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Argument with physics teacher

  1. Nov 15, 2007 #1
    Today, I had an argument with my physics teacher about the movement of electrons around the nucleus. I have read way more quantum mechanics than any normal high school student and my teacher is trained as an engineer, not a physicist, but I am not sure if I'm right.

    His argument was something like the following:
    Electrons move around the nucleus much like planets around the sun. They move in an elliptical orbit. The centrifugal force is what keeps them from crashing into the nucleus.

    My response was:
    We cannot know the precise position of an electron around the nucleus because of the uncertainty principle (note my name). It is therefore impossible to establish the electron as orbiting (elliptically) the nucleus.

    Other thoughts:
    If we approach the electron as a wave, I doubt any of his classical stuff makes sense. The centrifugal force thing even seems to be in conflict with the erroneous Rutherford and Bohr models.

    I am not too familiar with the following concepts but I believe they also have a role:
    The electron cannot fall into the nucleus because of the exclusion principle.
    If the electron glued itself to the nucleus, its position would be more or less certain, giving it an enormous momentum.
    These classical concepts where an issue after the discovery of the Rutherford model and the only way that an electron would stay out of the nucleus was if it accelerated because it would radiate energy.

    Anyways, those are some arguments that come to mind.

    Could someone please sort this out for me?
    Last edited: Nov 15, 2007
  2. jcsd
  3. Nov 15, 2007 #2


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    He/she is completely wrong!
    The electron and nucleus are different types of particles so exclusion principle does not apply here
    correct so it cannot be "glued" there. Same thing prevents white dwarves or neutron stars from collapsing
    This sounds a bit confused. It was that a classical electric charge in an elliptical orbit would necessarily radiate away energy and so a classical nucleus would be unstable.
  4. Nov 15, 2007 #3


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    Wow, that's painful.
  5. Nov 15, 2007 #4
    Feels good to know I'm right.

    Yeah, thats what I meant, but I'm not a physicist... ...yet.

    What kind of scared me though was when my teacher flatly said something along the lines of "No you're wrong." and then to justify himself, "I have a course prepared on nuclear physics/ I teach nuclear physics."

    Guess that sometimes happens when engeneers teach physics. Still respect the dude though.

    Anyone with a clear, reputable, simple reference that I can print to prove my point to him?
  6. Nov 15, 2007 #5

    simplest case of atomic theory

    granted wikipediaisn't the best reference, but any book or set of lecture notes will say the same. do a google search on hydrogen atom to get more info.

    also a highschool chem book should contain similar information at a simpler level
    Last edited: Nov 15, 2007
  7. Nov 15, 2007 #6


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    Yep. Dead wrong.
  8. Nov 16, 2007 #7


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    How about you just show him a couple of diagrams of http://en.wikipedia.org/wiki/Atomic_orbital" [Broken] and ask him to identify the ellliptical orbit for you?

    Or ask him to explain why a water molecule is bent? No classical explanation does this. Orbitals do with ease.
    Last edited by a moderator: May 3, 2017
  9. Nov 16, 2007 #8


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    I'm not sure whether the conditions below justify what your teacher said. Actually, I don't know what is your teacher thinking. It's inappropriate to think of an electron as a body revolves around the nucleus same as planet moving around the Sun and says its path is deterministic. But, I think it's appropritate to say that the centripetal force balances the electric potential. CORRECT ME IF I AM WRONG! Your answer to your teacher sayings sounds like your teacher is asking a question where you answer other things. What you said regarding uncertainty principle is correct, but he's saying other thing.

    Let's do a semiclassical way of approaching hydrogen atom.

    [tex] T=\frac{1}{2}m v^2-\frac{e^2}{ r} [/tex]

    Well, you should have no question about this, the total energy is the sum of kinetic and potential energy.

    [tex] \frac{m v^2}{r}=\frac{e^2}{r^2} [/tex]

    Well, it's saying the centripetal force is equal to the electric force!!! surprise

    [tex] m vr=n\hbar [/tex]

    The third equation expresses the quantization condition, introduced empirically by Bohr in order to explain the existence of discrete energy levels. he postulated that only circular orbits satisfying this condition are possible trajectories for the electron.

