Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thermionic diodes ?

  1. Jun 19, 2010 #1
    I am reading about thermionic diodes and trying to understand how they work ,
    it uses a heated cathode and the electrons are ejected from it via thermionic emission
    and i could see how the electrons couldn't go the other way because it would require a large voltage to get them off of a non-heated cathode , but when the electrons are ejected from the heated cathode why would they get attracted back to that heated filament when the current flips on AC current .
    Any input will be much appreciated .
  2. jcsd
  3. Jun 21, 2010 #2
    someone has to have something on this .
  4. Jun 21, 2010 #3


    User Avatar
    Science Advisor

    You're asking why the electrons get attracted to a +ive electrode?
  5. Jun 21, 2010 #4
    So when the voltage flips , what the electron just got emitted off wouldn't , it seems like it would be attracted to what it just got emitted off when the voltage flips ,
  6. Jun 21, 2010 #5


    User Avatar
    Science Advisor

    Because before the voltage "flipped" it (the cathode) was negative and after it "flips" it becomes positive. I still don't quite get what you're asking.

    Or did you mean to ask :

    "but when the electrons are ejected from the heated cathode why [strike]would[/strike] wouldn't they get attracted back to that heated filament when the current flips on AC current."?

    In which case the answer is "they would". Which would give a small displacement current but no continuous current as no new electrons are being emitted from the cold electrode at the other end.

    Seriously I'm trying to guess your actual question or doubt here. Could you make it a bit clearer please.
  7. Jun 21, 2010 #6
    yes that is my question , so you are saying that it will briefly go back but over all it will still progress away from the heated cathode .
  8. Jun 21, 2010 #7
    Last edited: Jun 21, 2010
  9. Jun 21, 2010 #8
    When the anode voltage is positive (relative to cathode), the electrons are attracted to the anode and you sense some current in the external circuit.
    When the anode voltage is negative, the emitted electrons cannot reach the anode because the electric force points toward the cathode. Thus, no current is measured in the external circuit. This is the way tube diodes work, current flows only when the anode voltage is positive.
  10. Jun 21, 2010 #9


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    The (thermionic) electrons which are produced in the region of the cathode have been given enough kinetic energy to escape from the surface due to thermal agitation. If the cathode is not connected to anything, there will be a cloud of electrons around the cathode and the cathode will be slightly positively charged (overall, the number of protons and electrons will be the same), keeping the electrons from going off on their own. This will be an equilibrium situation.
    A nearby Anode (connected so that it is +wrt the cathode) can supply enough field to make the electron cloud flow towards it. If the cathode is held positive wrt the anode, the cloud of electrons will flow, momentarly, back towards the cathode until a different (smaller) cloud of thermionic electrons is around the cathode (a new equilibrium situation). Two heated electrodes would allow AC to flow between them, natch.
  11. Jun 21, 2010 #10
    thanks for all your responses , i think it is clear now .
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook