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Author Topic: Angular momentum opposing charge momentum?  (Read 3153 times)

Offline jeffreyH

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Angular momentum opposing charge momentum?
« on: 08/02/2015 22:07:28 »
It has bothered me for a while that the electric and magnetic fields are considered static unless acted upon by a force since particles have spin. What if the angular momentum of the mass acts counter to the 'angular momentum' of the charge such that the charge is held static with regard to the external environment. Is this possible?


 

Offline PmbPhy

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Re: Angular momentum opposing charge momentum?
« Reply #1 on: 08/02/2015 23:21:09 »
Quote from: jeffreyH
It has bothered me for a while that the electric and magnetic fields are considered static unless acted upon by a force since particles have spin. What if the angular momentum of the mass acts counter to the 'angular momentum' of the charge such that the charge is held static
That makes no sense to me. Where did you hear that? You can't exert a force on a field.
 

Offline jeffreyH

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Re: Angular momentum opposing charge momentum?
« Reply #2 on: 09/02/2015 00:00:43 »
I was referring to moving fields rather than static fields. The word force was misused here, sorry.
 

Offline PmbPhy

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Re: Angular momentum opposing charge momentum?
« Reply #3 on: 09/02/2015 02:56:57 »
Quote from: jeffreyH
I was referring to moving fields rather than static fields. The word force was misused here, sorry.
In that case, please restate your question for me. I don't understand what spin has to do with this. Spin is a purely quantum mechanical quantity with no classical analogy.

You wrote
Quote from: jeffreyH
What if the angular momentum of the mass ...
What mass?

Quote from: jeffreyH
..acts counter to the 'angular momentum' of the charge ...
What charge?

Quote from: jeffreyH
such that the charge is held static with regard to the external environment. Is this possible?
If you'd state this in a particular case then it'd be easier to understand what you're looking for.
 

Offline evan_au

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Re: Angular momentum opposing charge momentum?
« Reply #4 on: 09/02/2015 17:38:33 »
Quote from: jeffreyH
What if the angular momentum of the mass acts counter to the 'angular momentum' of the charge such that the charge is held static with regard to the external environment.

The classical image of an electron is of a tiny charged ball which is spinning around the nucleus like a little solar system. If you did build such a charged spinning ball, it would create a magnetic field. This classical model can be used as a rough explanation of some effects in magnetism and spectroscopy. You can even model a nucleus with an odd number of protons as a spinning charged ball, and this gives some concept of Magnetic Resonance Imaging.

Perhaps the scenario here is imagining:
Quote from: Paraphrase
You have a lump of solid material, with all the electrons spinning around their nuclei at a certain radius and identical speed. If you spin this mass at the right speed in the opposite direction, could you cancel the rotation of the electrons, and eliminate the magnetic field of the electron?
I imagine there is a Nobel prize waiting for anyone who could cancel the magnetic field of an electron!

However, there are some severe problems with this simplified "solar system" model of an atom when you try to compare theory and practice. For example, if you try to calculate how fast the electron is spinning around the atom, it comes out at somewhere close to the speed of light.

So in this classical model, you would need to spin the macroscopic lump of matter at greater than the speed of light to cancel the sub-microscopic motion of the electron. Any real material would disintegrate long before it reached the required speed, even if Einstein hadn't told us that you can't exceed the speed of light.

In reality, all the electrons aren't all in the same orbit, they don't all orbit in the same direction, and they don't orbit at the same speed. They don't all orbit in a flat plane like a solar system, but are distributed in 3 dimensions to produce the various 3D shapes of molecules. So there is no single axis and direction and speed that you could spin a macroscopic lump of matter to cancel the motion of the constituent electrons. (The solar-system model also produces false predictions like "the electron should spiral into the nucleus, emitting a blaze of radiation" - see separate thread.)

To get a more accurate model of an atom, electron or nucleus, you need to include quantum effects, which say that the electron is not a point particle orbiting around the nucleus, but a wave function which is distributed in orbitals around the atom, with quantized angular momentum. In this model, spinning the lump of matter will have no effect on the magnetism of the electron (but linking relativity and quantum theory still leaves some gaps...)
« Last Edit: 09/02/2015 17:41:43 by evan_au »
 

Offline acsinuk

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Re: Angular momentum opposing charge momentum?
« Reply #5 on: 25/02/2015 12:04:04 »
IMHO the positively charged proton bundles must be held away from the outside electron enclosure by a magnetic angular momentum force that holds them apart
 

Offline JohnDuffield

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Re: Angular momentum opposing charge momentum?
« Reply #6 on: 25/02/2015 13:22:27 »
In that case, please restate your question for me. I don't understand what spin has to do with this. Spin is a purely quantum mechanical quantity with no classical analogy.
See The Einstein-de Haas effect which demonstrates that "spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". Also see The discovery of the electron spin by S A Goudsmit: "When the day came I had to tell Uhlenbeck about the Pauli principle - of course using my own quantum numbers - then he said to me: "But don't you see what this implies? It means that there is a fourth degree of freedom for the electron. It means that the electron has a spin, that it rotates".

