17 Jan 11
1 edit
Here is a link to a new technology in electron microscopy:
http://www.physorg.com/news/2011-01-electron-vortices-potential-conventional-microscopes.html
My question is this: What are the forces involved that would make an electron beam spiral around itself like that? I can see if there was a magnetic field line, that's what they do in regards to magnetic fields, but if there is no magnetic field, how can it spiral? Can electrons spiral around electric fields? I suspect there is more to the show than appears in the article.
Anyone know about this kind of thing?
17 Jan 11
1 edit
Originally posted by KazetNagorraBut if it is just spinning, if you did that with some kind of weight, say a 100 mg sphere, and imparted the same motion, as soon as it was let loose it would just continue on a straight line. How can you manage that feat with electrons?
From the article:
"Passing electrons through a nanometer-scale grating, the scientists imparted the resulting electron waves with so much orbital momentum that they maintained a corkscrew shape in free space."
I.e. the "vortex" motion is caused by conservation of angular momentum.
Even if the sphere was spinning at 10,000 rpm, when it was let loose it would not continue in a spiral motion.
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17 Jan 11
Originally posted by sonhouseErm. It would, somewhat, if it were in a medium. That's how baseball pitchers, tennis players, golfers and so on play effect shots. I must admit that I don't see how this could affect an electron, though, unless its spin causes a quantum-dynamic force in nearby particles (and thus also in itself).
But if it is just spinning, if you did that with some kind of weight, say a 100 mg sphere, and imparted the same motion, as soon as it was let loose it would just continue on a straight line. How can you manage that feat with electrons?
Even if the sphere was spinning at 10,000 rpm, when it was let loose it would not continue in a spiral motion.
Richard
17 Jan 11
Originally posted by sonhouseApparently they manage that feat with that nano something thingy. Don't ask me how it works though!
But if it is just spinning, if you did that with some kind of weight, say a 100 mg sphere, and imparted the same motion, as soon as it was let loose it would just continue on a straight line. How can you manage that feat with electrons?
Even if the sphere was spinning at 10,000 rpm, when it was let loose it would not continue in a spiral motion.
18 Jan 11
Originally posted by KazetNagorraYeah but there you get polarization, circular, linear, etc. That does not change the path the photon takes through space. Electrons, having a negative charge, would tend to fly apart in reaction to their own electric field, that might have something to do with it but just how that would be involved I haven't a clue.
I suppose it's analogous to gratings used in (ye olde) experiments with light.
19 Jan 11
Originally posted by sonhouseGratings do change the path the photons take through space.
Yeah but there you get polarization, circular, linear, etc. That does not change the path the photon takes through space. Electrons, having a negative charge, would tend to fly apart in reaction to their own electric field, that might have something to do with it but just how that would be involved I haven't a clue.
Electrons do repel each other, but in these kind of experiments this expansion of the cloud/beam is limited due to the speed of the electrons.
19 Jan 11
2 edits
Originally posted by KazetNagorraNot sure what the actual velocity of the electron beam, it can't be going more than about 25 KEV. We regularly accelerate arsenic, boron or phos to 200 KEV in our ion implanters but of course electrons, being so much lighter will go a hell of a lot faster volt for volt of accel. If I remember right, Arsenic, AMU 75, at 200 KEV is going about a half million miles per hour, 800,000 km/hr, so I imagine electrons at 25 KEV would be going a lot faster than that considering the huge difference in mass, about 140,000 to one compared to arsenic. That would indicate some kind of restoring force to keep the path of the beam in a spiral.
Gratings do change the path the photons take through space.
Electrons do repel each other, but in these kind of experiments this expansion of the cloud/beam is limited due to the speed of the electrons.
21 Jan 11
As far as I can tell, the electrons are travelling so fast that there is a wave effect as a result of quantum dynamics. If so, then the spiraling too may be a quantum wave effect rather than a Newtonian type spiral motion. I am not sure though where the 'angular momentum' comes in as I don't think that should apply in quantum dynamics (but then I could be wrong).
21 Jan 11
Originally posted by twhiteheadYep, you're wrong. First of all, the wavelength of electrons decreases as they move faster, so you will notice less "wavelike" effects in general. Secondly, conservation of angular momentum holds both in quantum and in classical physics.
As far as I can tell, the electrons are travelling so fast that there is a wave effect as a result of quantum dynamics. If so, then the spiraling too may be a quantum wave effect rather than a Newtonian type spiral motion. I am not sure though where the 'angular momentum' comes in as I don't think that should apply in quantum dynamics (but then I could be wrong).
21 Jan 11
Originally posted by KazetNagorraPlease explain more.
Yep, you're wrong. First of all, the wavelength of electrons decreases as they move faster, so you will notice less "wavelike" effects in general. Secondly, conservation of angular momentum holds both in quantum and in classical physics.
Are these waves quantum waves? If so, surely the slower the electron is going the greater the certainty of its position and thus the smaller the wave length? Or am I wrong again?
Is there any momentum involved in quantum waves? Is the electron moving side to side or forwards and backwards in a wave like motion, or is it really a probability function of its location?
21 Jan 11
Originally posted by twhiteheadRead this:
Please explain more.
Are these waves quantum waves? If so, surely the slower the electron is going the greater the certainty of its position and thus the smaller the wave length? Or am I wrong again?
Is there any momentum involved in quantum waves? Is the electron moving side to side or forwards and backwards in a wave like motion, or is it really a probability function of its location?
http://en.wikipedia.org/wiki/De_broglie_wavelength
http://en.wikipedia.org/wiki/Angular_momentum#Angular_momentum_in_quantum_mechanics
Note that angular momentum is not the same as momentum. Angular momentum is given by the operator L = r x p, with p the momentum operator and r the position.
24 Jan 11
Originally posted by KazetNagorraThanks for the links. I don't think I understand it all, but I did learn something new.
Read this:
http://en.wikipedia.org/wiki/De_broglie_wavelength
http://en.wikipedia.org/wiki/Angular_momentum#Angular_momentum_in_quantum_mechanics
Note that angular momentum is not the same as momentum. Angular momentum is given by the operator L = r x p, with p the momentum operator and r the position.