    [tex] E_n=-\frac{1}{n^2}E_1 [/tex]

    [tex] r_n=n^2a_0 [/tex]

    Where E_1 and a_0 are ground state energy and Bohr radius.
    Last edited: Nov 16, 2007
  10. Nov 16, 2007 #9


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    Er.. you are wrong.

    The centripetal force IS the "electric force"!

    "Centripetal force" is a GENERIC term given to a cental force. ANY force can be a centripetal force. Gravity is the centripetal force in planetary motion. The Lorentz force is the centripetal force when a charged particle is moving in a magnetic field. In the case of a charge being pulled in due to coulombic force, then the electric field IS the centripetal force.

    Think about it, if what you claim is true (" ....centripetal force balances the electric potential..."), then what is your NET force? Zero! They balance out, as you claim. Then why would it move in a circular path in the first place? That would violate Newton's first law, wouldn't it?

  11. Nov 16, 2007 #10


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    I just think it's inappropriate to put the analogy of revolving around the sun is same as electron revolving around the nucleus. His argument that says centripetal force acts and balanced by electric potential is correct to some sense.
  12. Nov 16, 2007 #11


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    Ok, I see, I admitted I didn't use the word centripetal force correctly. I agreed with what you said that centripetal force is indeed electric force. How about centrifugal force balanced by electric force.
    Last edited: Nov 16, 2007
  13. Nov 16, 2007 #12


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    Er.. invoking a fictitious force doesn't make it any better. There is no "centrifugral force" unless you intend to sit in the electron's reference frame. I thought this whole issue here is that there is no such ability at tracking an electron's path when we solve for the atom's orbital?

    Why are we making this more complicated than it is? Write down the free-body diagram of a body in a simple circular motion, and that's that.

  14. Nov 16, 2007 #13
    This line of reasoning fails once you try to explain why there can be multiple angular momentum states corresponding to each energy level. In a sense you can trace the problem to the assumption of circular orbits.

    The fact that this model leads to correct prediction for the energy levels, however, suggests that, mingled in with the oversimplifications, there must be some bit of good physical intuition. In this case, it can be found in a physical justification for the introduction of the Bohr quantization condition. If you start by assuming that the electron has wave properties and will interfere with itself if it's orbit is not an integral number of wavelengths, you can actually derive that condition. And, as it happens, this is essentially an over-simplified version of what you're doing when you solve the Schroedinger equation for the hydrogen atom.
  15. Nov 16, 2007 #14
    Teacher's who want to use Bohr's semi-classical model of the hydrogen atom as a teaching tool should at least be clear that this was "thought at one time to be a complete description, but it has its shortcomings, and has since been replaced by quantum mechanics...but the mathematics for that is very difficult, so we are going to work with Bohr's model".
  16. Nov 16, 2007 #15


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    That's incorrect, as others have already said. Electromagnetic radiation from the
    accelerating electron would make it lose energy and spiral inwards.

    But perhaps the best thing to do (to maintain a constructive teacher/student relationship)
    is to point your teacher at this forum and get him to say what he was really thinking (in
    case there was some misunderstanding about the debate). I'm sure the heavy-hitters
    around here will quickly set things straight.
  17. Nov 18, 2007 #16


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    I have taught physics at all levels, from high school tutorials to graduate courses. Let me say that as a physics teacher and researcher, I am deeply disturbed by this post. When a student asks for justification, and the teacher's response is to pull rank ("I teach physics, you don't, so shut up!"), that is criminal! If any teachers are reading this: if your students ask questions that you can't answer, then be honest - admit that you don't know the answer and encourage them to find out, as DpDx>hbar/2 (who I'll refer to as HUP) did. HUP: I cannot praise you enough! I truly feel very sorry for your colleagues who aren't as inquisitive as you are, and I very much hope that they turn out alright. As a active member of APS, I truly worry for them. Keep up the great work - don't trust anyone!

