Carl E. Mungan
- In the October 2009 issue of The Physics Teacher, H.K. Wong points out on page 463 a flaw in a simple explanation of a unipolar motor (made of a battery, nail, rare-earth magnet, and wire) that I have often demonstrated in class. The torque which rotates the magnet cannot be due to the internal current flowing through the magnet. Instead it must arise from a reaction to the force that the magnet exerts on the wire near the point at which they touch each other.
- I enjoyed Jeremy Bernstein’s biographical ruminations about Dirac (and other physicists of his era) on page 979 of the November 2009 issue of the American Journal of Physics.
- I find simple demonstrations of atmospheric buoyancy to be amusing and instructive. The November 2009 issue of Physics Education discusses two. On page 668, a person stands on a scale while wearing a Santa suit that can be filled with air. Does the scale reading change noticeably? On page 569, a syringe (with its tip capped off) is placed on a sensitive balance. Does its measured weight depend on whether the plunger is pressed in or pulled out? In one case the answer is no and in the other the answer is yes. If you add a volume of air to an object, both the gravitational and buoyant forces increase by the same amount, unless the added air is at a substantially different pressure than the surrounding atmosphere. (Now you’re ready to try the “alka-seltzer in a latex glove demo” at http://stokes.byu.edu/alkaseltzer.html that Harold Stokes presented at an AAPT meeting a few years ago.)
- A couple of papers caught my eye in the most recent two issues of the European Journal of Physics. On page 1173 for September 2009, Agrawal discusses a simplified version of the Curzon-Ahlborn (CA) engine. Unlike a Carnot device which optimizes the efficiency but at the expense of infinitely slow operation, a CA engine maximizes the rate at which work is output. Secondly, using a numerical wind-tunnel model on page 1365 of the November issue, a Spanish pair of applied physicists show that bicyclists traveling as a tight group benefit not only the behind riders (by drafting) but even the cyclist at the front of the pack!
- A pair of Russian researchers present a detailed vector kinematics solution to the dog-and-rabbit chase problem starting on page 539 of the September 2009 issue of the Latin-American Journal of Physics Education.
- A brief overview of photoacoustic spectroscopy of nanomaterials can be found on page 1238 of the October 2009 issue of the Journal of Chemical Education. This technique is particularly appropriate for materials that scatter light too much to be easily studied by conventional absorption spectroscopy. The idea is to place a sample in an air-tight chamber, hit it with a chopped laser so that the sample and hence the surrounding air is periodically heated, and measure the resulting pressure oscillations with a microphone.
Carl Mungan is an Associate Professor of Physics at the U.S. Naval Academy in Annapolis, MD.
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