Students are placed into small groups and asked to calculate the force on a rectangular loop of current-carrying wire due to an external magnetic field.
After finding a general expression for the force, the groups must then calculate the amount of energy needed to rotate the loop 90 degrees.
Having students calculate the force on a current-carrying rectangular loop due to an external field also provides a good transition point into a discussion on the magnetic dipole moment.
Students can then be shown the difference between the classically expected and measured results in the Stern-Gerlach experiment, which indicate the directions of the magnetic moment are quantized. Performing this activity before discussing the Stern-Gerlach Experiment will help provide some context for why the silver atoms originally used were forced either up or down relative to the device's orientation.
This activity also requires them to pull together things that they learned in their introductory courses, as well as the early paradigms.