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Thread: Hydrogen atom

  1. #101  
    Bullshit Intolerant PhDemon's Avatar
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    ..and your questions here show you are still hung up on classical ideas, exchemist has given some very good answers to which I can add nothing meaningful (not surprising as we learned this stuff at the same university, from the same lecturer, albeit a few years apart). It is not a problem with the theory (to really understand it though you have to get into relativistic QM, the Dirac equation etc. which is beyond my competence) it is as exchemist said you are asking for an advanced undergraduate (and possibly parts of a post-grad) course in QM, this is not something that can be explained in a few forum posts. Also if as you state in post #3 "I do not wish to learn QM or QM model of the atom." you are going to have difficulty understanding this and as exchemist seems to be explaining it more to your liking I'll bow out and leave him to help you.

    You may find this links useful:

    Electron spin
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  2. #102  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by exchemist View Post
    1 & 5) No, it is NOT "circling" in the ground 1s state.
    You really have to grasp that the electron in an s orbital is not circling: it is moving but has no average revolving component to its motion, hence no angular momentum..
    Thanks,
    - is there an explanation why this should happen only in 1s? and what is the evidence?or is it just a theory?
    - do the laws of electrostatics apply in 1s? how can the motion of the e be not-circular and respect them?
    OK 2 questions here.

    To the first, yes there is an explanation and evidence. The evidence that electrons in s subshells have no orbital angular momentum is bound up with what we have been discussing, viz. that they exhibit no orbital magnetic moment. (Remember I said the two thing go hand in hand).This is revealed in the fine structure of spectra, since spin-orbit coupling splits all energy levels except the s.

    The explanation is that, when you combine these spectral observations with the form of the solutions of Schroedinger's equation for the H atom - which as I've said before is related to the possible resonant vibrations of the system (spherical harmonics) - you find that the lowest energy solution (the one we call s) has an azimuthal quantum number , l, = 0. Since this is the state that is not split by spin-orbit coupling, the l quantum number must be the one that relates to how many units of angular momentum- and hence orbital magnetic moment about the nucleus - the electron has got.

    As to your second question, again I have to refer you to what I said right back at the beginning of all this. Yes electrostatics applies but, because of the wave nature of matter, the lowest possible energy state for an electron confined by the electrostatic attraction of a nucleus is not zero (i.e. it cannot drop into the nucleus and stay there). This is an example of zero point energy: an inherently quantum phenomenon.

    If it helps, you can think of the s electron as swinging like a pendulum through the nucleus and out the other side and back, i.e. continuing to exchange potential and kinetic energy (as expressed in the Hamiltonian operator in Schroedinger's equation) but without circulating - on average at least - round the nucleus. As I've said several times before, the motion is indeterminate, so you can't know its position in space at any given time, only the cloud of probability, which is spherical in shape.

    This cloud, by the way, does indeed go right up to (and possibly through) the nucleus in s orbitals, whereas in all orbitals with angular momentum (l > 0) the probability density is zero at the nucleus. Thus you can, sort of, see that this corresponds to the s state being a bit like an electron that has "fallen in" (except it keeps coming back out again because it can't lose any more energy), while the others , with l>0, are a bit like like states where it is "circling round" and consequently doesn't "fall in".

    This is what the wave nature of matter does to classical mechanics on the atomic scale. You may find it hard to picture but you just have to live with it, as it really seems to be the way matter behaves at this scale. Personally I find the mystery of it wonderful, which is why I have been prepared to devote so much time trying to communicate it to you. But you really, really have to:

    1) let go of your classical, mechanistic, orbiting particles,
    2) think of waves first, then
    3) get used to quantum indeterminacy, which tells you there are limits to which properties of the system are even defined at any one time, then finally
    4) allow yourself to see how the waves kind of approximate back to the classical particles, though in some respects only, whereas in others totally new phenomena emerge.
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  3. #103  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by PhDemon View Post
    I'm sorry but your questions indicate to me that this is something you are confused about and in the way I suggested.
    this implies to me you are still thinking of spin in a classical sense. It has "spin" (i.e. intrinsic angular momentum) but does not spin (in a classical sense).
    I told you I accept that e has an intrinsic property called 'wibble'. Fair enough. it is like L. OK.
    But please tell how do you calculate wibble? L is mvr, right? to what parameters is it related? how does it react to torque? how does it generate a magnetic field, a magnetic moment , how is this latter related to wibble by what ratio?
    If someone provides all this answer wibble is acceptable, a scientific concept, and not just a name.
    I know exchemist is doing his best, probably it is the theory that is incomplete
    No the theory is pretty solid, it's just that you are being unreasonable in demanding that the explanation should conform to 6th form Newtonian mechanics. mvr etc. The theory behind all this stuff is called Quantum Mechanics (or sometimes Wave Mechanics) precisely because you have to chuck out all the mvr stuff and start again. Dirac was the first to account for electron spin and his way of doing so - which was itself imperfect - was not taught to us undergraduate chemists at Oxford, even those like me that took the supplementary QM course, because it was too difficult and was not considered relevant to chemical applications of QM. So I am afraid you are asking the impossible. You can read about it here: Dirac equation - Wikipedia, the free encyclopedia
    but don't ask me about it as it is beyond me.

    But do not let me put you off. The urge to know is precious and to be nurtured. However you will just have to return to the explanation of electron spin when you read physics (not chemistry!) at a really good university. Until then, my advice is to park the issue, without forgetting it, and be patient.
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  4. #104  
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    Thanks , exchemist, for your excellent seminar on H atom, I do appreciate your effort.
    I still might have many questions, but realize you have done more than enough.
    If you could just conclude the issue just explaining how we get the total momentum= hbar/2 +, I'd be grateful and I'll start a new thread , where L and mu concept might incidentally be perfected.
    Thanks again!!
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  5. #105  
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    Quote Originally Posted by whizkid View Post
    Thanks , exchemist, for your excellent seminar on H atom, I do appreciate your effort.
    I still might have many questions, but realize you have done more than enough.
    If you could just conclude the issue just explaining how we get the total momentum= hbar/2 +, I'd be grateful and I'll start a new thread , where L and mu concept might incidentally be perfected.
    Thanks again!!
    Yes, it's probably time to call a halt soon. The thread is getting long and unwieldy. But I have quite enjoyed the trip down memory lane that you have forced me to retrace.

