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Carl Mungan, United States Naval Academy
Every month Boris Korsunsky comes up with a challenging problem for physics teachers and students. Page 446 of the October 2015 issue of The Physics Teacher has one concerning a hanging rope sliding frictionlessly around a cylindrical peg. To extend the problem, can you find the tension in the rope as a function of the angle around the peg? Can you find the normal force on a differential segment of the rope at that angle?
An article on page 506 of the June 2015 issue of the American Journal of Physics provides an accessible explanation of how electromagnetic waves propagate through a plasma starting from Maxwell’s equations, with application to radio waves interacting with the ionosphere. On page 567 of the same issue, a remapping of a charged line segment onto a circular arc is used to provide a geometric method to obtain the direction and magnitude of the electric field due to the segment. In the July 2015 issue, an article on page 621 considers what happens if a set of capacitors are initially charged and then placed in series with a battery and optionally a resistor; this configuration results in trapped charges that could make for interesting new classroom or textbook problems on capacitor circuits. Page 646 of the same issue presents the results of a PER study that shows that allowing too many tries on online homework problems encourages counterproductive tactics; the author recommends setting the maximum number of tries to five. Turning next to the August 2015 issue, an article on page 703 gives a clear exposition of radiation reaction and the resolution of associated energy paradoxes when a charged body accelerates. A short article on page 719 of the same issue uses some clever scaling laws and molecular parameters to deduce the maximum speeds of running and swimming of animals ranging in size from bacteria to elephants and whales. Finally, page 723 presents a marvelous analysis of the following puzzle: If the same amount of heat is transferred to two identical balls, one hanging from a thread and one on a tabletop, which ends up with the larger final temperature, neglecting all heat losses? The standard answer is that it is the hanging one, because the ball on the table has to convert some thermal energy into gravitational potential energy due to its thermal expansion. However, that would violate the second law because we could attach a thread to the ball on the table after its rise and then cool it so that it would rise again, thereby converting thermal energy into useful potential energy with an efficiency that can exceed the Carnot limit.
A short article on page 564 of the September 2015 issue of Physics Education presents experimental results for the nifty effect known as a chain fountain: If the end of a long chain coiled inside a beaker is dropped over the edge of the beaker, the chain in the beaker rises up in a loop above the top of the beaker. The same issue also has an interesting discussion on page 568 of the fact that if one connects ideal batteries in parallel with each other and a resistive load, then the currents through the individual batteries cannot be determined. Turning to the European Journal of Physics, Bringuier has a remarkable analysis of damping by entropic forces in article 055024 of the September 2015 issue. In addition, article 055033 about experimentally verifying the Sackur-Tetrode equation caught my eye. Both journals are accessible at http://iopscience.iop.org/journals.