The counterintuitive behavior of a Genecon connected to a capacitor as described by Prof. Mungan in his article “Motor demonstration using a hand-cranked Genecon” [TPT 52, 422 (Oct. 2014)] certainly presents a wonderful teaching opportunity. He also could have mentioned how the effort needed to turn the crank changes as the capacitor is charged.
One way to help students understand it is to ask a volunteer to turn the crank at a steady rate, keeping a small light bulb lit. When the instructor shorts out the bulb, the crank suddenly becomes harder to turn. The instructor asks the volunteer why he/she suddenly slowed down. Students quickly learn that the cranking effort is directly related to the current, which explains why the effort gradually decreases when the Genecon is being used to charge a supercapacitor. If friction can be ignored, the effort diminishes to zero when the capacitor is fully...