Catálogo de publicaciones - revistas

<jats:p>The Ehrenfest paradox for a rotating ring is examined and a kinematic resolution, within the framework of the special theory of relativity, is presented. Two different ways by which a ring can be brought from rest to rotational motion, whether by keeping the rest lengths of the blocks constituting the ring constant or by keeping their lengths in the inertial frame constant, are explored and their effect on the length of the material ring in the inertial as well as the co-rotating frame is checked. It is found that the ring tears at a point in the former case and remains intact in the latter case, but in neither of the two cases is the motion of the ring Born rigid during the transition from rest to rotational motion.</jats:p>

<jats:p>We present a simple, cost-effective, yet instructive spectrometer for use in undergraduate instructional laboratory courses. Deliberate design choices are made to enhance the learning experience provided by the setup, where every component is accessible to students, allowing them to fully understand the function of each individual item. The result is a state-of-the-art spectrometer, built from commercially available components, which balances pedagogical simplicity with the potential for a wide range of applications. Our setup prepares students for future spectroscopy work in research labs. Furthermore, data-taking by means of a CCD camera and the subsequent analysis teach students fundamental computational skills. Within one image, the spectrometer can cover a spectral range of 40 nm and its spectral resolution is about 0.1 nm, limited by the imaging optics. Systematic uncertainties arising from mechanical play of the grating's rotation stage limit the reproducibility of the setup to 0.65 nm. While these parameters can be improved, we decided to maintain the pedagogical and straightforward nature of the presented setup, as any increase in cost or complexity would undermine its educational benefits. Using the spectrometer in an undergraduate instructional laboratory makes possible a variety of valuable experiments, such as calibration measurements, investigation of different types of uncertainties and measurements errors, and historically important measurements (e.g., the Balmer series or solar spectrum). We are convinced that the presented spectrometer will greatly benefit the learning experience of students for many years to come.</jats:p>

<jats:p>The Tyndall effect describes light scattering by suspended solid particulates with sizes smaller than or comparable to the wavelength of light. This work describes the design and construction of an instructional kit for observing the Tyndall effect. The kit consists of a 3D printed holder that can be combined with low-cost and readily available materials to enable student laboratory experiments.</jats:p>

<jats:p>In electromagnetism, materials with a polarization density P→ or a magnetization density M→ are known to exhibit a bound surface charge density σb=P→·n̂ or a surface current density κ→b=M→×n̂, respectively, where n̂ is the unit vector perpendicular to the material boundary surface, directed outward. These expressions can be obtained from volume integrations for the electric potential V, or the magnetic vector potential A→, in which the integrals are restricted to the material volumes delimited by their respective boundaries. In that case, applying the divergence theorem leads to surface integrals on material boundaries and to the above-mentioned surface quantities. In this paper, a simple derivation is presented, which shows that both σb and κ→b are included in the expressions for the volume charge or current densities, provided that the divergence and curl operators are evaluated at the boundary so as to account for discontinuities at interfaces.</jats:p>

<jats:p>Extensible or expandable garden hoses have become popular. However, their behavior may be surprising to physicists. In this paper, the stress–strain equations that explain this behavior are derived and compared with experiments. The results clearly demonstrate that, since deformation is very large, conventional definitions of strain and stress are not useful and so-called real definitions are needed instead.</jats:p>

<jats:p>We show how a single metal guitar string of fixed length can produce a musical scale. The string is placed near a permanent magnet, and by applying an AC to the string at the frequency of the desired musical note, the Lorentz force creates vibrations in the string at that frequency. The tension of the string is set so that its harmonics correspond to the desired notes. A one-octave scale can be approximated by using these harmonic frequencies, allowing several melodies to be played using our non-contact monochord. This project could be adopted for demonstration or laboratory projects.</jats:p>

<jats:p>We present a simple experiment to demonstrate the concept of propagation of errors by measuring resistors on printed circuit boards (PCBs). We devise a method for connecting the resistors on a PCB that allows a relatively quick measurement of 200 resistor pairs to show that the distributions of the measurement of individual resistances, as well as the total resistance, are Gaussian-distributed, as expected from the central limit theorem. The measurement demonstrates how individual uncertainties propagate to the total uncertainty. The experiment is part of a laboratory course on statistics for physics students that emphasizes the application of statistics in data analysis.</jats:p>