doubles in size is it v*2 or v/2? Thanks
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