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How does the quantum model justify accelerating electrons not losing energy?

  1. May 28, 2009 #1
    A serious flaw in the Planetary model was that it theorized that electrons accelerated around the nucleus which would cause them to continuously lose energy as EMR and crash into the nucleus. The Bohr model also I think, fails to explain WHY the accelerating electrons don't emit EMR . How does the quantum model of the atom justify the electrons not losing energy though? I am aware that electrons act like waves and that the orbitals can only have a circumference that is an integer multiple of the wavelengths; even so, dont accelerating electrons emit EMR?
    I'm confused! help please!
    Thanks! =D
     
  2. jcsd
  3. May 28, 2009 #2
    In Quantum Mechanics, you can't take logic or reasoning into your equations.

    We have just as barely scratched the surface of QM, so it is still a long way to go to find out exactly why things like this happens, we have theories, but not proof.
     
  4. May 28, 2009 #3

    Vanadium 50

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    I don't think you can always get an answer to "but WHY"? Classical physics has a certain set of behaviors, and quantum physics has a different set - one that more accurately describes observation. That's really all one can expect science to provide.
     
  5. May 28, 2009 #4
    There is no acceleration in the ground state: such a state is stationary. The space distribution of the charge is still. And the other (excited) states are nearly stationary.
    To make a transition between stationary states, you have to disturb the atomic constituents (push electrons or nucleus).

    Bob_for_short.
     
  6. May 28, 2009 #5
    In my personal view, I do not think the Bohr model fails to explain why the accelerating electrons don't emit energy.

    In the Bohr model, when the orbital is an interger times the wavelength of an electron,
    the motion is stable.

    By only the Maxwell's law, the atomic behavior can't be completely explained.
    For example, Davisson Germer and two-slit experiments.

    Your model is granpa's model. If one electron is so big as the electron cloud, the scattering experiment will give different results.
     
  7. May 28, 2009 #6

    ZapperZ

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    You may want to start by reading an entry in our FAQ in the General Physics forum.

    Zz.
     
  8. May 28, 2009 #7
    Yes, the scattering experiments do give different results. The negative cloud sizes in atoms are described with the elastic atomic form-factors. They depend on the initial and final atomic states <n|, |n>, it is an experimental fact. Moreover, there are positive charge atomic form-factors describing the nucleus de-localisation in atoms (see "Atom as a "dressed" nucleus" by Vladimir Kalitvianski).

    Bob.
     
  9. May 28, 2009 #8
    The Bohr model is not the same as quantum mechanics. If you treat an atom according to quantum mechanics, you have to take into account the coupling of electrons to the elecromagnetic field. If you atart with an electron in the first excited state and the radiation field in the vacuum state (no photons present), then this state will evolve over time into a superposition of the electron in the initial exicted state and some states with lower energy. Roughly speaking this amounts to the state becoming

    A |excited state and no photons> + B|lower energy state plus photon>

    The coefficient A will gradually diminish while B will grow over time. In relality, the photons can be in many possible states, of course.
     
  10. May 28, 2009 #9
    If the electron loose smoothly its energy radiating, than the orbit is no more with an integer number of wavelength, but this non periodic orbit is not allowed by the Bohr condition. The electron in an atom can only loose energy by discrete amounts equal to the difference between two possible periodic orbitals i.e. the photoelectric effect.
     
  11. May 28, 2009 #10

    woah, didnt see tht XD
    thanks!
     
  12. May 28, 2009 #11
    I think in QM, it is difficult to image the atomic behavior.

    Are you trying to image the electronic motion concretely?
    Do you think one electron can be divided into pieces?
    In QM, the electron can change to any form as a magician.

    Bohr model, I think, is more natural if we image the electronic motion.
     
  13. May 29, 2009 #12

    Fredrik

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    That didn't make any sense. Of course you can use logic and reasoning in QM. And proof of what?
     
  14. May 29, 2009 #13
    The experiments show that the negative charge is smeared in atoms. It is "seen" as smeared.
    Division into pieces is possible only in mind. A projectile, as I said, "sees" the negative charge smeared (elastic atomic form-factor). Why then should we appeal to a simpler notion of a point-like and turning around electron? Planetary model is wrong and misleading rather than "natural". It fails theoretically and experimentally.

    Bob_for_short.
     
  15. May 29, 2009 #14
    I am sorry that I'm fussing over small details.

    OK. I understand you say that the electron is seen as smeared, is not actually smeared
    (the electron is not actually divided, enlarged or continuously distributed).

    So first you said the below,

    If the electron is not actually smeared (just seen as smeared), how do you explain the constant distribution of the charge of one electron? (which fact is indispensable for justifing that accelerating electrons not losing energy in your first opinion.)
     
  16. May 29, 2009 #15
    Yes, it is actually smeared, that is why it is seen as smeared. Properly carried out experiments on elastic charge scattering show namely that.

    But there is also the so called inclusive picture where all elastic and inelastic cross sections are added. Then the inclusive cross section coincides with the Rutherford cross section as if the target charge were point-like and situated in the center of inertia of a compound system. So the notion of point-like charge is illusory, not true.

    Bob_for_short.
     
    Last edited: May 29, 2009
  17. May 30, 2009 #16
    I imagine your model is probably based on the quantum mechanical theory (Is it right?)
    In QM there are some incredible problems such as below.

    1. In the ground state of hydrogen atom, the orbital angular momentum of the electron is zero, Dose
    the electron crush or penetrate into the nucleus?
    (If you say the electronic charge is still and moving around the nucleus, that is contrary to the fact that orbital
    angular momentum is zero.)

    2. In 2P, 3P ,or 3D... , the eddy of the spinning electron is flowing through the static charge.
    How can you keep the state of electonic charge still?

    3. An electron is too small, so by equating the angular momentum of the spinning electron to 1/2 hbar, the spinning
    sphere leads to 100 times the speed of light (In your model,
    the size of the electron is variable.)

    4. The spinning electron must be rotated by an angle of 4π in order to return to their original configuration. It is
    ridiculous!

    How do you explain these state in your model. Please answer as clearly as possible.

    In Bohr model, All these problems do not occur.
     
    Last edited: May 30, 2009
  18. May 30, 2009 #17

    Hootenanny

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    I haven't read the majority of this thread or any of your posts in their entirety. However, this comment just jumped out at me:
    What is the size of an electron?

    I have also noticed that all of your points hinge on the notion that the QM orbital angular momentum corresponds to the classical concept of angular momentum.
     
  19. May 30, 2009 #18
    Yes, that is right, it is based on QM, it is a QM result.

    The problems your are mentioning are not problems in QM. They are failures of classical picture applied to the quantum world. The smearing is quantum mechanical, not classical mechanical. In CM the charge smearing is described with charge density. In QM it is probability which is smeared.

    I cannot give here lessons of QM. For that there are textbooks.

    Regards,

    Bob.
     
  20. May 30, 2009 #19
    But it does not really explain the question: "WHY DON’T ELECTRONS CRASH INTO THE NUCLEUS IN ATOMS". It only points out what's wrong with classical and Bohr's models. The only explanation is that the electron is "smeared" in the atom. But it fails to explain why "smeared" electron won't crash into the nucleus and why the problem of electrons radiating due to an accelerated motion is no longer meaningful, just because no more “orbits” in the conventional sense and "smeared" electrons. In quantum mechanics, what is "smeared" is the "wavefunction" of electron, not electron itself. Since electron is a particle and can't have fractional charge when measured
     
    Last edited: May 30, 2009
  21. May 30, 2009 #20
    https://www.physicsforums.com/showpost.php?p=1287217&postcount=8

     
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