Reason for force in Stern-Gerlach experiment I'm currently working at an assignment, and I'm having some trouble understanding how the magnetic field deflects the silver atoms passing trough it. From what I understood, the atoms are deflected up or down of a specific amount according to their magnetic moment, but I can't understand what is causing this phenomenon. I'm an engineering student and physics isn't my main field, so I would prefer that the explanation is not too specific. Thanks in advance for your help.
 A: If an atom has a magnetic moment (not momentum), that means it acts like a tiny dipole magnet -- like a tiny bar magnet.  A uniform magnetic field will not exert a net force on a dipole magnet, because (thinking of it as a bar magnet) the magnetic field will push on one pole and pull on the other pole with equal but opposite force.
However, if the magnetic field is nonuniform - if it diverges upward, for example - you can imagine that when the dipole is aligned with the field, the field is stronger at the location of one of the poles than at the other because of the divergence.  Let's say the nonuniform field is due to the N pole of an electromagnet.  Then the N pole of the dipole is repelled (upward in this scenario) and the S pole of the dipole is attracted downward.  But the net force (upward minus downward) depends on whether the dipole is aligned with its North pole up or its S pole up.  In the former case, the net force is downward and in the latter case the net force is upward.
The key result of the Stern-Gerlach experiment is not that the atoms are deflected, but that they are deflected by a fixed amount upward or downward - with nothing in between.  This is proof of a quantum property of spin, which relates to the magnetic moment: projection of the spin onto any axis (in this case an axis aligned with the diverging magnetic field) is quantized.  "Quantized" means the spin angular momentum along an axis can only have certain discrete values.
