Catálogo de publicaciones - revistas

<jats:p>When central forces follow a power law, Cameron Reed [Am. J. Phys. 90(5), 394–396 (2022)] showed that the resulting force on a test particle located outside an object with spherical symmetry is the same as if the source were located at the center of the sphere, if and only if the potential is either Newtonian, F ∝ r−2, or Hookean, F ∝ r. This Note shares another simple proof of this result and comments on the result in the light of the so-called transmutation law of central forces.</jats:p>

<jats:p>When two flat surfaces approach each other, the fluid in between is accelerated and ejected from the sides at large speeds. This situation occurs often in everyday life, such as when you step in a puddle and create splashes of water, or when you clap your hands or close a book and create jets of air. For these systems, the inertia of the fluid resists the acceleration, creating large nonlinear forces on the flat surfaces. In this work, we study the case of a closing book. The fluid motion in this case is relatively easy to model, using the conservation of mechanical energy, and to measure using a MEMS gyroscopic sensor. This study reveals the unusual forces that occur when two plates collide in an experiment that can be performed by students at home.</jats:p>

<jats:p>This paper presents the calculation of the electrical power transported by the electromagnetic fields of two parallel wires carrying opposite DC currents. The Poynting vector is developed in bipolar coordinates and symbolically integrated over different surfaces. For perfectly conducting wires, the purely longitudinal power in the space surrounding the wires is shown to be equal to that which is produced by the battery (and consumed by the load resistor). For resistive wires, the longitudinal power transported by the fields is shown to diminish according to the distance traveled, and the loss is proved to be equal to the power entering the wires via the fields at their surfaces.</jats:p>

<jats:p>Among the several methods to compute the perihelion precession for bounded orbits in Schwarzschild spacetime, the simplest is to ignore a term in the equations of motion. This is currently justified under the assumption that the eccentricity of the orbit is small. For cases such as Mercury in our solar system, whose eccentricity is not small, this method seems not to be applicable. Yet it gives the right result, the reason being that the term that has been excluded, although responsible for first order—in the ratio of the Schwarzschild radius over the radial coordinate—corrections of the orbit, only produces completely negligible higher order corrections for the perihelion precession. We show this result by two different procedures. We claim, therefore, that as long as the aim of the computation is the perihelion precession, one can safely drop that term regardless of the magnitude of the eccentricity.</jats:p>

<jats:p>The mean relative speed and relative speed distribution function of molecules play important roles in kinetics and related topics. College textbooks frequently present a simplified derivation of these quantities that, while yielding the correct result, are not well justified. This paper presents a detailed physical picture of collision-related processes based on the microscopic kinetic theory. Formulas for the mean relative speed and mean collision rate along with the relative speed distribution function and the distribution function of relative speed of the paired particles in collisions (collision possible distribution function) are derived for molecules from two different systems that are each at thermal equilibrium.</jats:p>

<jats:p>Measuring the relative humidity of air is an important challenge for meteorological measurements, food conservation, building design, and evaporation control, among other applications. Relative humidity can be measured with a psychrometer, which is a hygrometer composed of two identical thermometers. The bulb of one thermometer is covered by a wick soaked with water so that evaporative cooling makes it indicate a lower temperature than the dry-bulb thermometer; it is possible to determine the relative humidity from the difference between these readings. We describe both a model and an experimental setup to illustrate the principle of a psychrometer for a pedagogical laboratory. The science of psychrometry could be more broadly taught at the undergraduate level to help introduce students to aspects of measurement techniques, fluid mechanics, heat transfer, and non-equilibrium thermodynamics.</jats:p>

<jats:p>We present a technique to observe stellar inteferograms, as well as the detailed analysis of those created by three bright stars: Betelgeuse, Rigel, and Sirius. It is shown that the atmospheric turbulence is responsible for the reduction of the long-exposure fringe visibility of the obtained interference patterns. By using different baselines in our interferometer, we are able to distinguish the decay of the visibility with the baseline, observe how different parameters such as the diameter or the distribution of the holes in our interferometer affect the pattern, and measure the atmospheric turbulence. This experiment can be performed by postgraduate students to gain practical experience in optical interferometry in astronomy.</jats:p>

<jats:p>We give a simple introduction to the properties and use of ultrastable optical cavities, which are increasingly common in atomic and molecular physics laboratories for stabilizing the frequency of lasers to linewidths at the kHz level or below. Although the physics of Fabry–Perot interferometers is part of standard optics curricula, the specificities of ultrastable optical cavities, such as their high finesse, fixed length, and the need to operate under vacuum, can make their use appear relatively challenging to newcomers. Our aim in this work is to bridge the gap between generic knowledge about Fabry–Perot resonators and the specialized literature about ultrastable cavities. The intended audience includes students setting up an ultrastable cavity in a research laboratory for the first time and instructors designing advanced laboratory courses on optics and laser stabilization techniques.</jats:p>

<jats:p>Acoustic trapping is used in modern biophysics laboratories to study cell adhesion or aggregation, to sort particles, or to build model tissues. Here, we create an acoustic trapping setup in liquid for an undergraduate instructional laboratory that is low-cost, easy to build, and produces results in a 1-hour laboratory period. In this setup, we use a glass slide, cover slip, and double-sided tape to make the sample chamber. A piezo-electric transducer connected to a function generator serves as the acoustic source. We use this setup to measure the node spacing (millimeters) and the acoustic trap force (picoNewtons). We anticipate that the simplicity of the experimental setup, the tractability of the theoretical equations, and the richness of the research topics on the subject will lead to an undergraduate laboratory with many interesting student projects.</jats:p>