However as evan_au said, you shouldn't think of it as some spinning ball. See atomic orbitals on Wiki where you can read that "electrons do not orbit the nucleus in the sense of a planet orbiting the sun, but instead exist as standing waves". It's more like a wave going round and round, and again like evan said, not on a flat plane.

Quote from: jeffreyh
...What if the angular momentum of the mass acts counter to the 'angular momentum' of the charge such that the charge is held static with regard to the external environment.
Like evan_au said you can't really do this. However maybe you can do something like this, just a tiny little bit. I'm thinking of what happens when a proton captures an electron: the mass of the electron+proton system is reduced by 13.6ev. For an analogy, imagine a wave going round and round reflecting off the four walls of a mirror-box. If you jiggle this box round and round, you need a longer wavelength to make a standing wave. That means less energy and less mass. And because the wave isn't going round in such a tight path, the angular momentum is reduced. Other analogies feature hula hoops and sparklers. None are ideal, but IMHO they're better than the billiard balls you usually see.
 

Offline yor_on

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Re: Angular momentum opposing charge momentum?
« Reply #7 on: 26/02/2015 20:45:02 »
Actually "Spin was first discovered in the context of the emission spectrum of alkali metals. In 1924 Wolfgang Pauli introduced what he called a "two-valued quantum degree of freedom" associated with the electron in the outermost shell. This allowed him to formulate the Pauli exclusion principle, stating that no two electrons can share the same quantum state at the same time.


The physical interpretation of Pauli's "degree of freedom" was initially unknown. Ralph Kronig, one of Landé's assistants, suggested in early 1925 that it was produced by the self-rotation of the electron. When Pauli heard about the idea, he criticized it severely, noting that the electron's hypothetical surface would have to be moving faster than the speed of light in order for it to rotate quickly enough to produce the necessary angular momentum. This would violate the theory of relativity. Largely due to Pauli's criticism, Kronig decided not to publish his idea."

So it can't be 'classical' in any way, as that would place this spin 'ftl'.

And this one wondering in terms of its spin vectors.

"Mathematically, quantum mechanical spin states are described by vector-like objects known as spinors. There are subtle differences between the behavior of spinors and vectors under coordinate rotations. For example, rotating a spin-1/2 particle by 360 degrees does not bring it back to the same quantum state, but to the state with the opposite quantum phase; this is detectable, in principle, with interference experiments. To return the particle to its exact original state, one needs a 720 degree rotation.

A spin-zero particle can only have a single quantum state, even after torque is applied. Rotating a spin-2 particle 180 degrees can bring it back to the same quantum state and a spin-4 particle should be rotated 90 degrees to bring it back to the same quantum state.

The spin 2 particle can be analogous to a straight stick that looks the same even after it is rotated 180 degrees and a spin 0 particle can be imagined as sphere which looks the same after whatever angle it is turned through."

Spin is nice, but confusing :)
« Last Edit: 26/02/2015 20:54:07 by yor_on »
 

Offline yor_on

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Re: Angular momentum opposing charge momentum?
« Reply #8 on: 26/02/2015 21:09:14 »
There might be some way around it though? At least when it comes to 'ftl'. It depends on how you think of entanglements. You 'split' one into two, both have a spin, and they know the other photons 'spin instantly'. How can it be 'instantly known'? Seems one argument is the one between what is 'meaningful information' relative 'non meaningful'. All meaningful information must obey 'c', according to that interpretation, which makes sense to me. Now, if those photons, although instantly correlating (in your measurement) can't be meaningful in terms of useful information, what does that make their spin? If it's not meaningful, does it have to obey 'c'?
=

although, I better admit to not liking that argument at all :)
I agree on 'c' being a limit for what is useful information, but this?
Nah.

Well, unless we invent electrons as small 'balls' able to exist inside a relativistic universe spinning 'ftl' ::))
« Last Edit: 26/02/2015 21:23:48 by yor_on »
 

Offline jeffreyH

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Re: Angular momentum opposing charge momentum?
« Reply #9 on: 27/02/2015 03:30:04 »

The physical interpretation of Pauli's "degree of freedom" was initially unknown. Ralph Kronig, one of Landé's assistants, suggested in early 1925 that it was produced by the self-rotation of the electron. When Pauli heard about the idea, he criticized it severely, noting that the electron's hypothetical surface would have to be moving faster than the speed of light in order for it to rotate quickly enough to produce the necessary angular momentum. This would violate the theory of relativity. Largely due to Pauli's criticism, Kronig decided not to publish his idea."


Yor_on thanks for this. It was this that prompted this thread. However as usual it was very badly phrased. I have no answer to my own question and do not expect one.
 

Offline acsinuk

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Re: Angular momentum opposing charge momentum?
« Reply #10 on: 05/03/2015 22:04:15 »
Pauli exclusion principal is a little reminiscent of a dipole electron that takes 2 revolutions to return to the same position as yor-on states 720 degrees for 1/2 a spin.  From this can we assume that 3/2 spin will take 6 revs to return to same position?
 

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Re: Angular momentum opposing charge momentum?
« Reply #10 on: 05/03/2015 22:04:15 »

 

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