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Electron gun drains electrons

  1. Dec 7, 2006 #1
    Will an electron gun firing electrons into the air end up having no more electrons to fire eventually?
  2. jcsd
  3. Dec 7, 2006 #2
    eventually any emitter will not be able to excite electons off... but i think it is because something in the shape of the source changes not that you run out of electrons..... I am consulting with my SEM designer friends at the moment.....
  4. Dec 7, 2006 #3


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    Er.. no. Most electron "guns" are thermionic guns, and to operate something like that, the filament must be grounded (or else you'll end up with nothing very quickly).

    So unless you want to argue that the "ground" will run out of electrons, it'll never happen. The filament might break, etc.. but it won't stop running due to lack of electrons.

  5. Dec 7, 2006 #4
    so the electrons don't come from the mains supply? the ground could eventually run out of electrons if the electrons are fired into space... but i guess that when that happens we'll have bigger problems to worry about than electron guns... :P
  6. Dec 11, 2006 #5

    so an old friend of mine got back to me and here is an edited reply on this topic.....
  7. Dec 11, 2006 #6
    That sounds confused. It doesn't matter where "ground" is.

    It's not that you run out of electrons, but you might get into the situation where the electrons left in the cathode are all too tightly bound or the electrons in the anode/target create too repulsive an electric field. This is avoided by connecting a high voltage power supply (or battery) between the cathode and anode. If you google for a cross-sectional diagram of your CRT monitor, you'll see how the electrons are being collected back from the screen/anode and pumped hard toward the cathode gun again.
  8. Dec 11, 2006 #7


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    Er... and my post sounded confusing?

    If you do any amount of photoemission experiment, for example, and your cathode or sample isn't not well grounded, or if you are using something that isn't a good electrical conductor such as a semiconductor, then you will see something called the "charging effect". This is where your cathode could not replenish itself as fast as the electrons leave its surface. What will occur is that the effective workfunction will start increasing and at some point, you will not get anymore photoelectrons!

    Furthermore, you can't simply connect the cathode and the anode to a power supply that is floating. This is because you are not guaranteed that all the electrons leaving the surface of the cathode will get to the anode. The electrons are attracted to anything at a higher potential than the cathode, and the grounded walls of the vacuum chamber is such a thing. Again, look at most setup for thermionic and photoelectron guns. The source is always grounded.

  9. Dec 11, 2006 #8
    I would have thought it safer and more practical to ground the target and the surrounding apparatus (whilst typically making the source -ve with respect to "ground", at least for CRTs - the photoemission experiment I've done uses more than two electrodes..), but I'll defer to your experience. My confusion was as to whether your phrasing implied charge accumulates anywhere ad infinitum.
    Last edited: Dec 11, 2006
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