    Regarding your final question, I was wondering - with a slightly sinking feeling - when we were going to get to this. The quantity hbar/2 again relates to angular momentum only. It is not the total but the projection of the angular momentum vector along a specified axis. In QM, you can't perfectly align, say, the spin of an electron exactly with the angular momentum due to its orbital motion (if it has any), as that would be specifying more than you are allowed to know about the system. Quantum indeterminacy strikes again!

    Let's call the direction of the orbital angular momentum the z axis. The spin angular momentum vector can only obliquely align along this z axis, thus defining a z component which can be defined and measured exactly, but leaving a remaining component in the x-y plane that is indeterminate - unknowable in direction. Using a semi-classical pictorial analogy, one often speaks of the spin vector precessing round the z axis, but the so-called "precession" is once more not a well-defined circular motion, it's more of a probabilistic pseudo-precession.

    So, you get a z component of spin angular momentum equal to hbar/2, which is less than the total spin angular momentum of the electron, since you may recall this is given by √(1/2(1/2+1) hbar = √3/2hbar. The remaining part of the projection is mysteriously somewhere in the x-y plane but is constantly shifting in a way you cannot specify. Every time angular momenta are combined, or every time an external axis is defined, e.g. by getting angular momenta aligned in an external magnetic field, this pseudo-precession and formation of a z component, less than the total, applies.

    So when we talk of spin "up" or 'down" or aligned with something or opposed to it, we mean the z component is up or down, aligned or opposed - the rest being unknown.

    But this is hard. The entire first term of my physical chemistry tutorials at university were devoted to atomic spectra and structure and it was quite a shock to me after school level. There is a lot to it and it is not easy to take it all in in one go. All I can do here is give you a rough idea of how it works and the main concepts behind it.

    We've now left the Bohr model of the atom a long way behind, I think you will agree.
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  6. #106  
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    [QUOTE=exchemist;551210]
    Quote Originally Posted by whizkid View Post
    I think you will agree.
    Great!!
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    Quote Originally Posted by PhDemon View Post
    ..and your questions here show you are still hung up on classical ideas,
    I am not, I said and confirm.
    If you are a physicist and are willing to discuss some principles while exchemist takes a rest, please let me know.

    Actually, it is QM that is still hung up on classical model, I am just stressing its contradictions, for the little I know.

    Suppose you show someone a mysterious suitcase (in which there is a spinning wheel) and you tell it has a 'wibble': a mysterious phenomenon that has intrinsic angular momentum. Lifting, rocking,rolling,rotating ,turning it one can find out the direction of the pseudovector L, then,by rotating it, one can find out the value of L etc.
    All the principles of physics and its laws are being respected, anyone would accept your theory (that a suitcase that is not spinning has intrinsic momentum) without hesitation.

    But if I take the wheel out, and tell you to show me the 'wibble' ( intrinsic L) of the suitcase you fail, the same as - you fail to show the intrinsic spin of an electronin all other cases when it is on its own, outside an atom. Then theory is in trouble.
    - If you say that an electron is not orbiting, has no orbital momentum, and then say that the 'wibble' is not aligned to the orbital momentum, that is a patent contradiction.
    - If you say that the electron can pass, linger by or pass through a proton, that is not confirmed anywhere else in the world. Even if you shoot an electron at a proton it deviates and goes round, and if you exceed with energy it produces a neutron.
    ...... and so on
    Please tell me if these notions are incorrect
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  8. #108  
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    I got as far as "it is QM that is still hung up on classical model" and stopped reading my patience level for nonsense of that magnitude is miniscule. Exchemist has more patience wait for him.
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  9. #109  
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    Quote Originally Posted by whizkid View Post
    Please tell me if these notions are incorrect
    Yes.

    - you fail to show the intrinsic spin of an electronin all other cases when it is on its own, outside an atom. Then theory is in trouble.
    I wasn't aware of this before. It appears to be a measurement problem, in that the magnetic moment of a single electron is too small to measure in relation to the charge and mass. I don't know why this would contradict theory; at worst it means some consistent evidence is not available.

    However, it seems it isn't that clear cut. Bohr's argument was semi-classical and so:
    It is widely held that Bohr has shown that the spin of a free electron is not measurable. We point out that Bohr's argument has some important ifs and buts. A concrete configuration is calculated to produce a clear spin separation. This is then shown not to contradict Bohr's reasoning.
    http://www.phys.tue.nl/ktn/Wim/bohrfopl1.pdf

    - If you say that an electron is not orbiting, has no orbital momentum, and then say that the 'wibble' is not aligned to the orbital momentum, that is a patent contradiction.
    It does have orbital angular momentum (see post #8, for example).

    - If you say that the electron can pass, linger by or pass through a proton, that is not confirmed anywhere else in the world. Even if you shoot an electron at a proton it deviates and goes round, and if you exceed with energy it produces a neutron.
    Because the position is not well-defined/localised it must spend some time in the nucleus.

    ...... and so on
    You seem very reluctant to accept quantum theory. This seems to be nothing more than argument from incredulity, despite the detailed and excellent explanations from exchemist.
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  10. #110  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by PhDemon View Post
    ..and your questions here show you are still hung up on classical ideas,
    I am not, I said and confirm.
    If you are a physicist and are willing to discuss some principles while exchemist takes a rest, please let me know.

    Actually, it is QM that is still hung up on classical model, I am just stressing its contradictions, for the little I know.

    Suppose you show someone a mysterious suitcase (in which there is a spinning wheel) and you tell it has a 'wibble': a mysterious phenomenon that has intrinsic angular momentum. Lifting, rocking,rolling,rotating ,turning it one can find out the direction of the pseudovector L, then,by rotating it, one can find out the value of L etc.
    All the principles of physics and its laws are being respected, anyone would accept your theory (that a suitcase that is not spinning has intrinsic momentum) without hesitation.

