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Drift Velocity: missing 1/2?

  1. Dec 7, 2011 #1
    If a is the acceleration of an electron in a conductor due to an electric field, and t is the mean time between collisions, than v_d, the average drift velocity, is

    v_d = at

    1. There should be a 1/2 there! If you followed one electron, its average v_d would be at/2 by kinematics.
    2. Book says: We're averaging over many electrons. For each one, at some instant, v_d = aT where T is the time since the last collision. And what's the average of that? Just v_d = at.

    Which one is right? They both sound correct to me...
  2. jcsd
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