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B Related to - Thermionic emission

  1. Feb 26, 2017 #1
    Hey currently im doing O levels and in my text book I read that when a metallic wire (in CRO - the cathode) is heated up, it starts emitting its free electrons (thermionic emission). And also its a fact that electrons are responsible for electrical conductivity. A question just popped up in my mind that, does the electrical conductivity of metals decrease when a high temperature is reached (as they must have emitted their free electrons)?

    -Thanks
     
  2. jcsd
  3. Feb 26, 2017 #2

    anorlunda

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    :welcome:
    Conductivity is probably not the right word for what you are asking. If the metal emits electrons, and there is no current flow, then it will become positively charged. The positive charge would resist further emission.

    However in the usual case, the metal is connected in a circuit (which eventually connects to earth ground). The electrons are replaced, and the metal remains neutral in charge, so that the thermionic emissions can continue indefinitely.

    Does that answer your question?
     
  4. Feb 26, 2017 #3

    ZapperZ

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    The resistivity of a conductor increases when the conductor is heated. So the conductivity decreases (conductivity is inversely-related to resistivity).

    This behavior has nothing to do with thermionic emission, but rather the behavior of a standard metal. In a thermionic emission, the metal is connected to a circuit, and current flow is how it is normally heated. So there is always a supply of electrons, since one end of it can be grounded. There is no "charging" effect.

    Zz.
     
  5. Feb 26, 2017 #4

    sophiecentaur

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    In the case of a simple thermionic diode with positive bias so that it conducts, the resistance drop across the filament will be caused by the current in the heater circuit but also the additional electrons that flow from the cathode to the Anode.
    So, for someone measuring the voltage across the filament and the current flowing into it, the Resistance they would measure would, I believe, be measured as different. This is quite a sophisticated question for an O level student - Wasif may be getting a more confusing answer than he(?) expected. :smile:
     
  6. Feb 26, 2017 #5

    ZapperZ

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    Are you sure? The OP never asked anything about a "thermionic diode". So he/she isn't asking a "sophisticated question", but a rather straight-foward one especially considering the level that we are talking about. Are we the ones making it more difficult than it really is?

    Zz.
     
  7. Feb 26, 2017 #6

    sophiecentaur

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    The question was probably based on the limits of what the student was told but when the thermionic emitter is included in a circuit, there can be a net flow of electrons and what I wrote would be true. And, actually, during the initial heating process, even if there is no other connection than a 'battery, heating the wire', there will be a small excess flow of electrons onto the wire, causing a finite (but time) increase or decrease in the measurement of its resistance.
    Yes we could be making it more difficult than intended but my description of the heating up transition would apply in any case.
     
  8. Mar 20, 2017 #7
    Thankyou soo much guys. Yeah i do understand till that part that the circuit is earthed and electron deficiency can be fulfilled from electrons from earth and hence the metal never runs out of electrons. I didnt understand the rest of the part XD.
     
  9. Mar 20, 2017 #8
    I am interested in your statement
    And, actually, during the initial heating process, even if there is no other connection than a 'battery, heating the wire', there will be a small excess flow of electrons onto the wire, causing a finite (but time) increase or decrease in the measurement of its resistance.
    How do you explain this?....are you considering reactive elements or the circuit?
    Electrons are not a concrete concept and as presented to anyone under the age of 11 probably inhibit consonance of understanding.
     
  10. Mar 20, 2017 #9

    sophiecentaur

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    Sorry - for "time" read "tiny". There would be, I guess, the equivalent of a small capacitance as the charge ('space charge') forms around the hot wire. I imagine there would be a finite net leakage of electrons into the surrounding air - being carried away in air currents, forming -ions and going to other neutral objects. A permanent change in measured resistance would need a constant flow of electrons onto an anode, nearby. (a parallel path) I immediately interpreted your post as relating to a diode. But we're talking angels on a pinhead here.
     
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