    But if I take the wheel out, and tell you to show me the 'wibble' ( intrinsic L) of the suitcase you fail, the same as - you fail to show the intrinsic spin of an electronin all other cases when it is on its own, outside an atom. Then theory is in trouble.
    - If you say that an electron is not orbiting, has no orbital momentum, and then say that the 'wibble' is not aligned to the orbital momentum, that is a patent contradiction.
    - If you say that the electron can pass, linger by or pass through a proton, that is not confirmed anywhere else in the world. Even if you shoot an electron at a proton it deviates and goes round, and if you exceed with energy it produces a neutron.
    ...... and so on
    Please tell me if these notions are incorrect
    Whizkid, after all this time, I am frankly both astonished and a little depressed to find you still saying wrongly that because these particles have no well-defined motion they do not have angular momentum. I have told you until I am blue in the face that they do, and that it is just like the way the electron has kinetic energy without having a well-defined speed.

    I have repeatedly pointed out that the absence of a well-defined speed or circular motion does not mean that the thing is not moving or circulating or spinning. In both cases it is simply that we can't know how they are moving: the only properties we can measure are angular momentum and energy respectively.

    This is quantum indeterminacy and, like it or not, it is an observed fact of how the world seems to work at the atomic scale, exactly as predicted by the theory of Heisenberg and others. Until you can get this (admittedly counterintuitive) idea into your head, further discussion is pointless.

    This is what we mean when we say you are still stuck in a classical mindset. Nobody blames you for finding it hard. We all did when we first encountered it.

    As for your incredulity about the electron spending time inside the nucleus, why should it not? What barrier would prevent this, in your view? There is a Coulomb attraction. Where is the repulsion that would stop it going in?
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  11. #111  
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    Quote Originally Posted by PhDemon View Post
    I got as far as "it is QM that is still hung up on classical model" and stopped reading my patience level for nonsense of that magnitude is miniscule. Exchemist has more patience wait for him.
    Phew, yes, it is a struggle. But I try to recall what it was like in November 1972, in my cold room in Christ Church, wrestling with G. Herzberg before my next tutorial with Richard.

    It's a big conceptual - and philosophical - leap to recognise that nature limits the amount you can know about a system, in contradiction to one's school physics upbringing on the confident certainties of Newton. I don't know how old Whizkid is or what level he or she has reached.

    (But a lot more productive than trying to engage with some others currently active on this forum, ahem.)
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  12. #112  
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    But, I'm guessing when RPW explained it you didn't repeat the same errors at the next tutorial (as whizkid is essentially doing). PWA certainly had no patience for students who did that and I can't imagine Richard being impressed either
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    Quote Originally Posted by exchemist View Post
    You really have to grasp that the electron in an s orbital is not circling: it is moving but has no average revolving component to its motion, hence no angular momentum.
    Quote Originally Posted by exchemist View Post
    Phew, yes, it is a struggle.
    Thanks for your sympathy, exchemist .I do not want to abuse your patience, I apologize for replying to PhD, but I thought I might get some information I am surely lacking in orderto grasp your explanation.

    I replied to PhD as I do not accept the judgement that I am refusing QM or that I am stuck to classical model.That is superficial, snobbish and derogatory. I am surely ignorant but not stupid or prejudiced, I am trying to decide if I am to study physics or not.

    I read in another forum the nice motto of some scientists 'if you cant tell it to your grandmother you have not really understood it', and I agree ,I hope you can tell it to your grandson too.
    There is nothing difficult in the laws of this universe, everything is simple and has a geometric, plastic representation. Any scientific and coherent theory can be explained and understood by a picture. I hope you agree.

    I noticed Strange is more willing to give me some supplementary information, I'll try to ask him some questions and hope other member do not mind.
    Thanks again, exchemist.
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  14. #114  
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    So it's "superficial, snobbish and derogatory" to point out why you seem to be having trouble grasping this :shrug: My assessment is based on the fact you are STILL asking about classical concepts, if your approach to learning is to label advice as snobbish etc. I pity your tutors if/when you go to university. I'll leave you to it...
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    Quote Originally Posted by Strange View Post
    I wasn't aware of this before. It appears to be a measurement problem.... at worst it means some consistent evidence is not available. .
    It does have orbital angular momentum (see post #8, for example).
    Could you explain how the angular momentum is measured concretely?
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    Quote Originally Posted by PhDemon View Post
    So it's "superficial, snobbish and derogatory" to point out why you seem to be having trouble grasping this :shrug: My assessment is based on the fact you are STILL asking about classical concepts, if your approach to learning is to label advice as snobbish etc. I pity your tutors if/when you go to university.
    I do not wish to polemize with you, but you are only confirming your snobbish and contemptuous attitude to me.
    I am more humble than you can fancy, I have trouble grasping this, as I said, because as I stated long ago I am ignorant or thick, or both.
    If you are intolerant, don't waste your time to figure out why I am having trouble.
    Thanks.
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  17. #117  
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    Quote Originally Posted by whizkid View Post
    Could you explain how the angular momentum is measured concretely?
    Of a single particle? As far as I know, it hasn't been (yet). The paper just points out that there is no fundamental reason why it can't be, given appropriate technology.

    I can't see this as being very important, though. There are many things we can't measure. That doesn't falsify the underlying theory. You need to measure something that contradicts the predictions of the theory, to do that.
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  18. #118  
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    I tried to offer advice as to what your conceptual stumbling block was, you didn't like it :shrug: Your problem not mine. I'll leave to the others to try and help you. Good luck.
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    Quote Originally Posted by Strange View Post
    Of a single particle? As far as I know, it hasn't been (yet)..
    I was referring to this post:
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by whizkid View Post
    But please tell how do you calculate wibble?
    It isn't calculated; it is measured.
    You read that that the 1H spin is 1/2, as the momentum of the spin L(e) on the z-axis is 1/2.
    How did they measure that? how do tou tell you are not measuring mu(e), or L(o) or L(p)?
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  20. #120  
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    The Stern-Gerlach experiment is the obvious example. (Hasn't that already been discussed?) I'm sure there are others, but this is not an area I am very familiar with. And of course, it can be measured indirectly by measuring the magnetic moment.

