Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

ANSWER me precisely

  1. Jul 11, 2009 #1
    1-is electron orbit about the nucleus ?
    2-is there is different between rotation and orbit?

    thanks in advance
  2. jcsd
  3. Jul 11, 2009 #2
    1. No, in physics we don't say that "electrons orbit the nucleus." The word "orbit" implies following a path through space, and electrons don't do that in standard QM.

    2. Yes, something can rotate even when no forces are applied (imagine throwing a stick in outer space and having it flip around and rotate as it travels away). Orbits require an external attractive force to make the thing which is orbiting stay bound to the thing it is orbiting around.
  4. Jul 11, 2009 #3
    Well if it doesn't orbit the nucleus,then how come electron has angular momentum?
  5. Jul 11, 2009 #4


    User Avatar
    Science Advisor
    Homework Helper

    Angular Momentum is due to rotational invariance of a system, so when we speak of angular momentum in QM (standard QM - Copenhagen interpretation) it reflects the shapes of the probability distribution functions.
  6. Jul 11, 2009 #5
    I was taught that they did, but that was a loooong time ago. Modern explanations are vague, but http://en.wikipedia.org/wiki/Atomic_orbital" [Broken] is good "information".
    Last edited by a moderator: May 4, 2017
  7. Jul 11, 2009 #6


    User Avatar
    Science Advisor
    Homework Helper

    Last edited by a moderator: May 4, 2017
  8. Jul 11, 2009 #7
    You can read about the quantum mechanical treatment of the Hydrogen atom http://en.wikipedia.org/wiki/Hydrogen-like_atom" [Broken]

    If you already know QM, then you know that the Hamiltonian for an electron in a spherically symmetric potential commutes with the angular momentum operator, and so the stationary states of definite energy have definite angular momentum as well.
    Last edited by a moderator: May 4, 2017
  9. Jul 13, 2009 #8
    3- what means Anglar moment then ? what thing that rotate to calculate it anglar monemt?
    4- is electron rotate about it self(spin) ?
    Last edited: Jul 13, 2009
  10. Jul 13, 2009 #9


    User Avatar
    Science Advisor
    Homework Helper

    3. I told you, in standard QM it means rotation symmetry (under SO(3))of the systems solutions (wavefunctions)

    4. Is and Is.. it has intrinsic angular momentum but that is again in standard QM just a property of its wavefunction under a rotation group (SU(2) in this case)

    much of this is covered in standard intro QM texts, I can give you some links to free pdf's if you want, it is not so efficient for us to give a full blown course here at the forum but rather discuss real specific issues.

    Don't give your email here, the discussion will take place at the forum - what we write here is meant for others to read and be learnt from so that we don't have to consider the same questions again again and again (compare with using a textbook)
  11. Jul 13, 2009 #10
    if you has the text.pdf supply me please
  12. Jul 13, 2009 #11
    Not in the same way that the Earth orbits the sun. It was believed before the 20th Century that electrons *did* orbit this way. We know now that these theories are incorrect, but they are still taught because they are easy to understand and are a useful stepping stone to the modern theory.

    An orbit (or revolution) is when one object walks around another in an ellipse. A rotation is when an object spins around its own center. The Earth rotates, and because of it we have night and day. The Earth also revolves around the sun, and because of this, we have spring and winter.

    Angular momentum is a property of a physical system. It has two different, but historically related meanings.

    The first is classical angular momentum. A system with angular momentum rotates or revolves about an axis, and can be calculated by the position and velocities of all the masses involved. It is conserved. Because of this, things that rotate or revolve tend to continue rotating or revolving.

    The second meaning of angular momentum is a quantum phenomenon. It is called spin. It gets its name because it produces effects similar to what would happen if an electron rotated ("spun") around its own axis. However, this is just an analogy, and shouldn't be taken literally. It is treated as a quantum property and we usually only experience its effects in indirect ways (such as permanent magnets).

    A very intriguing idea behind angular momentum is that its units: [tex]\frac{kg m^2}{s}[/tex] are the same units as action, an extremely important quantity in physics. Action is what you get when you add up the kinetic energy minus the potential energy of a system at every instant of time. When the universe chooses a path that a particle takes, it always chooses the path which costs the least amount of action. This is called the Principle of Least Action, and among many, many other things, it explains why and how light bends when it goes between air and water.
  13. Jul 13, 2009 #12


    User Avatar
    Science Advisor
    Homework Helper

    Last edited by a moderator: May 4, 2017
  14. Jul 13, 2009 #13
    5- why electron not lost its energy and fall in nucleus ? i.e what source of energy for electron movement make it stand near nucleus???
  15. Jul 13, 2009 #14
    There is no need for a "source" of energy to hold the electron in place. An electron around a nucleus is a stable system. It doesn't consume energy, nor does it produce it.

