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Speed of free electrons in vacuum, plasma and superconductors?

  1. Dec 17, 2009 #1
    What is the speed of free electrons in vacuum, plasma and superconductors?

    Can we 'slow down' free electrons? Just how slow can we make electrons go?
  2. jcsd
  3. Dec 17, 2009 #2
    The speed of a free electron is based on its total energy, and is not a set speed. The constant speed of light is specifically due to its not having any mass, whereas an electron has mass
  4. Dec 17, 2009 #3
    A good example of free electrons in a vacuum is the cloud of electrons that are around a hot cathode (thermionic emission) in a typical vacuum tube. A grid in the tube controls the electron density, called space charge, which in turn controls the electron current reaching the plate.
    Bob S
  5. Dec 18, 2009 #4
    Is there a way to slow down electrons, bring them to complete stop?
  6. Dec 18, 2009 #5


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    Yes, although you obviously not stop the motion complettely, it is possible to trap electrons in a reasonably well defined positon using magnetic fields. The principle is essentially the same as for ion traps.
    Google Penning trap.
  7. Dec 19, 2009 #6
    What is electron velocity inside Penning trap?

    Penning trap do not slow electrons down, or does it?

    Can anyone give some links about experiments describing how to bring electron to complete stop? I had a feeling that would be impossible, so I'd really like to see some experimental measurements of electron velocities in the range of 100 - 0 m/s.
  8. Dec 19, 2009 #7
    You can slow down electrons in the conductor by reducing the electric field and cooling the conductor. Even if you nearly completely remove the electric field and reduce the temperature to near absolute zero, the electrons would still move around. The slowest would move at near zero velocities, the fastest at the Fermi velocity.
  9. Dec 19, 2009 #8
    the movement of a free electron in absolute zero and sans an electric field wouldn't be linear right? I would imagine at that point it would just be vibrating in place
  10. Dec 19, 2009 #9
    Imagine a single free electron at the center a cubic copper box. There are equal image charges on all six interior sides of the box (forget about fractional charges-it doesn't matter). As soon as the electron moves away from the center, the image charge on the closest side of the box increases. This increases the attraction of the electron to that side, and the electron accelerates away from the center of the box.

    If the electron is in intergalactic space, it just keeps moving along, subject to the uncertainty principle: h-bar/2 = δx δp.
    Bob S
  11. Dec 19, 2009 #10
    Yes, if you stretch the definition of vibrating. I guess better classical analogy would be a random walk.
  12. Dec 19, 2009 #11
    And to the gravitational, electromagnetic, [...] fields. Strictly speaking even intergalactic space is not empty. There is cosmic background radiation, radiation from the stars and galaxies, etc.
  13. Dec 20, 2009 #12
    I don't think temperature has influence on electron velocities, "heat" is more about atoms and molecules. I don't think electrons velocity in low temperature superconductors ever come even close to zero. But, can't we measure this? Are there some experiments to measure electron velocities in superconductors or plasma? Are there some experiments that measure LINEAR electron velocities, like in electron beams? Can we slow down electron beams?

    Can we make electrons in electron beam move at say 0.5 m/s?
  14. Dec 20, 2009 #13
    Although I would like to know some range of velocities in low temp. superconductors and plasma, I'm not interested in "drift velocity" or some average velocity of electrons scattering, bouncing or jumping around in some confined area. Mostly, I'd like to know about linear velocity, when electrons are traveling in straight line, like in electron beam or alternatively in long, straight, superconducting wire.

    Ok, so what speed is that?

    What is the minimum linear speed anyone ever observed electrons to move at?
    ...and has anyone ever managed to slow down electrons, what was the minimum velocity?
  15. Dec 21, 2009 #14
    The answer here is Fermi velocity. But you are not going to get one velocity, rather it is going to be some probability distribution.

    Part of the answer here is Drift velocity. See http://en.wikipedia.org/wiki/Drift_velocity
  16. Dec 21, 2009 #15
    I'm not interested in Fermi energy, nor drift velocity. I'm talking about FREE electrons and LINEAR VELOCITY measured in meters per second. I would like to see some actual numbers, like - what is the minimum linear speed anyone ever observed electrons to move at? ...and, has anyone ever managed to slow down electrons, what was the minimum velocity?
  17. Apr 7, 2011 #16
    can that electron be slow enough to be immersed in a condensate without depleting it ?
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