    I think it is also predicted by the relativistic Dirac equation. But this is even further outside my area of knowledge.
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  21. #121  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by exchemist View Post
    You really have to grasp that the electron in an s orbital is not circling: it is moving but has no average revolving component to its motion, hence no angular momentum.
    Quote Originally Posted by exchemist View Post
    Phew, yes, it is a struggle.
    Thanks for your sympathy, exchemist .I do not want to abuse your patience, I apologize for replying to PhD, but I thought I might get some information I am surely lacking in orderto grasp your explanation.

    I replied to PhD as I do not accept the judgement that I am refusing QM or that I am stuck to classical model.That is superficial, snobbish and derogatory. I am surely ignorant but not stupid or prejudiced, I am trying to decide if I am to study physics or not.

    I read in another forum the nice motto of some scientists 'if you cant tell it to your grandmother you have not really understood it', and I agree ,I hope you can tell it to your grandson too.
    There is nothing difficult in the laws of this universe, everything is simple and has a geometric, plastic representation. Any scientific and coherent theory can be explained and understood by a picture. I hope you agree.

    I noticed Strange is more willing to give me some supplementary information, I'll try to ask him some questions and hope other member do not mind.
    Thanks again, exchemist.
    OK that's fine Whizkid. I just suggest you do your best to phrase your question in terms such as "I don't understand how…..such and such", rather than implying there must be something wrong or inconsistent about QM, as you seemed to in the post that annoyed PhD. Because, while QM does of course have some very uncomfortable loose ends, the part we are discussing (atomic structure and properties) is very well worked out and accounts successfully for just about all atomic properties.

    I think some reading on quantum indeterminacy would be a good thing in your case. And also, if you have not covered it in depth yet in your studies, something about waves: wavelength and frequency, resonance, standing waves and harmonics, superposition of different wavelengths (mathematically a Fourier series). These are the tools you need in your head for QM to start to make some kind of intuitive sense, rather than just seeming "impossible" or "wrong". (The Oxford tutor PhDemon and I have been talking about had been an amateur radio enthusiast as a teenager and he told me that for someone with that background, the uncertainty principle and other wave aspects of matter make absolute sense, very naturally.)
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  22. #122  
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    Quote Originally Posted by exchemist View Post
    (The Oxford tutor PhDemon and I have been talking about had been an amateur radio enthusiast as a teenager and he told me that for someone with that background, the uncertainty principle and other wave aspects of matter make absolute sense, very naturally.)
    Discovering that the initial formulation of quantum mechanics was in terms of Fourier transforms was an eye opener for me; it suddenly became "of course".
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    Quote Originally Posted by Strange View Post
    The Stern-Gerlach experiment is the obvious example. .
    AfaIk the experiment only shows that there are 2 beams spin-up/down, and does not measure it.
    What about magnetic resonance? is it different from Stern-Gerlach?

    But that is what I do not understand: you can detect (and measure) spin only by a magnetic field , right? you cannot rotate a single electron mechanically (as you said)? right? why do you conclude that you are measuring L and not mu?
    Do you follow me?
    This is just physics, not QM, not Bohr!
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  24. #124  
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    Apparently, the angular momentum of electrons has been measured: http://www.ncbi.nlm.nih.gov/pmc/arti...00312-0017.pdf

    More here: electromagnetism - Measuring the spin of a single electron - Physics Stack Exchange
    (I think it is fantastic that there is a forum where questions get answered by the likes of Lubos Motl!)
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by Strange View Post
    The Stern-Gerlach experiment is the obvious example. .
    AfaIk the experiment only shows that there are 2 beams spin-up/down, and does not measure it.
    It does measure it. The amount of deflection depends on spin.

    Do you follow me
    Not really. Spin is an intrinsic property in quantum theory, and then there are experiments that measure the magnitude of the spin.
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    Quote Originally Posted by Strange View Post
    Do you follow me
    Not really.
    Thanks for the links and your help, Strange!
    I'll try to explain:
    suppose we have a coil in which there is current, and which,therefore, has magnetic moment (mu)
    if we want to measure mu we put it in a magnetic field (Stern-gerlach) B and we get a value mu=k
    suppose we now make the coil spin around the axis of mu, it acquires momentum L (mvr) = j
    if we apply same field B what value of mu do we find?

    Hope it's clear
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  27. #127  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by Strange View Post
    The Stern-Gerlach experiment is the obvious example. .
    AfaIk the experiment only shows that there are 2 beams spin-up/down, and does not measure it.
    What about magnetic resonance? is it different from Stern-Gerlach?

    But that is what I do not understand: you can detect (and measure) spin only by a magnetic field , right? you cannot rotate a single electron mechanically (as you said)? right? why do you conclude that you are measuring L and not mu?
    Do you follow me?
    This is just physics, not QM, not Bohr!
    Ah. Do you perhaps mean that all the evidence is in fact evidence for a magnetic moment, rather than being directly for an underlying angular momentum itself , like that which in QM is presumed to give rise to that magnetic moment?

    Not a bad question, I think and you may have a point. I admit I cannot offhand think of an experiment that detects spin angular momentum by a means other than magnetic moment. Though maybe other readers will be able to do so.

    If not, I suppose you conceivably might try to construct a theory in which the electron has intrinsic magnetic moment but not angular momentum.

    But then, you would need to account for the type of quantisation of spin magnetic moment that we see : the highly suggestive presence of the √(s(s+1))hbar - related magnetic moment, and its related z axis projection, both of which mimic exactly what we see for orbital angular momentum/magnetic moment.

    And I'm pretty sure Dirac' s model includes angular momentum itself, not just magnetic moment, so you would need to show why his model was wrong or unnecessary. Good luck with that!

    But I'll happily bow to an input from real physicists at this point.
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    This is more a question of philosophy and how our measurements relate to the real world (if such a thing exists), etc.

    You can apply the same doubts to the measurement of everything. After all, everything is measured indirectly. For example, we don't measure the wavelength of light using a tiny ruler to measure the distance between peaks. We know it is a wave and we can deduce values which correspond to the length of the wave.

    Similarly, quantum theory says that particles have a property with the same units as angular momentum (confusingly called "spin") and that this generates a known magnetic moment. We can measure the magnetic moment, in several different ways, and it matches prediction to extremely high levels of accuracy.

    Also, angular momentum is a conserved quantity and (I think) there are ways of confirming the value in that way (e.g. when particles decay).

    In the end, it comes down to the fact that nothing rests on a single theory or measurement. Some people come to the forum with their personal ideas and think that if they can disprove one thing, then the whole of science will come down like a house of cards. But science isn't like that. It is more like a complex and very rigid structure of mutually reinforcing components. This is why it is so hard for new theories; they can't just change one bit, they end up having to change nearly everything.
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    Quote Originally Posted by exchemist View Post
    Ah. Do you perhaps mean that all the evidence is in fact evidence for a magnetic moment,
    I admit I cannot offhand think of an experiments that detects spin angular momentum by a means other than magnetic moment.
    That's just what I have tried to tell you many times over. You are detecting by all experiment the existence of a magnetic moment mu, or , at most the existence of a mu and an L (which is undetectable and unmeasurable).
    I think I remarked that an intrinsic mu in a natural magnet in a charge is more plausible than an intrinsic L without an actual spinning or rotation.
    If then the theory insists that there is an L , then what you detect must needs be the sum of mu and L.
    Lastly you keep forgetting that there surely is a mu and an L of the proton which has allegedly the same value (sterngth/importance/relevance) of the electron spin.
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    Quote Originally Posted by Strange View Post
    This is more a question of philosophy a.
    No Strange, this is a very down-to-earth question or problem.Could you please answer my concrete example? It's just plain, very simple beginner's physics. I can reply to you only when I check if my basic knowledge is correct.

    I have really no problem to put up with quantum indeterminacy, nor have any philosophical bend, just looking for a clear picture.
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    The thing is, we can measure spin of uncharged particles (which obviously have no magnetic moment) and relate this to the spin of charged particles through conservation laws. I don't see the point of trying to argue that particles don't have intrinsic spin when nearly 100 years of experiment have confirmed it.
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  32. #132  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by exchemist View Post
    Ah. Do you perhaps mean that all the evidence is in fact evidence for a magnetic moment,
    I admit I cannot offhand think of an experiments that detects spin angular momentum by a means other than magnetic moment.
    That's just what I have tried to tell you many times over. You are detecting by all experiment the existence of a magnetic moment mu, or , at most the existence of a mu and an L (which is undetectable and unmeasurable).
    I think I remarked that an intrinsic mu in a natural magnet in a charge is more plausible than an intrinsic L without an actual spinning or rotation.
    If then the theory insists that there is an L , then what you detect must needs be the sum of mu and L.
    Lastly you keep forgetting that there surely is a mu and an L of the proton which has allegedly the same value (strngth) of the electron spin.

    Hold on, hold on. You CANNOT add apples and oranges. Angular momentum does not have the same units as magnetic moment. You can't add them, any more than you can add the charge on the electron to its mass. Think clearly, please!

    I"m glad I've understood your point, but as I've also said in my reply, (a) others may recall demonstrations of angular momentum that do not depend on magnetic moment, (b) the maths of the size of the magnetic moment is consistent with an angular momentum mechanism, ( c) Dirac's formalism almost certainly treats it as having angular momentum. And further, which I did not mention, (d) if it does not spin, you need another way to account for how the magnetic moment arises, whereas spin accounts for it nicely. You say an intrinsic mu is more plausible, but how do you justify that, seeing as magnetism arises from moving electric charges?

    So do not run away with some wacky idea that QM is wrong and there is no spin! You will have a lot of explaining to do if you want such an idea to gain traction.
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  33. #133  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by Strange View Post
    This is more a question of philosophy a.
    No Strange, this is a very down-to-earth question or problem.Could you please answer my concrete example? It's just plain, very simple beginner's physics. I can reply to you only when I check if my basic knowledge is correct.

    I have really no problem to put up with quantum indeterminacy, nor have any philosophical bend, just looking for a clear picture.
    But you clearly DO have a problem, Whizkid. You still cannot take in that in QM a thing can have angular momentum without having a defined orbital motion. Or has that penny now really, really, dropped?
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    Quote Originally Posted by exchemist View Post
    You CANNOT add apples and oranges. Angular momentum does not have the same units

    So do not run away with some wacky idea that QM is wrong .
    I cannot anwer you if someone does not reply to my post #126.

    You may rest assured, no whacky ideas about QM, nor did I say that you cannot have intrinsic L, please read me right, I just ventured that, in case, an intrinsic mu is more plausible than intrinsic mu, don't you agree?

    So, it's OK, there is an intrinsic L and an intrinsic mu, without a definite orbital motion, alright, I'd go even one step further, I'd agree if you said that they exist even without any motion at all: Strange said it does not really spin, you said it always spins. You all seem to think that this is my problem.THIS is absolutely irrelevant to my quest. Can we forget about this aspect?

    What happens to our spinning coil when we apply same B field?
    Last edited by whizkid; April 12th, 2014 at 10:01 AM.
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    Quote Originally Posted by Strange View Post
    I don't see the point of trying to argue that particles don't have intrinsic spin when nearly 100 years of experiment have confirmed it.
    Where did I give you that impression?
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    Quote Originally Posted by whizkid View Post
    Where did I give you that impression?
    Quote Originally Posted by whizkid View Post
    an intrinsic mu is more plausible than intrinsic mu, don't you agree?
    No. Why would it be.

    Spin is a part of QM, it is quantized and conserved, and exists for both charged and uncharged particles. It plays an important role in the different behaviour of fermions and bosons and, importantly, the difference between them.

    None of that applies to magnetic moment so why would you consider it more fundamental?