    The reason the electron doesn't fall into the nucleus is because it's magic. If it did fall in, the universe would blow up. But we see a universe that hasn't blown up, so we know it doesn't fall in.

    More precisely, it's because of the nature of quantum mechanics. A rough argument of why this happens goes like this. The uncertainty principle says there is a lower limit to how well defined a particle's momentum and position can be. If you know where a particle is, it's momentum becomes almost random. If you know a particle's momentum, it's location is almost random.

    If we try squeezing an electron to the size of the nucleus, we know its position very well. By the uncertainty principle, it's momentum becomes randomized. Very soon afterwards, it will end up (by simple chance) with a very high momentum, allowing it to break free of the electric field.
  16. Jul 13, 2009 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    You may have a problem with understanding the responses being posted on here, at least based on how you are writing your questions.

    Please start by reading the FAQ thread in the General Physics forum. Some of the posts there may have already addressed a few of your questions.

  17. Jul 15, 2009 #16
    Hi malawi_glenn,
    Yes I understand that, but what business do electrons get upto really inside the atom? The n(quantum number) fits itself around the nucleus of the atom and there the electrons oscillate,rotate etc, right? If so, do they oscillate at fixed points or around the nucleus as they move?
  18. Jul 15, 2009 #17


    User Avatar

    Staff: Mentor

    There is no generally accepted answer to this question. The mathematics of QM allows us to calculate the probabilities of getting various values for physical quantities when we measure/observe them, but it does not address the question of what is "really going on" when we don't measure/observe them. This is the subject of interpretations of QM, about which people argue vigorously (in this forum and elsewhere). We cannot as yet distinguish between these interpretations experimentally, even in principle, as far as I can tell.
  19. Jul 15, 2009 #18


    User Avatar
    Gold Member

    I have never heard of HUP being applied as the reason electrons do not physically interact with the nucleus. This may be my oversight. I'd like to ask for independent clarification.
  20. Jul 15, 2009 #19
    In other word,it's like asking why atom doesn't collapse.okay according to intro chemistry,when you have say 5 electrons atom,no more than two electrons can share a 'spot',so the filling will be 1s^2 2s^2 2p^1.lowest energy orbitals get filled first and the next electrons move to the next orbitals(i.e no one electron sitting on top of other electron).I guess this makes the atom 'stable'.I may be wrong!
  21. Jul 15, 2009 #20
    Yes. Modern interpretations are vague when it comes to these issues. (angular momentum, electron "movement" in atom etc.)

    Using buzzwords or writing down the mathematical theory doesn't make it clear.
  22. Jul 15, 2009 #21
    Great post. Enlightening and insightful. Had I seen this, I wouldn't have posted my previous one.
  23. Jul 15, 2009 #22


    User Avatar
    Gold Member

    These things are not vague; they are exactly the opposite of vague.

    You're up against something that needs a set of definitions beyond day-to-day language. In order to discuss these things meaningfully, you need to learn the language.

    How do you suppose you would discuss the details of an integrated circuit with someone who thought that voltage and resistance were "buzzwords"?
  24. Jul 15, 2009 #23
    No. That's what some brilliant people around here want to believe. They want to believe that QM is well-understood and Copenhagen is the way to go, although much of the interpretation is still vague. Worse yet, the understanding and the MACHINERY can change depending on the interpretation.

    Voltage and Resistance can be physically defined in day-to-day language, I'll give you one if you want a layman explanation. And that's what makes Voltage and Resistance CLEAR - that they can be expressed simply.

    Using rotational invariance to explain electron's angular momentum and pretending (or naively believing) that mathematics of it is the reason behind the physics is misleading.
    and incorrect.
  25. Jul 15, 2009 #24


    User Avatar
    Homework Helper

    Sure, that is arguably true. But nobody is pretending that.
  26. Jul 15, 2009 #25


    User Avatar
    Gold Member

    You claimed, or at least implied, that terms such as 'angular momentum' are buzzwords. They're not. I was arguing your complaint of unclear terminology, not unclear theories.

    Nice try. The fact that something cannot be expressed simply does not invalidate it. It is a matter of the listener's level of competence in the subject. Good luck explaining how a computer works to a caveman - in a form he finds acceptable - while using only his own vocabulary. To understand QM, you need to understand the math.

    Yeah, that attitude will go far... "I don't understand all this. Obviously, I'm smarter than the rest of you delusional ones..."
    Last edited: Jul 15, 2009
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook