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Old Nov 6th 2009, 01:23 AM   #1
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mystery of electron spin(spin??)

a few days ago a teacher in chemistry told us that ms(spin quantum number) is due to z component of spin angular momentum of electron. i asked her that if electron spins about some imaginary axis(like a spinning ball), how are we fixing the z axis about the sphere?the teacher warned me repeatedly to get rid of the idea of electron as a spinning ball. when i asked her why, she said its due to relativity and passed to some other(boring) topic. now i have read relativity from resnick , halliday and cannot find a word about electron spin there. how can relativity give rise to spin? did she tell me the truth? or is this the usual chemistry nonsense?

Last edited by r.samanta; Nov 6th 2009 at 01:31 AM.
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Old Nov 6th 2009, 03:21 AM   #2
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Let us hope no chemist is reading this.

Spin is a quantum mechanical property and has no classical analog.

It was found that the electron has a magnetic moment other than due to its orbital motion. This happened during the study of hyperfine structure of the Hydrogen spectrum in quantum mechanics. To explain this the electron spin was postulated which can have only two values

+ or - (1/2)(h/2pi).

So at first like the Bohr model where electrons move around the nucleus it was thought that the electrons also spin around their own axis.

I remember reading that it was Lorentz who showed (in his beautiful handwriting!) that this could not be possible because for the value of magnetic moment which was found,the electron would have to be spinning so fast, its surface would have a velocity many times that of light which relativity does not permit. I have often seen relativity quoted as the reason without further explanation.

So your teacher was right you see. Relativity does not cause the electron to spin. Rather it forces us to abandon the classical idea of the electron actually spinning around an axis like say the earth.
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Old Nov 6th 2009, 08:29 AM   #3
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perhaps this will increase my faith in chemistry . its not totally loopy science afterall.i still have some problem regarding the last line. please explain it a bit more.(i am a nutter)
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Old Nov 9th 2009, 08:45 PM   #4
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This is really strange . I remember posting a reply for this which sems to have vanished. What i had posted was something to this effect.

You have company. Relativity and Quantum turns many of us into nutters.

Lorentz ( of the Lorentz trasnsformation) calculated that to obtain the measured magnetic moment , the electron would have to spin at such a rapid rate that its surface vel would exceed that of the vel of light which is not permitted by relativity. Thus relativity tells us that the magnetic moment is not due to the electron physically spinning.

It is now treated as an intrinsic (quantum) property of the electron in the sense it has no classical analog.
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Old Nov 10th 2009, 03:17 AM   #5
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i think your reply vanished due to a relativistic "thread" annihilation with some other anti thread.
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