    What happens to our spinning coil when we apply same B field?
    There are no spinning coils in electrons.
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  37. #137  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by exchemist View Post
    You CANNOT add apples and oranges. Angular momentum does not have the same units

    So do not run away with some wacky idea that QM is wrong .
    I cannot anwer you if someone does not reply to my post #126.

    You may rest assured, no whacky ideas about QM, nor did I say that you cannot have intrinsic L, please read me right, I just ventured that, in case, an intrinsic mu is more plausible than intrinsic mu, don't you agree?

    So, it's OK, there is an intrinsic L and an intrinsic mu, without a definite orbital motion, alright, I'd go even one step further, I'd agree if you said that they exist even without any motion at all: Strange said it does not really spin, you said it always spins. You all seem to think that this is my problem.THIS is absolutely irrelevant to my quest. Can we forget about this aspect?

    What happens to our spinning coil when we apply same B field?
    Sorry I've lost you. Where does a spinning coil come into it?

    Classically, the magnetic dipole due to a coil comes from the circulatory motion of the charge in the wires wound round in a circle that make up the coil. You don't need to make that spin as well. But (again classically) if you have a charged sphere, it needs to move in some kind of circulatory manner to create a magnetic dipole. This can be either by making it spin or orbit something.

    In the QM world you get some of the features of spin and orbital motion, including angluar momentum and magnetic moment, but not others (well-defined angular speeds and positions for example). When Strange or I talk of "spin", we mean it in the same sense as the orbital motion of an electron in an orbital with l>0. ​That is to say, it has some of the properties of a spinning (or orbiting) object, but not others.
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    Quote Originally Posted by Strange View Post
    None of that applies to magnetic moment so why would you consider it more fundamental?
    I am not entitled to produce theories or make hypotheses, the little I know just tells me that it is more likely that a (charged) particle be (for some obscure reason) a natural dipole/magnet than have the properties of mechanical momentum without motion. That's all

    There are no spinning coils in electrons.
    Of course I know that, Strange, I need that basic info to shape my general ideas, aren't you able to answer that simple technical question?
    - what about our supercilious physicist?
    Exchemist says I cannot add apple and oranges, but that is a practical, real example, so, something must happen, maybe a milk shake?
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    Quote Originally Posted by exchemist View Post
    Sorry I've lost you. Where does a spinning coil come into it?
    .
    Does not, I need that info in order to avoid saying stupid things
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    Quote Originally Posted by exchemist View Post
    But (again classically) if you have a charged sphere,
    Is e considered an elementary (indivisible) charge or is it made up by a number of many elementary ones?
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  41. #141  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by exchemist View Post
    Sorry I've lost you. Where does a spinning coil come into it?
    .
    Does not, I need that info in order to avoid saying stupid things
    OK but having re-read your post 126, about putting a coil into a Stern-Gerlach experiment, I can't for the life of me see why you want to make it spin, seeing as it ALREADY has a magnetic moment due to its circular or toroidal shape.

    Making the coil spin will make the magnetic moment tumble, I suppose. But isn't what happens in an atom. The spinning (or rather "spinning" since it is QM pseudo motion) of the electron does the job of the shape of the coil, to produce a magnetic moment pointing in a direction. If the Stern Gerlach field is energised it goes into one or other of only 2 allowed states, with a z component aligned or opposed to the field. The moment can't tumble as this would mean taking other orientations which are not allowed in QM.

    There: I've done my best, but I can't see how this thought experiment of yours is going to help you.
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  42. #142  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by exchemist View Post
    But (again classically) if you have a charged sphere,
    Is e considered an elementary (indivisible) charge or is it made up by a number of many elementary ones?
    Elementary.
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    Quote Originally Posted by exchemist View Post
    putting a coil into a Stern-Gerlach experiment,
    Making the coil spin will make the magnetic moment tumble, I suppose..
    Is the Stern_Gerlach machine the only means we have to test a magnetic moment?
    I am not thinking of 1H now, in general I'd like to know how we in practice measure a magnetic moment:

    suppose in the coil (#126) flows 1 Ampere, that C is 1 meter long, we have a formula that tells us what is B(c) and mu. How do we get the actual value of mu?
    -what is the apparatus we need , what should the external B(x) be? what its orientation?can we get the exact value in one go? etc
    Then, if the coil weighs 1 Kg and the speed of rotation is 1m/s (C is 1 m) what value do we get of mu with same B(x)? it is the same?
    - I suppose not, you hinted we cannot add k+j , so what is the value we register , how would you interpret it, could you discern apples from oranges?

    The only part that concerns our present discussion is that I assumed (so far) that, if you are not allowed to touch the coil, you cannot tell what part of the value you get is due to L and what part to mu.
    If I was wrong I should have to reconsider many posts, and I do not want to cause any more useless trouble. Do you follow me?
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  44. #144  
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    When you want to get back to quantum theory, here is a description of an experiment to measure the magnetic moment of a single electron.
    http://gabrielse.physics.harvard.edu...ticMoment.html
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  45. #145  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by exchemist View Post
    putting a coil into a Stern-Gerlach experiment,
    Making the coil spin will make the magnetic moment tumble, I suppose..
    Is the Stern_Gerlach machine the only means we have to test a magnetic moment?
    I am not thinking of 1H now, in general I'd like to know how we in practice measure a magnetic moment:

    suppose in the coil (#126) flows 1 Ampere, that C is 1 meter long, we have a formula that tells us what is B(c) and mu. How do we get the actual value of mu?
    -what is the apparatus we need , what should the external B(x) be? what its orientation?can we get the exact value in one go? etc
    Then, if the coil weighs 1 Kg and the speed of rotation is 1m/s (C is 1 m) what value do we get of mu with same B(x)? it is the same?
    - I suppose not, you hinted we cannot add k+j , so what is the value we register , how would you interpret it, could you discern apples from oranges?

    The only part that concerns our present discussion is that I assumed (so far) that, if you are not allowed to touch the coil, you cannot tell what part of the value you get is due to L and what part to mu.
    If I was wrong I should have to reconsider many posts, and I do not want to cause any more useless trouble. Do you follow me?
    Strange's post describes it being done for an electron.

