spins_unit_sg_experiment.ppt Pages 5-11

• Introduce the Stern-Gerlach experiment, discuss physical details of the experiment, needing 1s atomic state and a non-ionized atom, because they were testing the predictions for a neutral 1-s orbital based on the Bohr model. That prediction is that the 1-s state has angular momentum.
• Idea of an oven producing atoms with kinetic energy and random orientation, with a slit to select out those going in a specific direction (but not any particular orientation of the electron) - this is a good place to talk about experimental techniques
• Motivate why the experiment needs an inhomogeneous magnetic field: this can be done using the idea of magnetic fields wanting to be aligned, and reminding them of the magnetic field of the current loop, or using F=iLxB - only in an inhomogeneous field will you get deflection, depending on the orientation of the current loop
• ask students to predict what they anticipate would be seen in the experiment if the atom had angular momentum and could be modeled by a simple current loop. Any predictions should be discussed in terms of what reasoning was used to get there – and encourage sound reasoning – the most reasonable prediction is a some sort of random deflection up and down.
• Show experimental results of two explicit spots on the screen using a photo of the actual historical data from the famous postcard. The fact that two precise spots showed up SHOULD be surprising.
• Optional discussion: this leads terrifically into the included supplemental document about the history of this experiment. Stern and Gerlach thought their experiment showed the Bohr model was indeed correct because they saw deflection, but not until later was it pointed out that two spots was not the expectation, that it should be a smear of spots. So they were trying to falsify Bohr by saying no deflection would happen, but couldn’t falsify Bohr without that realization. Only 5 years later when the idea of intrinsic spin was introduced by Pauli was an explanation found that was consistent with the experiment results.
• Alternate discussion: derive the magnetic dipole, force on a dipole from an inhomogeneous B-field, energy of a dipole interacting with a B-field
• Discuss spin: why was it given that name? is something actually spinning? How do we know? What kind of calculation could we do to determine if it is or not? (they might know about radiated energy and instability of Bohr 1-s state, or they might think about the speed the electron would need to be going,..)
• Discuss quantization: this is a good chance to introduce the idea that quantization isn’t odd – they’ve seen it in classical physics, for example with harmonics on a string. It simply means from this data that whatever is happening with the electron is not a simple circular loop, but something that would produce two specific deflections: one upward and one downward.
• Optional SWBQ - “Find the magnetic moment of an electron orbiting the nucleus of an atom in terms of it's angular momentum” 15 min