# Why do animations of Stern Gerlach show the tiny magnets in fixed orientation? [duplicate]

Anyone who's played with bar magnets knows that if you bring a bar magnet close to another tilted at some angle off axial to the other magnet, the one you're not holding will immediately flip to line up with the other that you're holding.

This would mean that every single tiny magnet sent in to the SG apparatus no matter the angle would immediately flip in to a vertical orientation in line with the applied field and be sent in a straight line through the apparatus, resulting in a single clump at the center.

Yet in most animations of the Stern Gerlach experiment (like this one here http://www.thephysicsmill.com/2015/02/22/the-stern-gerlach-experiment), and indeed in explanations of the classical expected outcome of the experiment, we see the tiny magnets maintain their orientation throughout the trajectory through the magnetic field, and this is used as an argument as to why the tiny magnets are evenly distributed at the other end.

This has always bothered me why this is not reconciliated with our childhood experiences of playing around with bar magnets. Please note that I am NOT talking about the actual result of the experiment wherein the electrons formed two clumps one at the top and one at the bottom. I'm asking about the part where they explain the classical expectation of an even distribution.

My classical expectation is for only a single clump, right in the center because they all rotate line to up immediately entering the field and have equal magnetic forces up and down.

What is wrong with my expectation?

## marked as duplicate by sammy gerbil, Kyle Kanos, peterh, John Rennie, David HammenJul 28 '17 at 13:49

The reorientation of your bar magnets require: force, torque, and especially damping. An important contribution for damping comes internal processes, which are not present in an ideal "classical" elementary magnet. But torque is definitively present as it is in the quantum case (where it leads to precession). So what would happen to a semi-classical magnet. If it is tilted there would be no force in the beginning as $F=p\nabla E$. It then would, as you said, turn and a force would act. If it would enter already tilted, the force would act immediately. We can, therefore, still assume, that the displacement would depend on the angle at which the magnets enter. In detail it would depend on how fast they get turned, though. On the one hand, you are, therefore, right that the animations are somewhat incorrect showing a classical moment that does not reorient in a magnetic field. On the other hand, what would the moment of inertia be of this magnet and how would it turn exactly? As you are free to choose any value there, no turning is valid as well.