The ultimate demo day!

I teach both physics 1 and physics 2 each semester at Ishkabibble CC, but I don't recall ever doing all four of these demos on the same day, ever. A combination of factors (room scheduling, availability of staff to get LN for my morning class, a helium balloon) conspired to allow me to do 4 of my favorite demos today. Listed in the order they were done:


1) Talking with helium.

A review topic from last week, but an ICC club event had some helium balloons available that made it convenient to do it today. Great fun, and probably worth having to defer one example to the next class day.

2) Liquid Nitrogen experiments, including the best experiment that never !! works.

I use a constant volume gas "thermometer" filled with air to test the ideal gas law prediction of the change in pressure with temperature. The result is wrong because ... air is not an ideal gas. The Oxygen condenses out at LN temperatures, and that 20% change in the number of moles exactly explains the error. Great fun, because the students always think "experimental error" rather than "interesting physics beyond the standard model" when I ask what might be wrong with the prediction I made before doing the experiment.

3) Rotation of polarization with Karo syrup.

Always a crowd favorite, but I put two orthogonal Polaroids side-by-side on the bottom after doing the usual experiment and ask what will happen. They correctly guess "two different colors", but hardly expect to see two complementary colors (such as lime green and magenta) as the result.

4) Illustrating a real image produced by a concave mirror by putting the image of light bulb in an empty socket.

They can't believe this one. Some even think the bulb is real. I like it best because I built it myself, from memory, to replicate the one at Enormous State University where I did my PhD.

And, best of all, some of them learned a few things that won't even be on the test!


[Yes, any of my students will immediately guess who wrote this, but I'm not that paranoid about what I write here. Besides, I doubt that anyone even reads this blog yet.]