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Drift velocity

  1. Jul 13, 2015 #1
    for an electron, randomly moving inside a conductor , having applied an external electric field we have those electrons moving with a net speed called drift speed , against the direction of field.
    so initially as electrons are moving randomly we consider their initial velocity o
    and after time t =at
    where a = acc. of electrons = e(field)/mass of electron
    t = mean time between consecutive collisions of electrons
    courtesy PHYSICS by halliday resnick krane vol 2
    but i don't understand why don't we average the initial and final speed of electrons ie
    drift speed = (0 + at)/2
    Last edited: Jul 13, 2015
  2. jcsd
  3. Jul 13, 2015 #2
    The acceleration of an electron in a real conductor is not constant. I think when subjected to an electric field, the speed of the electrons increases monotonically up to its "steady-state" drift velocity. If the applied field is not changing (DC), then after an initial transient time, the electrons are flowing at the drift velocity. If the applied field is a sinusoidal function (AC), then the current (and thus drift velocity) will also vary sinusoidally. An accelerating charge (i.e. a varying current) establishes an electromagnetic wave.
  4. Jul 13, 2015 #3


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    I think drift speed is already an average since every electron will be moving at different speeds and constantly interacting with the material and the applied field.
  5. Jul 13, 2015 #4
    For the acceleration happened just a moment (average), the drift velocity is the average speed electrons have in the conductor after the generation of the electric field.
  6. Jul 14, 2015 #5


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    The electron in a (normal) conductor is under the influence of the electromagnetic force and to a friction force. The notion of a friction force is already a coarse grained description of the full complicated dynamics of the many-body (quantum!) system. On the level of linear-response theory you come astonishingly far with very simple classical pictures introducing some phenomenological transport coefficients (like electric conductivity) and response functions. On a microscopic level, you have to calculate appropriate correlation functions in statistical many-body QFT. See Landau-Lifshitz vol. X for a very good introduction into both classical and quantum transport phenomena.
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