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Why is the net velocity of electrons (drift velocity) considered to be zero, before setting up any electric field? I mean in the derivation of the equation drift velocity:

$$ v =-\frac{Eet}{m} $$

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  • $\begingroup$ Can you expand your question to specify what drift velocity you are considering. Is this drift velocity in a metal? In a plasma? $\endgroup$ – John Rennie Aug 4 '14 at 6:21
  • $\begingroup$ In metallic conductor. @John Rennie $\endgroup$ – user146181 Aug 4 '14 at 6:28
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In a metallic conductor the most energetic conduction electrons are moving at the Fermi speed, which is surprisingly high. For example in copper it's around $10^6$ m/sec. However the electrons are continually scattering off the lattice of metal atoms so the direction of the electron motion is randomised and the average velocity relative to the lattice of metal atoms is zero. The drift velocity is this average velocity, so in the absence of any applied field the drift velocity is zero.

When you apply an external field you increase the velocity in one direction and decrease it in the other direction, so the average velocity is now slightly different from zero.

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