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Thermal motion

  1. Jun 6, 2013 #1


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    Gold Member

    In some texts about plasmas, the plasma oscillations are discussed at the extreme of no Thermal motion. One example is the one in wikipedia:
    But I can't accept that approximation.Because it is assuming that we have a kind of motion called thermal motion and other kinds which arise from other things.But that's wrong and its the motion of particles that causes a feeling of temperature and when there is motion there is a non-zero temperature.
    Can anyone explain?
  2. jcsd
  3. Jun 6, 2013 #2


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    Staff: Mentor

    I'm not an expert in plasmas, but I think that what you have to consider is the difference between "absolute" speed and a speed distribution. What temperature gives you is a distribution of velocities. Take for example a container full of gas and fly it in a jet airplane at mach 2: you wouldn't consider that the temperature of the gas has changed because of this, even though the gas molecules are going much faster than they normally do at room temperature.

    My guess is that this is the approximation made here: the distribution of velocity of the electrons due to temperature can be neglected, and you can consider the motion to be only due to the plasma oscillation, i.e., as the motion of the electrons in the collective Coulomb field of the ions and electrons.
  4. Jun 6, 2013 #3


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    The cold plasma approximation of course is not appropriate in all situations. But it can be useful for solving for plasma waves which propagate much faster than the thermal velocity. It doesn't matter if the plasma is pretty hot. It's still looks cold relative to a sufficiently fast plasma disturbance. The cold plasma equations give solutions for relatively fast waves in the plasma, which are approximately correct for a warm plasma.
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