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Bohr Model

  1. Nov 15, 2004 #1
    I keep reading that the Bohr model explained why the electron in orbit around the nucleus does not emit radiation even though it is accelerating. How does the Bohr model explain this?
     
  2. jcsd
  3. Nov 15, 2004 #2
    he said there are certian quatum stable states in which electrons do not emit EMR these states exsist in certain shells at determined radii
     
  4. Nov 15, 2004 #3
    Yeah, why don't the electrons emit at those radii?
     
  5. Nov 15, 2004 #4

    ZapperZ

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    Er.. the Bohr model does NOT explain why an electron "in orbit" around the nucleus does not emit radiation. While the Bohr-Sommerfeld quantization rule sort of "explains" this, there are still problems. This is because the Bohr model isn't correct! Electrons do not "orbit" the nucleus the way planets do around the sun. Refer to the solution to the Schrodinger equation for the hydrogen atom, for example.

    Why is this in the classical physics section?

    Zz.
     
  6. Nov 15, 2004 #5
    Okay, fair enough. As for why this was in the classical physics section, isn't the bohr model based on classical physics?
     
  7. Nov 15, 2004 #6
    Bohr formulated these in postulates, which means they were assumptions. These postulates clearly states that the chosen orbits are exceptions to classical electromagnetic theory. This is not new in physics. You are allowed to make assumptions and then lead to a theory. If your theory stands the test of experiment and verified by other proven theories then your theory wins.

    Think of the postulates that Albert Einstien proposed in his special theory of Relativty; stating that speed of light is constant if measured from any referance frame. This is unthinkable in Newtonian mechanics. Yet we know the STR to be so true.

    I hope this gives you a feeling of the stand a postulate takes to go out and explain a phenomenon.
     
  8. Nov 15, 2004 #7

    Integral

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    Albert Einstein was born within a few years of the completion of the Michelson Morely experiment. For his entire life the speed of light was known to be constant, and further, that there was no ether to propagate it. It was a known, but distressing, fact to all Physicists of the day. Einstein simply showed them how to square the constant velocity of Electro Magnetism with the rest of Mechanics.

    Einstein only postulated that which was already accepted Physics. It is a very dangerous thing to have a postulate which is not part of accepted Physics. It is that sort of thing that should be derived from known postulates. If your postulate are not understood, how can your results have meaning? Thus the questionable postulate of Bohr lead to an incorrect model.
     
  9. Nov 16, 2004 #8
    Theories or postulates , can turn out to be incorrect. Thats not the point. We are trying to state what exactly is a postlulate and what are the thoughts behind formulating one. Whether it turns out to be a correct one or not is a different story. Perhaps I was oversimplifying the idea of a postulate by saying you can assume "anything". But it is perfectly acceptable of a postulate to be contradictory and not accepted by a part of of physics and yet attempt to prove a phenomenon. The formulated postulates if true would go back and explain the contradictions.

    So Einstien's postulate for STR was aligned with MM experiment and Maxwells equations but surely a "dangerous thing" for Newtonoan Mechanics.
    MM expt and Maxwells equations were 20 years new to physics compared to hundreds of years of Newtonian mechanics and our eternal experiance how relative velocites add. So when you say "..Einstein only postulated that which was already accepted Physics...." thats not really true. It was revolutionary.

    Similarly Max Planck's idea of Quanta is an accepted postulate, was OK to explain Black Body energy distribution results but was a "dangerous thing" for classical physics. Now whether Planck was correct or we needed a Bose-Einstien :) statistics is a different topic. But I am trying to elaborate the role of a postulate.
    Follow the same argument with Bohr's theory as well...
     
  10. Nov 16, 2004 #9

    reilly

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    The problem of 1. atomic stability, and 2. discrete atomic spectra drove physicists absolutely nuts, until Bohr. His great insight was to accept as fact that "orbiting" electrons did not radiate. Given that, and acceptance of photons, as described by Einstein, Bohr was able to derive the formula for the Balmer series, and other hydrogen spectral series. Bohr started modern atomic physics, and, as a result, his work led to an explanation of the periodic table, and on-and-on.

    To say that Bohr provided us with an "incorrect model", as suggested by Integral, is like saying the first motion pictures were 'wrong' because they did not have sound, or that Adam Smith was wrong because he did not anticipate modern economies, or that Newton was wrong, or Maxwell was wrong -- they never got to quantum effects. And, the planetary model of Rutherford was really wrong, yet it survived, if only as a metaphor.

    In fact, once Bohr did his thing, atomic and nuclear physics prospered, in an astute enough fashion, that physics survived Bohr's 'incorrect ideas' to push on to better theories. As physicists, we owe a debt, beyond calculation, to Bohr. In a sense, Bohr opened up the microscopic world: he got the basics right, and thus set the stage for modern QM, QED, QCD and string theory--to be sure, he was not alone in this.

    I'm old enough to have worked with, and studied with men who knew Bohr, and Einstein. The reverance awe, and respect my seniors had for these two men is off the charts. These days, a banal description of creativity is: Think outside the box. Then there's; Go where no person has ever gone, and .... The point is that, almost of necessity, big and small advances in anything, break with conventional wisdom.

    I am in agreement with the comments of snbose.
    Regards,
    Reilly Atkinson
     
  11. Nov 16, 2004 #10
    The electrons are not orbiting....that's a classical picture being imposed on a quantum state. The electron "wave" is only telling you where you are most liekly or most unlikely to find the electron with that angular momentum and energy if you place a detector "in the way" and try to detect it! This will inevitably "knock" it out of your "orbit" by putting it into another bound quantum state or it will knock it out completely...like the photo-electric effect!! So the moment you try to view the "orbit"; you'll bring about it's destruction!!
     
  12. Nov 16, 2004 #11
    So the electron isn't orbiting; therefore it isn't accelerating and so it won't emit photons. The "wave" is truly a stationary state.
     
  13. Nov 16, 2004 #12

    ZapperZ

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    Since I'm one of those who also said that the Bohr model is incorrect, and somehow, this was taken to be a sign of disrespect towards Bohr and his accomplishment, let me be clear that THAT was NEVER the intention. Pointing out the Bohr model was incorrect was meant towards those who STILL haven't gotten beyond the "planetary model" of an atom, as if our knowledge of this hasn't progressed beyond that. It wasn't a denigration of Bohr at all! Every ideas that we have in physics started out in the most naive, elementary, and restrictive form. Even the photoelectric effect model isn't correct if one were to view it today with our advanced electron analyzers. But this doesn't detract from the accomplishment of Einstein in enabling the opening of the door that led to this progress.

    The Bohr model served its purpose as the starting point of a more complete description of an atom. It's primitive idea is still being taught in intro Modern Physics courses. However, it would be wrong to answer questions like this by giving the impression that THIS is what the CURRENT understanding of an atomic description, because it isn't! This will only perpetuates what is simply isn't correct. We certainly do not need to tell people about the caloric theory of heat even though that idea played a role in the formation of the theory of thermodynamics.

    Bohr deserves all the recognition that he received. The Bohr-Sommerfeld model should be looked upon as a historically a significant milestone of physics. However, it should not be used to explain present-day phenomena when a better and more accurate picture can be used.

    Zz.
     
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