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Steady current of orbiting electron

  1. Aug 28, 2010 #1

    I know that the a constant magnetic field is created by a loop of DC. Steady current does not create any EM waves. At the same time, it is told that an electron orbiting around an atom should emit an EM field. In both cases we have a loop but in one case no radiowaves are emitted, in other, they are. What is wrong?

    I mention that an electron is alone while the current is a plurality of charges. The charges on the opposite side of orbit could compensate the dM created by the first. Indeed, the electron is distributed into a continuous cloud of charge in QM picture. Is it right to guess that an ideal current loop is similarly made of a distribution of infinitesimal charges that all emit radio waves but completely cancel out each other?
  2. jcsd
  3. Aug 28, 2010 #2


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    Staff: Mentor

    Classical electrodynamics simply is not valid on the atomic scale. This is why physicists had to invent quantum mechanics, and later quantum electrodynamics.
  4. Aug 28, 2010 #3
    Do you lack of the opportunity to demonstrate your literacy (the trivial facts, actually) where it is appropriate? The current loops and instability of planetary model of atom are studied at normal scales!
  5. Aug 28, 2010 #4


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    No it isn't. Okay, it was told 100 years ago, but not any more.

    The problem is that if the electron radiates EM energy, the electron would have to lose that energy. But if the electron is in the lowest-energy quantum state -- as is usually the case -- then it cannot lose any energy, hence it does not radiate EM energy.
  6. Aug 28, 2010 #5
    One more idiot to show off his literacy. Don't you see that I'm aware of both models - Bohr and QM?
  7. Aug 28, 2010 #6


    Staff: Mentor

    Your responses have been very rude and inappropriate. You asked a question that would be typical for a student who had learned the Bohr model but not QM, and the other posters responded appropriately.

    If you are aware of QM then the question becomes rather confusing, what about QM are you not understanding?
  8. Aug 28, 2010 #7
    Electrons in non S-wave atomic states have orbital angular momentum l ≠ 0, which is the source of an orbital magnetic dipole moment. This orbital magnetic moment can be considered to be due to a dc orbital "loop current". This orbital magnetic moment interacts with the electron intrinsic magnetic moment s, creating the atomic fine structure j = l ± s = l ± ½, for example the 2p1/2 and 2p3/2 splitting in hydrogen.

    Bob S
  9. Aug 28, 2010 #8
    My responses were rude because of inappropriate teaching.

    What makes people think so? Should I keep silence about Bohr model and write about the CLOUD OF DISTRIBUTED CHARGE in big letters just for the students who just got to know why Bohr was superseded by QM despite this would distort the idea of my question?

    By "orbiting" in the original question, I imply "a ball that circles around heavy nucleus". It must emit EM waves according to classical dynamics. I understand that atom does not radiate because of QM. But, the Electrodynamic prediction is not wrong because atom is different. My question is why a (circular) loop of current, a meter-scale piece of wire, does not radiate! It must according to the same prediction classical electrodynamics!

    Who tells you that my question is about QM? Why are you sure that I am OK with Bohr?
  10. Aug 28, 2010 #9
    A point particle orbiting with constant speed is not a steady current. The current density is time dependent, because the point particle is at different points at different times.
  11. Aug 28, 2010 #10

    Ok, now I start to understand why guys turned to QM model of atom. Indeed, the cloud orbiting around the center must also create the magnetic field. I do not understand how the fine structure could help here.

    My question was not about QM model. My question was about a macroscopic loop with current. It is described by the same classical ED that tells us that electrons must radiate and fall into the nucleus. Yet, the static magnetic field of the loop suggests that nothing is radiated. I want to know why? The ideal current as electronic cloud was just my hypothesis. The lack of nucleus is another.

    Is here at lest one specialist on classical ED?
  12. Aug 28, 2010 #11
    Good. Does this confirm my picture that ideal current is made of a continuously distributed charge in space rather than a chain of distinct electrons?
  13. Aug 28, 2010 #12


    Staff: Mentor

    That is not a justification for rudeness. Nobody here knows you or your background nor can we read your mind. They were responding reasonably to the question you actually asked. When you get responses that are not what you wanted then the appropriate thing to do is clarify your question, not insult the attempted assistance offered.

    I think you will have trouble finding people willing to help after that initial exchange. All I can do is suggest that you search the site, this question (as I understand it now) has come up before. I remember one particularly good post about dipole, quadrupole, and multipole expansions.
    Last edited: Aug 28, 2010
  14. Aug 28, 2010 #13
    No. The laws of electrodynamics involve such abstractions as continuous charge and current distributions because they are obtained by averaging the microscopic electromagnetic fields over distances much larger than the characteristic inter-atomic distances. In any such volume there is a tremendous number of electrons (and nuclei) that the granular nature of the charge distribution can be "smeared out" into a jelly-like charge distribution.

    Nevertheless, there are processes in which this granular nature of the charge distribution becomes apparent. For example, the effect of http://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise" [Broken] is a direct consequence of the existence of charge carriers that perform random motion about their average motion.
    Last edited by a moderator: May 4, 2017
  15. Aug 28, 2010 #14
    Even if the existence of QM can be cited to discourage any number of intuitive lines of thought, I don't think it is so foolish to wonder what would happen to the electric charge as electrons orbit nuclei. Is it possible that electron energy at the atomic level conserves its charge within the orbital cloud/loops and would only radiate energy when the charge-loop was disturbed? Is that too simplistic a possibility?
  16. Aug 28, 2010 #15
    Why "No"? How this is different from my guess?

    I guess absolute zero - no motion to distort the current.

    This way to stabilize the atom is pretty natural to guess, isn't it? I would ask Bohr why he preferred the inherently unstable planetary model to ideal current distribution?
    Last edited by a moderator: May 4, 2017
  17. Aug 28, 2010 #16
    Please explain the process of ionization according to your "guess".
  18. Aug 29, 2010 #17
    Your post describes what I was asked for -- the look of ideal current, the constant not disturbed current that produces constant magnetic field. There was nothing about any ionization!
  19. Aug 29, 2010 #18
    Where does my post describe something that someone asked you ???
  20. Aug 29, 2010 #19
    Wait. In https://www.physicsforums.com/showpost.php?p=2856753&postcount=13", you quote my guess and rephrase it, which looks like a confirmation. Nevertheless, you start by "No", which means that there is some difference. But I do not see any difference between two descriptions of real vs. ideal current.

    Aside from that, you mention the Johnson–Nyquist noise. This is external disturbance that impedes the current. I think it is unnecessary to entail especially because the real current, made of distinct charges, has another inherent impedance: it always has magnetic field fluctuations, and, thus, radiates EM waves in the same way as Bohr's atom electron does. This will happen even at zero temperatures - superconductors must radiate and thus slow down and heat up faster and faster!
    Last edited by a moderator: Apr 25, 2017
  21. Aug 29, 2010 #20
    I can't understand a word you are typing.
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