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## 20 April 2015

### AP Physics 1 electrostatics assignment and video: PSSC Coulomb's Law

I've never, ever been able to get a quantitative Coulomb's Law demonstration to work.  The best, and only, quantitative electrostatics demonstration I know comes from Wayne Mullins, and is described here.  But that experiment measures electric potential; how can I directly measure the force between charged objects?

While *I* can't measure that force, the stereotypical 1950s scientists in this PSSC video can.

Watch them charge and discharge the foil-covered balls.  Watch them move the balls closer together and farther apart, and measure the change in electrostatic force between the balls.  Watch them cut the charge of one ball in half, and show that the force likewise is cut in half.

I've already had my students read up a bit on Coulomb's Law.  I don't see the point in lecturing on it -- what can I say that isn't in a standard treatment, since I can't do a live experiment?  But I can show this video.

Tonight I've assigned my students to watch the first 16:00 of the video as homework.  Then, in class tomorrow, we will work on this problem set.  The problem starts with a situation like in AP Physics B 2009 problem 2, where two charged balls hang from two strings.  Instead of asking about electric field lines and electric potential, I go straight to the equilibrium conditions.  Then I add a conceptual piece: I double the charge on one of the hanging objects; describe any changes.

Try the video and the assignment.  Let me know if you have other non-Van de Graff suggestions for a quick AP 1 level Coulomb's law treatment.  (Sorry, I gave up on Van de Graffs after seven years of never getting them to work right.)

1. The absolute best part of this video is at 10:08 where he takes off his glasses, turns to the camera, and says "well, what did you expect from an inverse square law?" It's amazing.

Cinema aside, I really like this idea and will be showing it to my students. It's way too difficult for me to set something like this up in my lab, but maybe we'll get lucky and the folks from SERC at Carleton will do a piece on it. Good find!

2. Thanks, Mike... I *think* it was Joe Stieve who first showed it to me. I'm not 100% sure, and I feel bad if I credit the wrong person, but it was found through networking, not brute force. Enjoy! :-)