Vol. 52 No. 2 Jul-Dec (2006): Revista Mexicana de Física E
Published:
2006-01-01
Artículos
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Medición de la susceptibilidad magnética de sustancias líquidas en el laboratorio de física
Abstract:Measurements of the magnetic susceptibility of the non-ionized water are reported. A simple experiment based on the interaction of a magnet with the substance, measured by an analytical balance, was employed. A NdFeB permanent magnet, which has a magnetic moment \mbox{$m = $0.58 $\pm $ 0.04 A$\cdot $m$^{2}$} was used. A mean value for the bulk susceptibility of the non-ionized water of $\chi$ (-6.8 $\pm $ 0.4) $\times$ 10$^{ - 4}$ (SI) was obtained, by using a theoretical model for the interaction between a dipole and a cylindrical sample. It is also demonstrated that a semi-infinite model is not a good description of the used experimental geometry. This is an experiment easy to be implemented into the basic physics laboratory, for the determination of magnetic susceptibilities of liquid substances.⬇️ Scroll down to see the full summary -
Mirror potentials in classical mechanics
Abstract:It is shown that for a central potential that is an injective function of the radial coordinate, a second central potential can be found that leads to trajectories in the configuration space and the momentum space coinciding, respectively, with the trajectories in the momentum space and the configuration space produced by the original potential.⬇️ Scroll down to see the full summary -
A computer model for a high temperature fuel cell
Abstract:A computer model is developed for determining the performance characteristics (e.g., current-voltage curves) of a solid oxide fuel cell. The model determines the fuel/oxidant concentration, the temperature profiles and the current density along the fuel and oxidant channels, respectively. Then, the average of the local current density distribution is used to determine the current-voltage curves (IV curves) over the entire range of cell potential, i.e., from open circuit voltage to short circuit voltage.⬇️ Scroll down to see the full summary -
Quantum bouncer with dissipation
Abstract:Effects on the spectra of the quantum bouncer due to dissipation are given when a linear o quadratic dissipation in the velocity of the particle is taken into account. Classical constants of motion and Hamiltonians are deduced for these systems and their quantized eigenvalues are estimated through perturbation theory. Differences were found comparing the eigenvalues of the constants of motion and the eigenvalues of the Hamiltonians. The cases when the dissipation parameters go to zero are compared with the nondissipative cases.⬇️ Scroll down to see the full summary -
An alternative method of solution to radiative transfer
Abstract:In this work, we propose an alternative method for solving a to radiative transfer equation in the four-stream approximation following the ideas of Jiménez-Aquino and Varela (2005). We use the Li and Ramaswamy (1995) proposal to establish the set of four coupled first-order differential equations associated with the radiances or radiative intensities. The method consists in transforming those four coupled differential equations into a set of four independent fourth-order differential equations associated with the quantities $M^s$ and $M^d$, which represent the sum and the difference respectively of two radiative intensities. As a consequence of this fact, the solutions for the radiative intensities are then easily calculated, and no matrix method is required.⬇️ Scroll down to see the full summary -
Análisis del grado de conocimiento declarativo y procedural de estudiantes en cursos de física universitaria
Abstract:The capacity for retaining the acquired knowledge of the students participating in one Physic course is analyzed as well as the ability to solve open problems that have a known solution. The analysis is focused to the degree of procedural and declarative knowledge; the last one was used using the categorization based in stages designed for analyzing simple written answers according with the standard methodology. From these results we inferred that the learning process of the students generate a routine semantic memory without meaningful semantic knowledge due to the absence of procedural work. This procedure favors a separation from the scientific teaching where the student is moved away from the kind of work made in the scientific environment.⬇️ Scroll down to see the full summary -
Maximum entropy principle, evolution equations, and physics education principio de máxima entropía como herramienta didáctica para discutir ecuaciones de evolución temporal
Abstract:The landscape of Physics is in a constant state of change and the structure of the University level Physics Curriculum needs to be adapted to this state of affairs. One of the most interesting current features of physics is the increasing importance of multidisciplinary studies. Methods and ideas from physics are being applied to diverse areas of science ranging from biology and economics to sociology and linguistics. Statistical Physics (SP) provides the most fertile set of methods for these kind of applications. The aim of the present contribution is to show how a powerful idea from SP that is widely applied in many fields, the maximum entropy principle (MaxEnt), can be integrated into the physics curriculum. First of all, the constrained maximization of an entropic measure provides an important illustration of the Lagrange multipliers technique, which is part of the standard calculus course for physics students. Secondly, MaxEnt provides the basis for an alternative foundation for statistical mechanics, which is nowadays being considered in some modern textbooks on SP. In point of fact, the main role usually assigned to MaxEnt (as a tool for teaching theoretical physics) is in connection with the Gibbs canonical and grand canonical ensembles. However, as we shall here explain, MaxEnt also constitutes a useful tool in the teaching of other aspects of theoretical physics: it provides an elegant and simple method for obtaining analytical solutions for several evolution equations, like the Liouville equation, the diffusion equation, and the Fokker-Planck equation. Last but certainly not least, MaxEnt belongs to the tool-kit that physicist use to solve concrete ``real-world" problems.⬇️ Scroll down to see the full summary -
