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Carl Mungan, United States Naval Academy
Poiseuille’s law says the flow rate through a pipe is inversely proportional to the length of the pipe. In the January 2014 issue of the American Journal of Physics, Michael Nauenberg explains why the flow rate nevertheless does not diverge if the outlet pipe connected to a hole in the side of a liquid tank is made vanishingly short. The February issue presents a surprising demonstration by the Naval Postgraduate School in which a styrofoam pendulum bob is attracted to a loudspeaker emitting high-amplitude low-frequency sound, rather than being jetted away from it as in the famous Maxell cassette tape ad. In the April issue, there are articles on page 280 making interferometric measurements of the collision of a steel ball with a rod on a rolling cart, on page 301 extending the Clausius-Clapeyron equation from first to second derivatives, and on page 306 discussing the advantages of plotting pressure-volume heat engine cycles on a log-log scale.
There is always so much good stuff in The Physics Teacher that it is hard to choose, but here is just one selection from each of the past five issues. Page 58 of the January 2014 issue demonstrates by breaking a light bulb and cutting off the filament that the glass of the base can be made electrically conducting by heating it with a blowtorch. Page 122 of the February issue challenges readers to construct a stable spinning top from a single paperclip. Page 142 of the March issue experimentally demonstrates the surprising fact that the turning of a paddle wheel in a cathode ray tube no more demonstrates electron momentum than does the turning of a radiometer demonstrate photon momentum. (In both cases, the momentum transfer is drowned out by heating of the residual gas in the tubes.) On page 241 of the April issue, two Portugese educators ask why it requires more work to run on an inclined than a horizontal treadmill? (The answer is simpler in a reference frame attached to the moving belt.) Finally, on page 286 of the May issue, two educators from an institution that I took a college physics course while in high school (Mount Royal University, although in my day it was Mount Royal College) point out that the traditional explanation is wrong for why Kelvin’s estimate of Earth’s age was so far off. (Accounting for radioactive minerals would only increase his estimate by about 10% which is still way too low.)
The May 2014 issue of Physics Education, accessible at the IOP Science Journals web page, considers the emf generated when a cylindrical bar magnet is dropped vertically through a flat coil on page 319. If the bar magnet is short, it can be modeled as a circular loop, whereas if it is long, it can be modeled as a solenoid, in principle permitting one to estimate the magnetic dipole moment by fitting to experimental data.
There are several notable articles on thermodynamics in the March 2014 issue of the Journal of Chemical Education, including a vacancy model for the entropy change of a thermal reservoir on page 380, and a discussion of whether one should use the system pressure or the surroundings pressure in calculating expansion work during an irreversible gas expansion on page 402.
Disclaimer – The articles and opinion pieces found in this issue of the APS Forum on Education Newsletter are not peer refereed and represent solely the views of the authors and not necessarily the views of the APS.