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Carl Mungan, United States Naval Academy
In the February 2016 issue of The Physics Teacher, Prentis and Obsniuk present an introductory-level discussion of how the Helmholtz free energy can be thought of as a competition between minimizing energy and maximizing entropy at equilibrium, depending on the temperature. I also learned something from the iPhysicsLabs column in the same issue about the oscillations in pressure (easily perceptible to passengers’ ears) as waves are reflected from the far end of a tunnel shortly after a train enters it. The April 2016 issue has a convincing experimental demonstration that the sound produced when a camera flash is discharged next to a cymbal is due to thermal expansion of the metal upon absorption of the light, and is not due to momentum transfer from the reflected photons. In the same issue, Mallmann has another of his “surprising facts” articles. His second fact asks how far apart two earths would have to be to experience the same gravitational force as two ping-pong balls in contact? His third fact shows that the circle of least confusion for light focused by a lens of large diameter is at a substantially different position than the focal plane, thus explaining a common systematic error in introductory lab measurements of focal lengths of lenses.
An article on page 113 of the February 2016 issue of the American Journal of Physics analyzes videos of a uniform thin rod as it tips over starting from a vertical position to see whether the bottom end slips or not, and if so in which direction. There is also an interesting exchange of letters on pages 146 and 147 of the same issue concerning how torques are transmitted along a rigid pivoted rod connecting two point masses located at different radii from the hinge. I am interested in the role that eddy currents play in damping the tumbling of small metal-frame satellites (such as CubeSats) in earth orbit; an article on page 181 of the March 2016 issue investigates this issue computationally and experimentally. On a similar note, the comparison between experiment and an analytic solution for a spherical magnet falling through a copper pipe on page 257 of the April 2016 issue is convincing. I also appreciated the study of supercooling of water samples of varying degrees of purity on page 293 of the same issue.
Article 015022 in the January 2016 issue of the European Journal of Physics gives an introductory derivation of the wave and beam equations starting from discrete ball-and- spring or accordion-like bending-slab models, respectively. Article 025301 in the March 2016 issue discusses how Snell’s law fails to describe the propagation of light in a medium where the surfaces of constant refractive index are not a set of parallel planes. Also, article 035602 in the May 2016 issue considers the idea (first mentioned by Feynman) that, because of gravitational time dilation, the center of the earth is a few years younger than its surface. Article 015005 in the January 2016 issue of Physics Education asks whether the 8-minute propagation time for light from the sun to reach the earth implies that the sun will already be 2° below the horizon when you see it on the horizon at sunset? Article 015010 in the same issue presents a simple approximate derivation of the temperature of a black hole. Both journals can be accessed at the IOPscience Journals webpage.
Page 213 of the March 2016 issue of Resonance has a well-written review of the theory and detection of gravitational waves. Another interesting review is of the physics of “singing” sand dunes on page 339 of the April 2016 issue. These articles can be freely accessed at http://www.ias.ac.in/listing/issues/reso.
An article on page 2094 of the December 2015 issue of the Journal of Chemical Education explains why combustion always yields about 418 kJ per mole of oxygen gas. A commentary on page 583 of the April 2016 issue proposes modifying the vague term “amount” of a substance with the adjective “stoichiometric” to make it clear one is referring to number of moles and not to mass or volume. The journal archives are at the ACS Publications List of Issues.
Article 010128 in Physical Review Physics Education Research recommends that Faraday’s law be taught by presenting (via worksheets or clicker questions) to students contrasting cases so that they discover the key features of a situation required to obtain a nonzero induced voltage or current.
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.