    For macroscopic objects with magnetic moment, here is a description of a commercial device for doing it: http://www.serviciencia.es/not-apli/NAS01-i.pdf

    But please don't ask me to go into either of these with you. We would be getting off-topic and I don't have the energy for such a digression.
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  46. #146  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by exchemist View Post
    You really have to grasp that the electron in an s orbital is not circling: it is moving but has no average revolving component to its motion, hence no angular momentum.
    Quote Originally Posted by exchemist View Post
    Phew, yes, it is a struggle.
    Thanks for your sympathy, exchemist .I do not want to abuse your patience, I apologize for replying to PhD, but I thought I might get some information I am surely lacking in orderto grasp your explanation.

    I replied to PhD as I do not accept the judgement that I am refusing QM or that I am stuck to classical model.That is superficial, snobbish and derogatory. I am surely ignorant but not stupid or prejudiced, I am trying to decide if I am to study physics or not.

    I read in another forum the nice motto of some scientists 'if you cant tell it to your grandmother you have not really understood it', and I agree ,I hope you can tell it to your grandson too.
    There is nothing difficult in the laws of this universe, everything is simple and has a geometric, plastic representation. Any scientific and coherent theory can be explained and understood by a picture. I hope you agree.

    I noticed Strange is more willing to give me some supplementary information, I'll try to ask him some questions and hope other member do not mind.
    Thanks again, exchemist.
    Whizkid, I'm sorry to have to disabuse you of this notion as well. It goes out of the window along with Newtonian mechanics, when we come to QM. (By the way, the same goes for General Relativity, it seems to me.)

    I do strongly agree that a pictorial explanation is by far the easiest for most people to grasp. It is also the case that science uses the principle of Occam's razor, i.e. the explanation chosen should be the simplest one that is consistent with the observations. But the sting in the tail of the Occam's razor principle is the latter part, highlighted. The sad fact is that one cannot do QM justice without a lot of maths. All the pictorial representations (and I have largely confined myself to these in this thread) can only give the reader an impression of what is going on. There are caveats about it being "a bit like" or "sort of like" something, or that "you can if you like picture such and such as...", all over the place in the thread.

    Chemists use these pictorial representations a great deal, as we are really after explanations of molecular properties and interactions, for which the rigorous mathematical QM solutions are almost impossibly complicated. But physicists modelling quantum reality accurately have to do it with maths.

    It depends what sort of grandmother one has, of course, but I found towards the end of the 6th Form I could no longer explain what I was studying to my parents, both of whom were Oxford arts graduates. My grandmothers would have been even more of a stretch. It was a bit sad but part of the loneliness of growing up.
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    Quote Originally Posted by Strange View Post
    Spin is a part of QM, it is quantized and conserved, and exists for both charged and uncharged particles...
    None of that applies to magnetic moment
    .
    Quote Originally Posted by Strange View Post
    we can measure spin of uncharged particles.
    Quote Originally Posted by Strange View Post
    , quantum theory says that particles have a property with the same units as angular momentum and that this generates a known magnetic moment.
    .
    Quote Originally Posted by Strange View Post
    The Stern-Gerlach experiment.... does measure it. The amount of deflection depends on spin
    Quote Originally Posted by Strange View Post
    here is a description of an experiment to measure the magnetic moment of a single electron.
    http://gabrielse.physics.harvard.edu...ticMoment.html
    charged and uncharged particles have intrinsic spin and angular momentum L,
    those with charge generate a magnetic moment mu, the others do not.
    an electron is charged then it has
    L(e) = 3/2 hbar with a component on the z-axis L(e)z = 1/2 hbar in an 1H atom
    mu(e) = 1.001156 hbar/2
    please correct any imperfection!
    spin in free electrons has been detected and measured?,
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by whizkid View Post
    Could you explain how the angular momentum is measured concretely?
    Of a single particle? As far as I know, it hasn't been (yet).
    one of you links seems to it has!
    Could you give me a link where an uncharged particle's spin is measured?
    Thank you
    Last edited by whizkid; April 15th, 2014 at 03:50 AM.
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  48. #148  
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by Strange View Post
    Spin is a part of QM, it is quantized and conserved, and exists for both charged and uncharged particles...
    None of that applies to magnetic moment
    .
    Quote Originally Posted by Strange View Post
    we can measure spin of uncharged particles.
    Quote Originally Posted by Strange View Post
    , quantum theory says that particles have a property with the same units as angular momentum and that this generates a known magnetic moment.
    .
    Quote Originally Posted by Strange View Post
    The Stern-Gerlach experiment.... does measure it. The amount of deflection depends on spin
    Quote Originally Posted by Strange View Post
    here is a description of an experiment to measure the magnetic moment of a single electron.
    http://gabrielse.physics.harvard.edu...ticMoment.html
    charged and uncharged particles have intrinsic spin and angular momentum L,
    those with charge generate a magnetic moment mu, the others do not.
    an electron is charged then it has
    L(e) = 3/2 hbar with a component on the z-axis L(e)z = 1/2 hbar in an 1H atom
    mu(e) = 1.001156 hbar/2
    please correct any imperfection!
    spin in free electrons has been detected and measured?,
    Quote Originally Posted by Strange View Post
    Quote Originally Posted by whizkid View Post
    Could you explain how the angular momentum is measured concretely?
    Of a single particle? As far as I know, it hasn't been (yet).
    one of you links seems to it has!
    Could you give me a link where an uncharged particle's spin is measured?
    Thank you
    Shouldn't the angular momentum be √(1/2 (1/2 +1)) h bar, and therefore √(3/4) h bar = √3/2 h bar ?
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    Quote Originally Posted by whizkid View Post
    Could you give me a link where an uncharged particle's spin is measured?
    I would just be googling it, so you could do that. Photon spin is measured via its polarization. Neutrino spin is determined from conservation laws. (I don't know if there is any more direct way of measuring neutrino spin.) Neutrons (and atoms) have a magnetic moment because they are composite particles.
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    Quote Originally Posted by exchemist View Post
    Shouldn't the angular momentum be √(1/2 (1/2 +1)) h bar, and therefore √(3/4) h bar = √3/2 h bar ?
    Sure, I'll correct the post
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    Quote Originally Posted by whizkid View Post
    an electron is charged then it has
    L(e) = √3/2 h bar with a component on the z-axis L(e)z = 1/2 hbar in an 1H atom
    mu(e) = 1.001156 hbar/2
    Quote Originally Posted by Strange View Post
    Of a single particle? As far as I know, it hasn't been (yet).
    L in a free electron has not been detected, but it is 1/2 hbar

