protons spin but electrons dont?

[bwanaaa]

If electrons were spinning, then they should generate a magnetic field. Since electrons in a metal are free to move, then a magnetic field should cause them to line up as well and the south magnetic poles of the spinning electrons should be pulled towards the north pole of the magnet. A static magnetic field should therefore generate a potential difference between two ends of a hunk of metal. But this does not happen-so that means electrons are not spinning? Right?

Also, consider that we have observed nmr. The principle behind NMR is that many nuclei have spin and all nuclei are electrically charged. If an external magnetic field is applied, the north poles of the protons line up. Adding a radio wave (playing music?) adds energy to the protons - an energy transfer is possible between the base energy to a higher energy level. Turning off the radio, allows the spinning protons to release the stored energy and return to their base orientation in the magnetic field. In returning to their base energy, they make radio waves of their own. So, the fact that nmr can be done proves that protons are spinning like tops. But what about electrons?Since electrons have opposite charge to protons, a magnetic field should cause the electrons of an atom to line up the opposite way to which the protons have lined up-these fields should cancel out and then nothing should be magnetic. Also, the electrons should do the opposite of the protons and a complementary radio wave should be emitted resulting in cancellation-no nmr signal would be observed if the electrons did this and this is not the case. So, electrons cant be spinning, right?

Is there any way to know if electrons are spinning?

 

[sao123]

some of this is way above my head... but, reading this
http://hyperphysics.phy-astr.gsu.edu/hbase/spin.html#c1 appears to answer your question.

 

[Born2bwire]

Real quick but I'm sure this was answered by sao123, but, electrons do have spin. They have a spin of 1/2 and this spin plays a role in magnetism like paramagnetism and ferromagnetism if I recall correctly.

 

[esun]

As far as I know (i.e., based on a few QM classes), the "spin" that elementary particles have is not a result of them actually physically spinning. It is a fundamental property of the particle much like mass or charge.

I don't see how electron alignment should cause a potential difference, though. If you align electron spins you'll certainly get some magnetic effect, but for a potential difference you need to move them, not change their orientation.

 

[bwanaaa]

As far as I know (i.e., based on a few QM classes), the "spin" that elementary particles have is not a result of them actually physically spinning. It is a fundamental property of the particle much like mass or charge...

yes, that is exactly what i remember. the concept of spin in an electron is analogous to the properties of quarks which are color, flavor, charm,etc. Electron spin is inferred from the magnetic dipole moment-not that wikipedia is an authority but the explanation there:
http://en.wikipedia.org/wiki/Electron_spin
says that electrons act like tiny bar magnets and therefore they are presumed to be spinning.

...I don't see how electron alignment should cause a potential difference, though. If you align electron spins you'll certainly get some magnetic effect, but for a potential difference you need to move them, not change their orientation.

if electrons are tiny bar magnets and they are free to move about in the electron cloud of a mass of metal atoms, then they should line up and move when you bring a big magnet nearby. just the way two magnets will snap together and move through physical space because of the attraction of the opposite poles, electrons should line up land move. since they can't jump out of the metal, they should accumulate at the ends of a block of metal nearest the spots of most magnetic intensity. The ends of the metal should become negatively charged relative to the middle. Would this not be considered a potential difference? the middle of the metal block has a weaker magnetic flux and the tendency for electrons to align and gather there is less.


btw, sao, i really agree with your signature. every job in a civilized world has performance measures and evaluations that govern it EXCEPT politicians and lawyers. Voting is insufficient to achieve behavioral changes in the body politic as new members are subjected to the same forces as the old members and will therefore exhibt the same maladaptive and inefficient behaviors. We need term limits and performance measures for our politicians. but that is off topic, sorry.

 

[Soleron]

You're still thinking of electrons as a tiny sphere. Don't, they are a spread out wavefunction with some particle and wave properties.

You can't take the bar magnet analogy very far. When a metal is magnetised, the domains do align but the electrons don't move to one end.

nmr relies on the radio wave frequencies being specific to the nuclei. Electrons would resonate at a different frequency (or not at all, not sure) so any emission from them wouldn't cancel but you'd still detect the characteristic frequencies of what you were trying to determine.

 

[Biftheunderstudy]

There are other complications too, for one, this spin is quantized. 2, the Pauli exclusion principle, 3) spin orbit coupling.

All of these things make it a little more complicated than just saying everything should line up. But, there are methods to map out the spin's of say, a crystal lattice. This can be done a number of ways but the most powerful and widely used method is neutron scattering. Since the neutrons are neutral but have a non-zero spin and thus magnetic moment, they probe the magnetic structure of a crystal.

Then there are terms like ferromagnet, antiferromagnet, frustrated antiferromagnet, etc. which all apply to the orientations of the spins.

None of those terms I've mentioned are descriptive, but they are good keywords to search up if your interested.

 

[silverpig]

Also, look up the hall effect.

The electrons don't have to move to cancel out the magnetic field. They can just flip to produce a corresponding opposite magnetic field.

