The Sticky Tape Lab

I had the first lab of the term yesterday in my introductory E&M class. This is the first time I've taught out of this book (Matter & Interactions by Chabay and Sherwood), which actually includes the basic elements of this lab as suggested activities in the second chapter of the text. The lab was more successful than I expected (I've done the lab before), and I even managed to add a somewhat more free-form element to it, that worked out well.

The materials for the lab are extremely simple: One roll of "invisible" tape for each group, plus some sort of stand for them to stick tape to.

It turns out that, if you put a piece of Scotch tape down on a table, and then lay a second piece of tape on top of it, when you pull the second piece off, it picks up some charge. If you put two pieces of tape down, pull them both up, and then separate them, you can make tapes with opposite charges.

This provides a simple and effective way to investigate interactions between charged objects. Of course, the conditions have to be right-- on a humid day, the tapes don't hold charge for very long, and it can be really difficult to see anything. The exact brand of tape also makes a difference-- of the two types we had kicking around the department, the wide, translucent tape charges up better than the narrow, transparent tape. Why? I have no idea, it just does.

Yesterday was a good sticky-tape day, and everything went very smoothly. The only real problem we had with the basic lab was that the tapes held charge too well in some cases-- a couple of the basic exercises ask the students to rub the tape to remove the excess charge, and look at the interactions of charged and neutral tapes. For some reason, it was damnably difficult to neutralize the tapes.

There are much worse problems to have, though. I've tried to do this lab on a humid day, and it was a disaster.

The basic handout we have leads the students through some really simple interactions with the tapes, but doesn't really ask them to measure anything, at least in the current version. Since I'm teaching the honors section, though, I added a couple of tasks to the end of the lab, asking them to develop a way to make a quantitative estimate of the number of electrons that moved to or from a tape during the charging process.

This requires them to come up with some way to measure the force between two tapes, and the students came up with a number of inventive ways of doing this. A few groups remembered that the book describes one method, and just copied that, but others came up with some novel methods (which I won't describe here, to thwart lazy Googlers. Do your own work, dammit!).

Every group managed to come up with the right methods, and they all got the right order of magnitude for the answer. They even finished the lab with the allotted time, meaning that for the first time in quite a while, I've ended three classes in a row on time.

I expect that to come to an end today, when I try to walk them through writing a VPython program to find the electric field of a dipole and simulate the motion of a test charge in that field, but, hey, you take what you can get...

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van de Graff generator, kids' bubbles. Fire up that puppy then blow some bubbles in the general vicinity. They slowly drift toward the bell until one touches and bursts. The others immediately fly away!

A psychologist would have a masters thesis, including behavioral training loops. A physicist notes the mostly invisible spray from bubble failure is charged and powerfully accelerates away from the bell. Every bubble that intercepts a charged bleb then follows. The inverse enables electrostatic coating (and remember the big ballast resistor).

Hmm, fun with Van de Graff. Have to try that when I'm teaching E&M...

As for the two tapes, I'd bet that they don't have the same glue along with having the different backings. These differences in materials could lead to a number of things (maybe lower surface tension of water droplets landing on the tape, so they spread out more, polarize on average less; less attraction with water droplets in the air, etc). I would bet the different surface areas might be part of it, but I have no idea whether area-dependent behavior would be anything near the dominant term.