What aspects of atom are discrete?

[giann_tee]

Experiments in nuclear magnetic resonance for example, demonstrate that precessing atomic nuclei do it so smoothly. At the same time, atoms have discrete magnetic moments presumably associated with spins. Would anyone care to comment on the difference?

 

[ZapperZ]

Experiments in nuclear magnetic resonance for example, demonstrate that precessing atomic nuclei do it so smoothly. At the same time, atoms have discrete magnetic moments presumably associated with spins. Would anyone care to comment on the difference?

1. You are confusing the bulk magnetization of a material with the individual nuclear spins of atoms that make up the bulk material.

2. NMR also involves an application of another external magnetic field in another direction. In pulsed NMR, an 90 degree pulse can also be applied. This temporarily changes the direction of the net external field.

Zz.

 

[giann_tee]

I presume, precessing magnetic moments have random phases and then they can collectively precess smoothly. Except "random" means random moment in time to turn discretely. (Ok I'll shut up).

Zapper after 15000 posts its time for you to look back, because you already wrote everything, including the ULTIMATE ANSWER.

 

[Bob S]

Zeeman effect lines (splitting of characteristic quantized atomic lines) vary smoothly (but not always linearly) with the magnitude of applied magnetic field. The splitting depends on J, L and S.

 

[giann_tee]

Zeeman effect lines (splitting of characteristic quantized atomic lines) vary smoothly (but not always linearly) with the magnitude of applied magnetic field. The splitting depends on J, L and S.

Very interesting. BTW, I encountered this tidbit about most precisely quantized thing...

"The quantization of the Hall conductance has the important property of being incredibly precise. Actual measurements of the Hall conductance have been found to be integer or fractional multiples of e2 / h to nearly one part in a billion. This phenomenon, referred to as "exact quantization", has been shown to be a subtle manifestation of the principle of gauge invariance. "
http://en.wikipedia.org/wiki/Quantum_Hall_effect

 

[ZapperZ]

I presume, precessing magnetic moments have random phases and then they can collectively precess smoothly. Except "random" means random moment in time to turn discretely. (Ok I'll shut up).

Zapper after 15000 posts its time for you to look back, because you already wrote everything, including the ULTIMATE ANSWER.

Huh?

Very interesting. BTW, I encountered this tidbit about most precisely quantized thing...

"The quantization of the Hall conductance has the important property of being incredibly precise. Actual measurements of the Hall conductance have been found to be integer or fractional multiples of e2 / h to nearly one part in a billion. This phenomenon, referred to as "exact quantization", has been shown to be a subtle manifestation of the principle of gauge invariance. "
http://en.wikipedia.org/wiki/Quantum_Hall_effect

What does the quantum Hall effect (which involves the supercurrent) have anything to do with your question on NMR, which involves the NUCLEAR magnetic moment?

Zz.

 

[giann_tee]

What does the quantum Hall effect (which involves the supercurrent) have anything to do with your question on NMR, which involves the NUCLEAR magnetic moment?
Zz.

Nothing I was just saying.

 

[saaskis]

What does the quantum Hall effect (which involves the supercurrent) have anything to do with your question on NMR, which involves the NUCLEAR magnetic moment?

This is a little off-topic, but I just had to ask. Are you saying that QHE is analogous to supercurrent due to the fact that there is no backscattering in a conductor with Landau levels? I mean the situation in which the left and right-moving states are spatially separated.

 

[ZapperZ]

This is a little off-topic, but I just had to ask. Are you saying that QHE is analogous to supercurrent due to the fact that there is no backscattering in a conductor with Landau levels? I mean the situation in which the left and right-moving states are spatially separated.

Oh, I had a brain malfunction. I have no idea why I associated QHE with supercurrent. For some odd reason, I was thinking of SQUIDs.

Still, QHE has nothing to do with nuclear spins.

Zz.

 

[giann_tee]

Quantum spin precession I am reading, is a rotation of spin vector around the axis of the external field H. Its a two-state system with energy difference E.
<s_z> = hbar/2 * cos(delta E * t / hbar)
delta E = 2 * mu_Bohr * H_z
resonant frequency = delta E / h.
After radio pulse at that frequency, some number of spins will flip to higher state. That vector will point to a new lower angle (greater declination from z), a state not possible to imagine with a singe discrete spin, but okay with bulk magnetization vector.

 

[ZapperZ]

It looks like you've answered your own question AND illustrated why I said you were confusing the individual nuclear spins with the bulk magnetism.

Zz.

 

[giann_tee]

I discovered one new book about magnetism, so good that I could study everything all over again. J. Stohr, H.C. Siegmann "Magnetism From Fundamentals to Nanoscale Dynamics". Its not specialized about spin lattices or MRI, but general.