It is actually very difficult to polarize particles in a solid like that. Even with a 15T magnet you'll only get a fraction of 1% polarization IIRC. (ie, 50.5% spin up, 49.5% spin down. Normally you're at 50/50, and a 15T magnet will suck the tin man through a keyhole).

 

[shortylickens]

My chemistry teacher just told me that they do spin, but only cuz it helps to explain certain things. Since they are truly elementary we cant actually observe them spinning.

 

[silverpig]

My chemistry teacher just told me that they do spin, but only cuz it helps to explain certain things. Since they are truly elementary we cant actually observe them spinning.

Electrons have angular momentum. What that physically means at those scales we don't know, but at our scales, it means something is going round and round and round.

 

[Farmer]

My chemistry teacher just told me that they do spin, but only cuz it helps to explain certain things. Since they are truly elementary we cant actually observe them spinning.

Electron's don't "spin," at least not in the sense that you are imagining it, (i.e., there is a hard sphere and it is rotating about some axis). Spin is a quantum mechanical idea that has no direct parallel in the classical interpretation, so if the intent of your chemistry teacher is to introduce you to QM, then saying an electron actually spins is a bit misleading.

Like silverpig said, QM spin can be called angular momentum in a more abstract, modern physics sense (i.e., not your r x p, see Noether's theorem). It's not as intuitive as, say, orbital angular momentum, which comes from quantizing r x p for an electron. I.e., unlike orbital angular momentum, which a classically-minded physicist might be able to conceive and transform into a quantized theory, spin is something that would be hard to make up (it wasn't made up, see Stern-Gerlach experiment).

"Spin" is just a name given to a state parameter of the electron (or all particles, for that matter). Because this "spin" is related to the magnetic dipole moment of the electron, it reminded scientists of a classical particle which was charged and spinning, and hence generates a dipole moment. Hence they decided to call this property "spin".

Mathematically, the "spin" state lives in a 2D Hilbert space, i.e., it's two additional dimensions to the Hilbert space which is the eigenspace of your ordinary, spin-less Schrodinger equation. Practically, you add terms to your normal Schrodinger equation to account for spin effects: in hydrogen in vacuum, that would be spin-orbit and spin-spin coupling, which leads (in part) to fine and hyperfine structure (the basis for 21cm radio astronomy). In NMR (the physical principle behind your MRI machine) and more elaborated stuff, it could be coupling between spin and external magnetic fields.

Moreover, via spin-statistics, you arrive upon "bosons" and "fermions" which have different statistical behavior, primarily Pauli exclusion, which leads to a whole bunch of other stuff.

In general, it is difficult to "find" classical parallels for a lot of things in QM, because QM is a totally separate and deeper theory. While some parallels exist, QM spin is a QM concept, you can't intuit it from classical stuff.

 

[LiuKangBakinPie]

http://scienceblogs.com/principles/2010/07/electron_spin_for_toddlers.php

Well, it's an additional property that electrons (and other particles) possess. Here's an analogy: think about the Earth orbiting the sun--

 

[Farmer]

Here's an analogy: think about the Earth orbiting the sun--

Again, misleading.

 

[Sunny129]

its extremely tough to talk about these things with the layman. one should really read and introductory book to quantum mechanics before looking for answers here. not that there aren't folks who understand it, but understanding the material and being able to pass it on onto others clearly and concisely are two different animals, the latter being just as much a challenge as the former.

one cannot "taste" the different flavors of quark for obvious reasons. one cannot "see" the different colors of quark b/c quarks are smaller in diameter than any wavelength of visible light. likewise, one cannot liken the "spin" of an electron to that of a rotating mass in classical physics/mechanics. it is simply just another parameter than describes a particular property of the electron, in the same sense that "colors" and "flavors" of quark are not actually colors and flavors in the classical sense - they are once again just parameters that describe particular properties of the quark.

 

[LiuKangBakinPie]

Again, misleading.

explain

 

[bwanaaa]

@Farmer

thank you for pointing out the Stern-Gerlach experiment.

in this experiment silver particles are vaporized and shot past a magnet. (how you get the silver particles to move in a coherent direction is unclear-maybe the kiln where you are cooking the silver is a closed vessel and only has a little hole). The silver particles stream down a tube and past a magnet. We expected a smear of particle impacts but actually observed discrete bands. The interpretation was that the particles (which were neutral in that expt) had discrete magnetic moments - there were magnetized to a specific amount in either one direction or the other (it's not clear how you can define direction on a particle that is symmetric and has no other reference marker) The magnet forced the the particles pointing one way to go one direction and the others were directed in the opposite direction. But the orientation of the millions of particles entering the magnetic field had to be random so the amount of deflection was different for each. Instead of thinking about magnetic moments being continuously variable and the particles being discrete, another experimental conundrum comes to mind. The double slit expt.

In the dbl slit expt, we see a discrete distribution of photon impacts on a screen when there are two slits-even when we fire one photon at a time. The expectation was that we should see a smear of photon impacts. But instead we see discrete bands. The concept of spin is nowhere to be found in this expt or its analysis. Rather, we speak of a probability distribution that the particle is actually a wave that that passes through both slits and interferes with itself. It's only a particle when we use our crude tools to try and observe it.