    Thanks Strange, could you please confirm this is the state of the art?
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    Quote Originally Posted by whizkid View Post
    L in a free electron has not been detected, but it is 1/2 hbar

    Thanks Strange, could you please confirm this is the state of the art?
    I'm not sure what you are asking. Does this help: Gabrielse Group Homepage
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    an electron is charged then it has
    L(e) = √3/2 h bar with a component on the z-axis L(e)z = 1/2 hbar in an 1H atom
    mu(e) = 1.001156 hbar/2
    L in a free electron (has not been detected, but it) is always 1/2 hbar

    ----
    I am simply asking you to say that these statement are correct so that I can make some factual observations
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    Quote Originally Posted by whizkid View Post
    I am simply asking you to say that these statement are correct so that I can make some factual observations
    OK. So the "not been detected" bit is no longer true. The rest seems accurate.

    BTW, not sure if this has been linked to before: Electron spin
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    Quote Originally Posted by Strange View Post
    this has been linked to before: Electron spin
    Does that value (sqrt3/2 gives) a clear indication of the angle of the axis of L with the z-axis?

    My first question (about L+mu), is unanswered,
    Now one further thing I do not understand:
    both Stern-Gerlach and Penning apparatus use a magnetic field, why does the first detect L and the second mu?
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by Strange View Post
    this has been linked to before: Electron spin
    Does that value (sqrt3/2 gives) a clear indication of the angle of the axis of L with the z-axis?
    I believe so, yes. It is the orientation of the remaining component, in the x-y plane, that is undefined.
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    Quote Originally Posted by whizkid View Post
    both Stern-Gerlach and Penning apparatus use a magnetic field, why does the first detect L and the second mu?
    As far as I know, they both measure the magnetic moment. Which depends on spin. (I am getting a feeling of deja vu )
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    Quote Originally Posted by Strange View Post
    they both measure the magnetic moment. Which depends on spin. (I am getting a feeling of deja vu )
    it's a wrong feeling, your links do not say that, moreover it is impossible since one measurement (L) is 1/2 and the other is 1.001156/2.
    Last edited by whizkid; April 15th, 2014 at 06:25 AM.
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    Quote Originally Posted by whizkid View Post
    moreover it is impossible since one measurement (L) is 1/2 and the other is 1.001156/2.
    I don't understand. What is impossible?
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    Quote Originally Posted by Strange View Post
    I don't understand. What is impossible?
    that they are measuring both the same thing as the values are different, of course.
    Quote Originally Posted by Strange View Post
    they both measure the magnetic moment.
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by Strange View Post
    I don't understand. What is impossible?
    that they are measuring both the same thing as the values are different, of course.
    Quote Originally Posted by Strange View Post
    they both measure the magnetic moment.
    Whizkid I do not think any experiment measures a value of "1/2".

    That is the value the quantum number has to take in the formula, in order to explain the measured value you actually get.
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    Quote Originally Posted by exchemist View Post
    I do not think any experiment measures a value of "1/2".
    That is the value the quantum number has to take in the formula, in order to explain the measured value you actually get.
    If I got it right, you are sayng that 1/2 is a theoretical value and 1.001156 the real value of L-mu and whenever they are talking of spin they are actually referring to mu, but 1/2 is also an indication of symmetry, if it is not precise can the full simmetry take place after 720°?

    Moreover,
    - gyromagnetic ratio is the ratio between L and mu so they should be different entities, are they calculating the ratio between a theoretical value and an actual one
    - in the second frame of your link Electron spin L and mu are mentioned and described separately, that caused my confusion
    Last edited by whizkid; April 15th, 2014 at 07:54 AM.
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    You are the on creating confusion. I think I'll give up on this.
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    Quote Originally Posted by whizkid View Post
    Quote Originally Posted by exchemist View Post
    I do not think any experiment measures a value of "1/2".
    That is the value the quantum number has to take in the formula, in order to explain the measured value you actually get.
    If I got it right, you are sayng that 1/2 is a theoretical value and 1.001156 the real value of L-mu and whenever they are talking of spin they are actually referring to mu, but 1/2 is also an indication of symmetry, if it is not precise can the full simmetry take place after 720°?

    Moreover,
    - gyromagnetic ratio is the ratio between L and mu so they should be different entities, are they calculating the ratio between a theoretical value and an actual one
    - in the second frame of your link Electron spin L and mu are mentioned and described separately, that caused my confusion
    Come on, for God's sake think straight, man! It's not a matter of theoretical vs actual, it's a matter of the quantum number in the formula versus the measured value of the physical quantity.

    To give a familiar example, weight = mass x g, the acceleration due to gravity, right?

    So if you measure a weight of 9.8 Newtons, and you know g, (as it is a well established physical constant) is 9.8m/sec², then you know the mass of your object is 1kg. Right?

    Similarly, if you measure a value for the magnetic moment and , knowing the value of Planck's constant, h , you plug this value into the QM formula for how magnetic moment is derived from the expression for angular momentum, you find it equates to a spin quantum number of 1/2.

    I really fail to see what is difficult about this. It is just a simple algebraic relation.

    Until you get this much straight I don't think we should even attempt to start discussing symmetry.
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    Quote Originally Posted by exchemist View Post
    To give a familiar example, weight = mass x g, the acceleration due to gravity, right?
    you plug this value into the QM formula
    .
    You mean sqrt(l*(l+1)) hbar?
    Last edited by whizkid; April 16th, 2014 at 07:20 AM.
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