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Where does the electron gain its energy from to keep orbiting?

  1. Sep 13, 2014 #1
    Basic leak in Rutherford's model of atom was that electron would emit energy while orbiting around nucleus and this energy loss would lead to electron's spiraling to the nucleus and collapse of both atom and atom model. Then Niels Bohr came with the idea that electron exists only in discrete orbits, what prevents it from falling. But still, in his model, electron does orbit nucleus and thus radiates energy. My question is - what keeps it standing in the same orbital? How is it possible that electron which stands in ground orbit keeps the energy associated with that orbit if it obviously radiates some energy just by orbiting around?
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  3. Sep 13, 2014 #2


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    Bohr's model cannot explain that, he just postulated it.
    Modern quantum mechanics (well, "modern" - 1925+) can give an explanation - there is nothing "moving" in the way it would lead to radiation.
  4. Sep 13, 2014 #3


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    The same way an electron parts ways with a proton in a chemical reaction, they can attract each other to form the (instable) atom. Stable Hydrogen is a 2-atomic molecule.
  5. Sep 13, 2014 #4
    Uhm, sorry, but I don't get it.. I thought, that electron moves within the "probability cloud" around the nucleus and so creates EM wave...
  6. Sep 13, 2014 #5


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    No, it doesn't. That's the point: the classical model, which says that an electron must radiate energy while orbiting the nucleus, is *wrong*. The correct model is the quantum model, where the electron does not radiate energy unless there is a lower energy state for it to go to. If the electron is in the lowest energy orbital (the ground state of the atom), there is no lower energy state for it to go to, so it can't radiate any energy.
  7. Sep 13, 2014 #6


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    The electron is not moving in the sense that its quantum state is not changing with time. This is the important sense for EM radiation: if the quantum state is not changing, there is no EM radiation (according to quantum electrodynamics, and in agreement with experiment).

    On the other hand, the electron is moving in the sense that, if you measure its velocity (or kinetic energy), you will not get zero.

    Weird, isnt't it? That's quantum mechanics! :)
  8. Sep 13, 2014 #7


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    That's a bad analogy - it doesn't move in a cloud etc - in fact what properties it has when not observed is a matter of debate strongly depending on interpretation. Best to simply think of it as not having any properties until observed.

  9. Sep 14, 2014 #8
    Everything depends on what we understand by "moving". The electron in hydrogen does not change position and its wavefunction is stationary, yet it has nonzero momentum.

    The analogy I use to visualize it is a circular pipe with circulating water. The pipe does not change position, the water does not change mass distribution, yet it has nonzero momentum.

    The important thing is to distinguish the 3 notions of "moving":
    - changing (non-stationary) probability density
    - changing position
    - nonzero momentum or kinetic energy.
  10. Sep 14, 2014 #9
    I think it also has something to do with the with the fact that there is a mimimum action principle at work. A particle cannot loose an arbitrary small amount of energy.
  11. Sep 14, 2014 #10
    No EM wave is created! What are you reading to get this impression?

    And I think your interpretation "electron moves within the probability cloud" is dodgy - though there are so many interpretations these days that I certainly wouldn't be prepared to dismiss it :).

    If you are new to this, I'd stick to trying to understand the Copenhagen interpretation for starters!
    In the Copenhagen interpretation, there is only the probability cloud to start with. (Another name for the probability cloud is the wave-function for the electron.). The electron only appears when a measurement is made - due to "wave-function collapse". Whether the probability cloud is real or not is a moot point, both positions are taken by different supporters of the Copenhagen interpretation. You can never "see" the probability cloud, if it exists, because a measurement always causes wave function collapse. I like to think there is something there (a fuzzy cloud around the nucleus) that instantaneously collapses into a little electron particle when a measurement is made. But maybe I'm being too committal in thinking this way:

  12. Sep 14, 2014 #11
    Yes, the probability cloud sits there like fog above London Bridge on a still day.

    What electron?! Until the measurement is a made there is only the probability cloud. Of course the measurement causes the cloud to implode into a electron with an actual velocity.
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