Students had much more difficulty with this problem than we anticipated. Very few were reasoning about charges moving. Many thought that the charge was distributed on the surface (instead of the charge polarizing the conductor) and more than one group were overgeneralizing the “balancing” and thought that the field inside the cavities would be zero.

Students also had a difficult time at the end of the activity articulating what they had learned from this problems, so assigned a homework problem that asked them to do so.