  The work done by a parallel plate capacitor is evaluated when the plate
separation is changed. Two cases are considered: 1) the capacitor has a
constant charge; 2) the capacitor is at constant voltage. The net work is
calculated when the device follows a closed cycle in the charge-voltage space.
For certain conditions a net mechanical work can be obtained from the cycling
capacitor. The analysis is simple enough to be explained in a general physics
course.
