# The magnetic field in Stern-Gerlach

## Main Question or Discussion Point

In the Stern-Gerlach experiement, what's so special about the magnetic field that causes the electrons to behave like they do? How would you explain this in layman terms?

We learned that whenever a charged particle passes through a 'normal' magnetic field, a force F = qvB acts upon it, unlike the magnetic field in that experiment which causes the electrons to divert based on their spin value.

Thanks,

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The important interaction in the Stern Gerlach experiment is not the charge of the electron with apparatus, but the magnetic moment of the electron with the apparatus. For a uniform magnetic field, there would be no displacement, but the Stern-Gerlach magnets produce a nonuniform field that is increasing along one direction.

Chen said:
In the Stern-Gerlach experiement, what's so special about the magnetic field that causes the electrons to behave like they do? How would you explain this in layman terms?

We learned that whenever a charged particle passes through a 'normal' magnetic field, a force F = qvB acts upon it, unlike the magnetic field in that experiment which causes the electrons to divert based on their spin value.

Thanks,
hi
this is rajarshi please can you tell me how the non uniform magnetic field is arranged for Stern-Gerlach experiement.
what is the mathematical analysis envolved in this?

Galileo
Homework Helper
It are silver atoms that are used in the Stern-Gerlach experiment, since these are neutrally charged as a whole, the Lorentz force is not the cause of this effect. Like slyboy said, it's the magnetic moment interacting with the strongly nonuniform magnetic field that does it.

I guess the magnetic field is created with an electromagnet, but I think a permanent magnet would also work. If the edge of the north pole of the magnet is very sharply peaked and the south pole is smooth, the field lines are much 'denser' near the north pole than near the south pole so you get a very nonuniform magnetic field.
Lots about it can be found on the net, just google stern gerlach.

CarlB
Homework Helper
To imagine what is happening in the Stern-Gerlach experiment from a classical point of view, consider what would happen if you threw a bar magnet into a region with a magnetic field.

The north end of the magnet would be attracted one way in the field, while the south end would be attracted in the other. If the field were uniform, these two forces would cancel. The effect of a uniform magnetic field would be to apply a torque on the bar magnet, but it wouldn't cause it's center of mass to change.

On the other hand, if the magnetic field were not uniform, then the two ends of the bar magnet would experience different strength magnetic fields, and it would be possible for the magnet to have its center of mass moved in one direction. Of course if the magnet were reversed, the force would reverse too, so there you have it, a non uniform magnetic field will spread magnets out while a uniform magnetic field does not.

If you happen to find a magnetic monopole, well it would be attracted to one end or the other of even a uniform magnetic field. But we know that these things are hard to get, so the best we can do is to use a magnetic "dipole". And as illustrated above, a dipole can only be influenced, in terms of an overall force acting on the center of mass, when the dipole is placed in a non uniform magnetic field.

The quantum effect is similar. Hope this helps your intuition on the problem. The Stern-Gerlach experiment is very very important to an understanding of quantum physics.

Carl

hmm, pardon my stupid question, but could it be that electrons are magnetic monopoles that can come in two different directions, a north and a south (spin up, spin down)? and a bar magnet has two directions due to the pretty much 50/50 split of different spin-oriented electrons? then would this mean that if you isolated a bunch of like spin electrons and protons (or atoms) made a magnet out of them, it would have assymetrical pole strength?

hmm. then again, viewing things this way would only cause problems with bosons...

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Why doesn't the electron "flip"?

I know this thread is older than the hills...

Why doesn't the "bar magnet" (the electron) "flip" if the poles are "backwards", which would have the consequence of showing one deflected beam instead of two? I would intuitively expect the force pushing the electron away would be unstable, like balancing a ball on the head of a pin. More like if you had a bar magnet resting on a low friction surface and approached with a repulsive pole, it would flip and move toward the magnet in your hand.

Is this because the electron is tied to the silver atom and can't flip?

Is it due to some QM equivalent of precession?

Or is it just a breakdown of the analogy, where there is no classical explanation?

None of the above?

I know I am on thin ice with the classical analogy, but I want to take it as far as I can before it blows up.