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Homework Help: Drift speed on copper cable

  1. Sep 20, 2008 #1
    1. The problem statement, all variables and given/known data
    A 150 km long high-voltage transmission line 2.0 cm in diameter carries a steady current of 1030 A. If the conductor is copper with a free charge density of 8.9 1028 electrons per cubic meter, how many years does it take one electron to travel the full length of the cable?

    2. Relevant equations

    I= nq(vd)A

    where A is the area
    vd is the drift speed
    n is the charge density
    and I is the current

    3. The attempt at a solution

    SO we are solving for the drift speed so i thought this equation would work. i found the area first using surface area of a cylinder 2pir^2+2pirh and i got 18840.0025 because i converted cm and km both to m.
    then i plugged everything else in and the answer was wrong

    oh yea and i used -1 for the q which is the charge. i assumed -1 b/c they are electrons
  2. jcsd
  3. Sep 20, 2008 #2


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    Homework Helper

    wrong area. you want the cross-sectional area only.
  4. Sep 20, 2008 #3
    ok so im not solving for drift speed, im slving for delta t... so now the equations im trying are Q=(nA(x)) q

    and then Q=(nAvdt)q
  5. Sep 20, 2008 #4
    ok so now i've got an A of the cross section which is pir^2.... i got .001256
    so i multiplied that by 8.9*10^28 and then multiplie that by 150000m .. to get the charge. then i divide the charge by the current to get the time.. still wrong though.. any ideas
  6. Sep 20, 2008 #5


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    solve for the drift velocity (v). How long does if take for an electron moving at v to travel [itex]\delta x=150000m[/itex]? I.e., what's [itex]\delta t[/itex] in terms of [itex]\delta x[/itex] and v.
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