    Especially me!:wink:
  18. Nov 18, 2007 #17
    I thought that Bohr-Sommerfeld theory led to an extremely awkward formalism and that quantum mechanics is much simpler.
  19. Nov 18, 2007 #18
    The simple Bohr model (with circular orbits) can be derived with a couple of simple physical assumptions and algebra. The full quantum version requires one to solve a non-linear partial differential equation (which can, in this particular case, be done analytically). I think there's really no question that the Bohr model is simpler. It's also quite wrong.
  20. Nov 18, 2007 #19

    Hans de Vries

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    Apparently one can still win a Nobel prize with such a picture...


    http://www.nobel-prize.org/EN/Peace/images/iaea.jpg [Broken]


    Regards, Hans.
    Last edited by a moderator: May 3, 2017
  21. Nov 18, 2007 #20
    Yes, but Bohr and Sommerfeld attempted to generalize the result for the hydrogen atom to general systems to obtain a general theory. What they came up with was much more complicated than Quantum Mechanics, http://en.wikipedia.org/wiki/Bohr_model#Refinements".

    Last edited by a moderator: Apr 23, 2017
  22. Nov 22, 2007 #21
    Well, I've been busy the last few days (don't do physics 3 and 2 at the same time kids). Anyway, I talked to my physics teacher the day after my first post and he was quite willing to accept my reasoning seeing as I researched it at length(I have yet to put together some proof). So thanks to nrqed for his rapid and clear response.

    It seems that he had familiarised himself with the Bohr model as is shown in his textbooks. If the so-called nuclear physics class was offered at my high school, the students would do calculations based on that model, that I understand are easy enough to carry out, if not mostly (completely) wrong physically.

    As for what’s criminal IMOHO, is that trough physics 1, 2, and 3, we do almost exclusively mechanics. I don't know if its like that elsewhere in the world, but it's a great way to get inquisitive young minds interested in physics (sarcasm). None of modern (after prehistory :-)) theory is touched: no relativity, no quantum mechanics, no thermodynamics, no electricity/magnetism (ph 4), no particle physics, no optics... ...nothing. I think that it is a problem that should be addressed if we expect to have a new generation of physicists. Students end up thinking that Newton is God and that all physicist do is push stuff and watch it move. I am the only person seriously considering a career in physics in my graduating class.

    …well that’s my rant for the day.
  23. Nov 22, 2007 #22


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    thanks for the update. And you are very welcome!
    What bothers me a bit is that you said tha he teaches nuclear physics!?!
    At what level? I am a bit shocked that someone would teach nuclear physics without a proper understanding of the most basic concepts of atomic physics.
  24. Nov 22, 2007 #23
    I don't really understand why he would have a nuclear physics class prepared that is not offered (most schools in my area don't even offer as many physics classes as we have available), seems to me that this stuff is standardised by the school district (province?). Not to mention that nuclear physics is usually a 3rd or 4th year class in university.

    He prepared the class from textbooks. I think he followed some kind of model. I do not see how this class would be fit for a high school at all though.
  25. Nov 22, 2007 #24
    I think you are mostly correct. The electron is like a fuzz cloud extending to infinity, but mostly concentrated in a region near the nucleus. It is most definitely not a billiard ball in shape or behaviour, but it does have some similar properties, and in this sense your teacher would be partially correct. When you solve the Schrodinger equation for the hydrogen atom and allow the electron to have orbital angular momentum, the math looks almost analogous to solving for elliptical orbits in Newton's theory, with some extra wrenches thrown in of course. Also, electrons don't radiate at the ground state energy, but at higher energies they do radiate and it gets close to classically predicted levels as you get up to the very high energy orbits.
  26. Nov 23, 2007 #25


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    Maybe it is more "elementary nuclear engineering", which is in fact really basic. You know, composition of the nucleus, mass defect (E = mc^2 oooh..... :smile: ) energy balance in reactions, elastic collisions (neutrons on nucleus such as ping pong ball on soccer ball), some elementary nuclear model (radius as a function of N and Z)... fission reactions, moderators, diffusion equation etc...

    You can do that without one single word of actual quantum mechanics (except maybe the concept of "excited state").
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