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Homework Help: Another Current and Drift Velocity Question

  1. Feb 5, 2008 #1
    [SOLVED] Another Current and Drift Velocity Question

    1. A car's starter motor draws 50 A from the car's battery during startup. If the startup time is 1.5 s, how many electrons pass a given location in the circuit during that time?

    3.As current = rate of flow of charge,
    I = q/t
    Charge, q = I.t = 50 x 1.5 = 75 C
    1 electron has charge, e = 1.6x10^-19 C
    And q = N.e
    Number of electrons which pass in 1.5 s,

    N = q/e = 75 C/1.6x10^-19 C = 4.69x10^20 electrons

    OR use this equation:I = ne/dt and get:

    n = 50 x 5 = 250
    e 1.6 x 10-19
    = 156.25 x 10^19
    = 1.56 x 10^21 electrons
    1.56 x 10^21 electrons pass during that time.

    Which solution/answer is correct?
  2. jcsd
  3. Feb 5, 2008 #2
    It would be better if you used units. I'm not sure I understand your numbers.

    In the second equation, what is the 5 in n=50 x 5?
  4. Feb 5, 2008 #3

    Tom Mattson

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    Staff Emeritus
    Science Advisor
    Gold Member

    The first method is correct. I'm afraid I don't understand the second one. What's "d" in that equation?
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