So, going back to the Stern-Gerlach setup, can we not say the same thing? That the silver particle is actually a wave whose orientation is defined by its motion. Since the silver vapor is collimated and travelling past the magnet in one direction, the waves are either crests or troughs as they pass the magnet. The magnetic moment of a crest is the opposite of a trough, and these are deflected in opposite directions. Now if a silver atom is a wave, its moment is flipping back and forth, so you might imagine that it all averages out. but the magnetic force is greatest when the silver is closest to the magnet-if the moment is pointing up as it passes close to the magnet-it gets pulled one way the most. By the time the moment has flipped, it's further away from the magnet and the force to go the other way is much less. To actually calculate the force, you'd need an integral summing up all the contributions during the silver's path-but they should cancel out. The bottom line is that the magnetic moment is pointing in only one of two directions as the silver passes closest to the magnetic field. In other words, the magnetic moment exists in discrete quanta-not just a quantized amount, but also a quantized direction.

Of course I am ignorant of the details of this experiment-for example what happens when you send only one silver atom at a time through the magnetic field-do you get the same 2 bands? Does the temperature of the silver have an effect? etc. The point is though that the magnetic moment exists even in neutral atom and has nothing to do with a spinning charge. It is an intrinsic property.

That we can generate magnetism by translational movement of a charged particle may simply be a red herring that is not relevant at the quantum scale. In fact, one might just as easily argue that angular motion of a charged particle should have no magnetic effect. After all, how could you possibly know when a charged particle is spinning?

 

[silverpig]

explain

We don't know for sure that anything is actually going around and around when we talk about electron spin. What we do know is that the electron has some property which acts exactly like a quantum spinning object should. It looks like angular momentum, and adds with other angular momenta in the appropriate ways.

The easiest way to conceptualize this is to imagine the electron as a little spinning charged ball, but we don't actually know what an electron looks like or if it physically spins (whatever that means at those scales).

 

[Farmer]

explain

In addition to what silverpig and bwanaa have said, I'll restate my main point from my previous post, which you can read if you like:

In general, it is difficult to 'find' classical parallels for a lot of things in QM, because QM is a totally separate and deeper theory. While some parallels exist, QM spin is a QM concept, you can't intuit it from classical stuff.

 

[Cogman]

The problem with talking about the "spin" of an electron is that electrons aren't purely particles. They are wave-particles much like light.

Imagining electrons as tiny balls of charge is somewhat fallacious as they have dual behaviors.

 

[bwanaaa]

I wonder if we would be better served by thinking of these small particles as ribbons with a twist. maybe left handed twists give a moment down and right handed twists give a moment up. and no twists result in no moment. But then, how do you keep flipping moments by adding more energy (like in nmr, or emr spectroscopy)?

Maybe the model is different, even numbered twists have no magnetic moment. Odd numbered twists give an up or down moment. Adding energy to the particle (rf, laser, whatever) just adds a twist. This fulfills the quantum requirement of discrete energy states. And leads to an interesting corollary-if you could forcibly twist in the opposite direction you could end up with 'negative energy'? or maybe just an antiparticle?

 

[Farmer]

I wonder if we would be better served by thinking of these small particles as ribbons with a twist. maybe left handed twists give a moment down and right handed twists give a moment up. and no twists result in no moment. But then, how do you keep flipping moments by adding more energy (like in nmr, or emr spectroscopy)?

Maybe the model is different, even numbered twists have no magnetic moment. Odd numbered twists give an up or down moment. Adding energy to the particle (rf, laser, whatever) just adds a twist. This fulfills the quantum requirement of discrete energy states. And leads to an interesting corollary-if you could forcibly twist in the opposite direction you could end up with 'negative energy'? or maybe just an antiparticle?

You can think of it however the hell you want to think of it. You are essentially saying the same thing, your "twists" are the energy states associated with the different spin states. You "flip moments by adding more energy" by transitioning between these spin states (which have different energy eigenvalues: since spin and Hamiltonian are simultaneously diagonalizable, your spin states can also be energy eigenstates). IMO, all of what you are describing could be described by some change of basis in the eigenspace of the spin operator. You are simply appending "analogies" to concepts that are already well understood in their own language.

How do you "forcibly twist"? Driving can be done electromagnetically, which again is something people have already considered (see optical Bloch equation). What other type of interaction do you propose for an electron? Gravitational, lol?

Yes, you could end up with negative energy. The zero value of energy is arbitrarily set due to freedom in choice of gauge in scalar potentials. If you are referring to the problem of infinite negative energy, that is never the case with spin. For instance, if you have spin-1/2, your energy is -1/2 hbar or 1/2 hbar.

 

[komatta]

What is your educational background Farmer? I just completed my BS in physics and astronomy and I appreciate your clarity on the subject(s).

edit: whoops didn't realize this was a few weeks old..

 

[Farmer]

What is your educational background Farmer? I just completed my BS in physics and astronomy and I appreciate your clarity on the subject(s).

edit: whoops didn't realize this was a few weeks old..

I also just have a bachelor's in physics. I will be starting a Ph.D. next semester but you could say it is in a different field.