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Minimum electron speed?

  1. Aug 6, 2014 #1
    What is the minimum speed at which electrons can be emitted? Why is it I never heard of really slow moving electrons, like 10 meters per second slow?
     
  2. jcsd
  3. Aug 6, 2014 #2

    davenn

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    electron drift in a copper conductor is really slow ... around 0.00029 m/s, or very nearly 1.0 m/hour.

    that is a figure based on a 1mm diameter copper conductor carrying 3 Amperes of current

    http://en.wikipedia.org/wiki/Drift_velocity

    now when you said emitted can you be a little more specific please



    Dave
     
  4. Aug 6, 2014 #3
    It means the question is not about wires and drifting, but free electrons like in electron beam.
     
  5. Aug 7, 2014 #4

    Drakkith

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    Electrons are very light and easy to accelerate. Even an acceleration voltage of 1 volt will give them a very high velocity. (thousands of m/s)
     
  6. Aug 7, 2014 #5

    davenn

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    OK, no probs. I don't know of any specific speeds
    what I would have to say is that without a "strong" positive plate ( anode) say in a valve ( electron tube) say a TV CRT tube, an emitted electron will happily just fall back onto the cathode filament.
    Its the high voltage positive potential on the anode that will attract and accelerate any free electrons towards it

    in the case of the TV and similar large CRT's the Anode voltage was often up ~ 25,000V ( in the case of a 26" screen size)

    The emission of electrons from a heated filament is called thermionic emission and as I said earlier .... without the presence of a large positive anode to attract the electrons. The electrons will just leave and fall back onto the filament.

    In a Triode valve a grid was added between the filament (cathode) and anode. It would have a small negative voltage applied to it which depending on the amplitude of that voltage, it would repel more or less electrons back to the filament.
    This way you could control the number of electrons that traversed the gap between the cathode and anode.

    cheers
    Dave
     
  7. Aug 7, 2014 #6
    I still don't see what's the limit to emitting slower electrons than that, or why we couldn't slow them down afterwards to even crawling speed like 1 meter per 10 seconds.
     
  8. Aug 7, 2014 #7
    There is no such limit.

    What makes you think that we cannot?
     
  9. Aug 7, 2014 #8
    Can you be more convincing?


    Not thinking really, it's more asking. Because I cannot find anything about it on the internet. I can tell you though, I'm not talking about Penning trap or anything other than electrons simply moving from point to point over some distance in some measured amount of time, with some speed less than, say 100 m/s.

    I also find it hard to imagine slow electrons, for some reason. If photons can't go any slower than the speed of light, then let's just say I wouldn't be surprised there are some bottom limits to electron speed as well. Maybe even for the same reason, whatever reason that is, and so I wonder. Do you know why photons can't go any slower than the speed of light?
     
  10. Aug 7, 2014 #9

    Drakkith

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    Photons move at the velocity c because they are massless. Electrons are not, though, and can go any speed below c, including zero.
     
  11. Aug 7, 2014 #10

    Nugatory

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    Here's an easy way of getting an electron to stand still (or move at any speed that I please):

    Take an electron source that emits electrons at some very high speed. Mount it in a car (or a plane, or a spaceship) pointing backwards. Drive the car forward at the same speed that the electrons are emitted backwards... And there you are.

    I just answered the question about photons and the speed of light in your other thread. If you try the same experiment with photons (mount a light source on a vehicle and then start the vehicle moving) you will find that the light ends up moving at speed ##c## relative to both the ground and the vehicle, even though the vehicle is moving relative to the ground. This seems pretty weird at first glance, but that's how it is - you'll have to learn some relativity before you can go any deeper.
     
  12. Aug 7, 2014 #11

    vanhees71

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    Of course, theoretically an electron can be put to rest as any particle with mass (ok, in the sense of classical physics, which we discuss here; the issue becomes more complicated when quantum theory is considered, which you must do in principle when dealing with elementary particles, but e.g., to construct an electron accelerator you are fine with the classical concepts; why is another deep story).

    In practice, it's not so easy to bring electrons or other subatomic particles to low speeds. Ironically deceleration is nearly as much a chalenge as is acceleration. Of course, nowadays one can trap particles in various ways.

    This comes pretty close to particles nearly at rest. An example are ultracold neutrons:

    http://en.wikipedia.org/wiki/Ultracold_neutrons
     
  13. Aug 7, 2014 #12
    It's probably better to separate those two questions, so I started a new thread in relativity section about the the speed of light. In any case, I don't see why would lack of mass prevent photons to go slower than c, or faster for that matter.


    I don't disagree, but I also don't see anyone has actually done it.
     
  14. Aug 7, 2014 #13
    I approve. That way, or whatever other way, but has anyone actually done it?
     
  15. Aug 7, 2014 #14
    Wouldn't it electron decelerator work on the same classical principles and perhaps with exactly the same equipment?


    Neutrons are neutral, electrons are different.
     
  16. Aug 7, 2014 #15

    vanhees71

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    Yes, the fact that neutrons are neutral and pretty much heavier than electrons in fact helps to achieve those low speeds for free neutrons as compared to free electrons. As I said, deceleration is almost as chalenging as acceleration.
     
  17. Aug 7, 2014 #16

    jtbell

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    Neutrons are uncharged, so they're (mostly) not affected by stray electric fields. Charged particles like electrons are another matter.

    Suppose you hold two electrons stationary 5 meters apart, then let one of them go. It flies off because of electrostatic repulsion. A quick calculation using conservation of energy says that it ends up traveling at about 10 m/s. Of course, if the electrons are closer together to start with, the final speed is higher.

    So if you want to maintain an electron moving at a very low velocity, you have to keep it very far away from other stray charges that can produce a net electric field at its location, or else ensure that the other charges are very precisely symmetrically arranged so that their fields cancel out.
     
  18. Aug 7, 2014 #17
    What about emission? As I gathered we can get faster or slower electrons in cathode ray tube by supplying more or less voltage to "electron gun" apparatus. Is there anything then preventing us to supply some low voltage corresponding to electron speed of, say 10 m/s? And if that is so easy, then why I can't find anything about it on the whole internet?
     
  19. Aug 7, 2014 #18

    Drakkith

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    Electron guns typically rely on a method called "thermionic emission" to get enough electrons to be useful. This requires heating up a filament so that electrons near the surface get enough energy to escape the metal. The energy each electron receives just from the temperature of the filament accelerates them to a much higher velocity than 10 m/s.

    Even the electron guns that don't use thermionic emission still impart enough energy into each escaped electron to accelerate them well above 10 m/s.
     
  20. Aug 7, 2014 #19
    Ok, so there is one of the limits I was looking for - electron bonding energy that must be exceeded in order to separate it from its atom. But I don't see how would that define electron's subsequent speed. And I also don't see why would supplying more energy produce faster electrons, because it seems to me they would fly off as soon as supplied energy reaches over its bonding energy, so then the speed of electrons would be defined by the material rather than voltage.
     
  21. Aug 7, 2014 #20

    Drakkith

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    The transfer of energy to each electron is not smooth and continuous. Random vibrations and collisions produce a wide range of electron velocities. Sometimes an electron gets barely enough energy to exceed the work function of the material and escape, while another electron could get smacked so hard that it blazes out of the material at several times the average velocity.

    An applied voltage can then accelerate the electron once it leaves the material to whatever velocity you desire. (Or reduce the work function on the material, allowing electrons to escape easier)
     
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