June 2025
Volume 93, Issue No. 6
Online “Advanced Labs” in physics
Here, we describe a set of experiments that enables undergraduate students to perform and understand nucleation and growth utilizing the concept of birefringence. Semicrystalline polymers, e.g., polyethylene oxide, exhibit a Maltese cross extinction pattern when they are imaged under polarizing optical microscopy with crossed polars (i.e., two polarizers whose axes are oriented at right angle to each other). This behavior is due to the birefringence of the polymer's spherulite, which is its building block, analogous to the grains of a metal. In this instructional laboratory experiment, samples of polyethylene oxide were crystallized at three different transformation temperatures of 295, 273, and 77 K. Taking advantage of the birefringence of spherulites, the dependence of the microstructure with undercooling is shown. In particular, smaller spherulite sizes were obtained with larger undercoolings. It is also shown that the temperature dependence of the resulting polymer microstructure is in agreement with the analytical model for a nucleation and growth process. Finally, a live transformation of solidification of polyethylene oxide from the melt was recorded under crossed polars at room temperature via video microscopy. The time-dependent data that describe the advancement of the phase transformation are in excellent agreement with the Avrami model.
EDITORIAL
In this issue: June 2025 by Beth Parks; B. Cameron Reed; Donald Salisbury. DOI: 10.1119/5.0276909
PAPERS
Understanding the oscillatory motion of Saxon bowls: An experimental and theoretical approach by Hui Xiang Sim; Bernard Ricardo. DOI: 10.1119/5.0096408 Editor's Note: The Saxon Bowl consists of a bowl with a small hole in its base. Similar to an hourglass, it can be used to indicate a time interval by setting it on a water surface so that water flows in through the hole and it slowly sinks. When the bowl is fully submerged, the allotted time has elapsed. This work explores why the timing device is remarkably robust to initial conditions. The analysis of this bowl could be an exercise in an introductory fluid dynamics course, especially as it nicely illustrates the difference between turbulent and laminar drag.
Enhancing conceptual understanding of Gauss's law through asymmetric infinite sheet/slab charge examples by Simarjeet S. Saini; Reza Kohandani. DOI: 10.1119/5.0218051 Editor's Note: Teaching Gauss's law is challenging. If you are facing a challenge, you should read this paper. It describes a teaching plan for applying Gauss's law to a variety of situations where the charge distribution depends on only one Cartesian coordinate. Students will understand Gauss's law more deeply after this presentation and will also be able to understand and solve problems related to practical devices.
Designing objects that are invisible to electromagnetic waves by Johan Helsing; Shidong Jiang; Anders Karlsson. DOI: 10.1119/5.0238542 Editor's Note: While invisibility still lies in the realm of science fiction, it is possible to achieve a form of “cloaking” where a dielectric rod does not scatter an incident plane electromagnetic wave of a specified frequency, angle of incidence, and polarization. This paper reviews the relevant electromagnetic theory and shows how commercial software can be used to carry out the calculations corresponding to such situations. This paper will be of interest to upper-level students in electromagnetism, microwave theory, communications, and optics classes.
Electromagnetic field momentum in theoretical magnetic monopole-like models by Isaiah Ruz; Douglas Singleton. DOI: 10.1119/5.0244726 Editor's Note: This paper is best advertised by a quotation from an anonymous reviewer: “This beautifully clear and accessible paper illuminates some subtle aspects of the Dirac string model for a magnetic (quasi-)monopole, along with two closely related models. The author calculates the (nonzero!) field momentum in the three models and shows that it is balanced by ‘hidden’ (mechanical) momentum in the currents. This paper brings an intriguing if esoteric subject to the attention of a wider audience.”
Daniel Davis, Jr., and his Manual of Magnetism by Thomas B. Greenslade, Jr. DOI: 10.1119/5.0220651 Editor's Note: In the nineteenth century when the theory of electromagnetism was developing, scientific equipment was developing rapidly to demonstrate these new principles. In the U.S., much of this equipment was produced and sold by Daniel Davis, Jr., whose Manual of Magnetism provided guidance to understanding and using it. Some examples of this equipment are still found in physics departments. The author urges us to seek them out, use them to better understand electromagnetism, and preserve them.
A flame war about entropy by Bruce D. Popp. DOI: 10.1119/5.0224315 Editor's Note: This is a fascinating account of disputes that arose in the early twentieth century concerning the nature of entropy and its role in the second law of thermodynamics. Of ultimate importance is Poincaré's assertion that entropy is not a fundamental quantity with which to state this law, and instructors in upper-level thermodynamics can profitably address this theme.
Investigation of student and faculty problem solving: An example from quantum mechanics by Alexandru Maries; Ryan Sayer; Chandralekha Singh. DOI: 10.1119/5.0207947 Editor's Note: In an investigation of student problem solving, the authors discovered that students often chose to answer problems with their gut feelings, rather than performing a calculation or supporting their answer with conceptual reasoning. This work explores the reasons for this choice, and the paper concludes with recommendations for improvements to physics instruction.
Detecting gravitational waves with light by Markus Pössel. DOI: 10.1119/5.0228933 Editor's Note: Einstein's general theory of relativity predicts an alteration of space itself through the propagation of gravitational waves. The effects of these waves on observed electromagnetic radiation are now yielding detailed insights into structural developments in the universe—from galactic interactions to black hole mergers. Progress has been immense since the first shorter period gravitational wave detection in 2015, and with longer period detection in 2023 employing pulsars opening a new window on the universe as explained in this article. Even greater sensitivity will result from the placement of the laser interferometer space antenna satellites around 2035. This is all opening up an exciting new world of observational astronomy to today's students, made accessible through the author's simple treatment of the modification of space. Various aspects of this work can be incorporated into a variety of courses, from first year physics to an undergraduate introduction to general relativity.
INSTRUCTIONAL LABORATORIES AND DEMONSTRATIONS
Online “Advanced Labs” in physics by Peter A. Bennett. DOI: /10.1119/5.0242200 Editor's Note: The rapid growth of online instruction creates a challenge for physics departments: Is it possible to replace the Advanced Lab with an online offering? This paper not only describes a successful online offering but also shares the complete set of materials with other instructors. Even instructors who teach in-person may choose to supplement their offerings with some of the exercises and techniques shared in this paper.
NOTES AND DISCUSSIONS
A Green function before your eyes by G. L. Lippi. DOI: 10.1119/5.0238550 Editor's Note: It is easy to get lost in the mathematics of the Green function. This paper points out a useful analogy between how we visually characterize a scene using white light and how a Green function similarly characterizes a system's response over frequency and position. Instructors who use Green functions in their courses may want to introduce them with this analogy.
Additional Resources
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