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Thread: Why do we have to heat the cathode in a CRT?

  1. #1 Why do we have to heat the cathode in a CRT? 
    Senior Member MaxPayne's Avatar
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    Does heating frees up more electrons?
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  2. #2  
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    They jump out off the metal surface when it is hot.
    Gave me an idea as to where the electrons come from in the Earth's core. Plenty of hot metal down there.
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    Senior Member MaxPayne's Avatar
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    [QUOTE=Robittybob1;12071]They jump out off the metal surface when it is hot. /QUOTE]

    Why?

    I will jump on a hot metal surface. Can't blame the electrons? Poor things! its too hot for them to just stand there.
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  4. #4  
    mvb
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    [QUOTE=MaxPayne;12098]
    Quote Originally Posted by Robittybob1 View Post
    They jump out off the metal surface when it is hot. /QUOTE]

    Why?

    I will jump on a hot metal surface. Can't blame the electrons? Poor things! its too hot for them to just stand there.
    The energy used to "heat up" the metal goes partly into raising the kinetic energy of the conduction electrons in the metal. Then when the electrons come to the edge of the metal with its potential-energy barrier to electrons leaving the solid, there is an increased chance that any given electron has enough energy to get over the barrier. Hence more electrons come out than when the metal is cooler.
    Last edited by mvb; 03-10-2014 at 10:32 PM. Reason: Change energy "mostly" goes into the electrons to "partly" goes into the electrons.
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    [QUOTE=mvb;12109]
    Quote Originally Posted by MaxPayne View Post

    The energy used to "heat up" the metal goes mostly into raising the kinetic energy of the conduction electrons in the metal. Then when the electrons come to the edge of the metal with its potential-energy barrier to electrons leaving the solid, there is an increased chance that any given electron has enough energy to get over the barrier. Hence more electrons come out than when the metal is cooler.
    How did you know the energy "goes mostly into raising the kinetic energy of the conduction electrons in the metal"? I can't recall that being discussed before.
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    mvb
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    [QUOTE=Robittybob1;12118]
    Quote Originally Posted by mvb View Post

    How did you know the energy "goes mostly into raising the kinetic energy of the conduction electrons in the metal"? I can't recall that being discussed before.
    Sorry, I put that too strongly. The energy goes partly into raising the kinetic energy of the ..., electrons. After all, every atom in the metal vibrates a bit more around its equilibrium point in the lattice, which is no doubt where most of the energy. Enough goes into the conduction electrons however.

    We know that the valence electrons must average a bit more energy because they are in thermal equilibrium with the rest of the metal. If the rest of the metal has more energy because of its temperature, so must the valence electrons, which have plenty of slightly higher-energy states available and aren't attached to the atoms and can't vibrate along with them.
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    [QUOTE=mvb;12121]
    Quote Originally Posted by Robittybob1 View Post

    Sorry, I put that too strongly. The energy goes partly into raising the kinetic energy of the ..., electrons. After all, every atom in the metal vibrates a bit more around its equilibrium point in the lattice, which is no doubt where most of the energy. Enough goes into the conduction electrons however.

    We know that the valence electrons must average a bit more energy because they are in thermal equilibrium with the rest of the metal. If the rest of the metal has more energy because of its temperature, so must the valence electrons, which have plenty of slightly higher-energy states available and aren't attached to the atoms and can't vibrate along with them.
    OK that sounds like a more standard explanation.
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