Tuesday, April 24, 2007

The Genecon-capacitor conundrum

I'm a big fan of Genecons. They're simple, versatile, and fairly rugged. They were designed for classroom use, and I use them in many classroom activities. Even jaded high school students love cranking the handle to light a bulb. A number of Genecon activities can be found in the circuits and magnetism sections of The Book of Phyz.

The high-tech one farad capacitors are also well-suited for classroom use. A Genecon and a 1 F cap? Two great tastes that taste great together!

One engaging activity is to connect Genecon to capacitor, crank, and release. The cranking charges the capacitor. Upon releasing the handle, the capacitor powers the Genecon as a motor. See the video below.

But there's something strange here. The handle rotates in the same direction is was being cranked.

We expect the handle to turn back from whence it came, as if our cranking had wound up a rubberband motor. Upon release, it should go back the other way! Nevertheless, the capacitor-powered Genecon motor turns the same way we cranked it to charge the capacitor.


I make my attempt to explain the conundrum in this interactive QuickTime presentation. It's a 24 MB download (it took about a minute to download on my DSL). I'm sure it needs polishing and further clarification. Your comments will help determine what's needed to improve it.


Peter Rea said...

Check out Paul Doherty's explanation at

Dean Baird said...

Thanks Peter,
Paul Doherty explains it in his characteristic excellent manner. I use more words and pictures to try the same thing. Nice to know that he and I agree on the mechanism.