Newton's missing experiment
Abstract:Some characteristics of Newton's philosophical method relevant to his works First Paper on Light and Colours (1672) y Opticks (1704) are discussed. It is shown from his prism experiments using different materials described in those works that it is possible that he may have carried out experiments with air prisms in water. This would have questioned the inductive conclusion that red rays are always less refracted than blue ones. Finally, and with a pedagogical intention, an experiment is reported to illustrate the result obtained depending on the material of the prism and of the medium.⬇️ Scroll down to see the full summary -
An analysis on the inversion of polynomials
Abstract:In this work the application and the intervals of validity of an inverse polynomial, according to the method proposed by Arfken [1] for the inversion$^{i}$ of series, is analyzed. It is shown that, for the inverse polynomial there exists a restricted domain whose longitude depends on the magnitude of the acceptable error when the inverse polynomial is used to approximate the inverse function of the original polynomial. A method for calculating the error of the approximation and its use in determining the restricted domain is described and is fully developed up to the third order. In addition, five examples are presented where the inversion of a polynomial is applied in solving different problems encountered in basic courses on physics and mathematics. Furthermore, expressions for the eighth and ninth coefficients of a ninth-degree inverse polynomial, which are not encountered explicitly in other known references, are deduced.⬇️ Scroll down to see the full summary -
Transfer matrices for piecewise constant potentials
Abstract:By expressing the time-independent Schrödinger equation in one dimension as a system of two first-order differential equations, the transfer matrix for a rectangular potential barrier is obtained making use of the matrix exponential. It is shown that the transfer matrix allows one to find the bound states and the quasinormal modes. A similar treatment for the one-dimensional propagation of electromagnetic waves in a homogeneous medium is also presented.⬇️ Scroll down to see the full summary -
Refractómetro de cubeta de sección cuadrada
Abstract:In this paper a simple and accurate method to measure refraction index for liquid substances is described. It is based on a cubic edge refractometer, where probes for several liquids such as common Water and Ethylic Alcohol were done. Also, a measure of refraction index for a Water-Ethylic Alcohol mixture at different concentration levels was carried out. We show results for all those experiments together with a detailed description of the refractometer operation. Finally a mathematical relation between refraction index and concentration level is given by the Water-Ethylic Alcohol experiment.⬇️ Scroll down to see the full summary -
Brownian motion of a charged particle in a magnetic field
Abstract:In this work we show that Taylor's description of Brownian motion in a magnetic field is equivalent to the situation in which the constant magnetic field is allowed to point along any direction. This can be achieved by means of a rotation of the Langevin equation given in the space of coordinates $ r$, to another space of coordinates $ r^{\prime}$, where the description of the problem is quite similar to that studied by Taylor. We use the over-damping approximation to show why, at equilibrium, the oscillatory behavior inherent in the system is not reflected in the diffusion process across the magnetic field, a fact not studied by Taylor. We also use the large-time approximation to study the effects of colored noise (small correlation time) on the diffusion processes across and along the magnetic field.⬇️ Scroll down to see the full summary -
Complete electromagnetic multipole expansion including toroidal moments
Abstract:The electromagnetic multipole expansion presented in this paper is complete on two accounts: i) It is valid for all points in space, and ii) it recognizes the existence of toroidal moments. The electromagnetic field due to alternating poloidal currents in a toroidal solenoid is evaluated exactly via the solution of the inhomogeneous vector Helmholtz equations, using the outgoing wave Green function technique and the Debye potentials for sources and fields. The physical meaning of the toroidal moments can be appreciated when they are compared with the familiar electric and magnetic moments; the analysis of the long-wavelength limit of the exact results also explains the previous neglect of the toroidal moments. The magnetostatic limit and the point source limit are also physically and didactically interesting.⬇️ Scroll down to see the full summary -
How to get mechanical work from a capacitor and a couple of batteries
Abstract:The work done by a parallel plate capacitor is evaluated when the plate separation is changed. Two cases are considered: 1) when the capacitor has a constant charge; 2) when the capacitor is at constant voltage. The net work is calculated when the device follows a closed cycle in the charge-voltage space. For certain conditions a net mechanical work can be obtained from the cycling capacitor. The analysis is simple enough to be explained in a general physics course.⬇️ Scroll down to see the full summary -
Diseño, construcción y aplicación de un sistema de electrodepósito químico: películas de quitosan sobre acero inoxidable
Abstract:Chemical electrodeposition is a cheap deposition technique with several advantages, between them we can mention: a wide range of low cost precursors available allowing easily to change pHs and concentration of the electrolyte, tight control of film thickness, uniformity, deposition rate and isothermal process. In the present work we report the design, construction and application of a low cost electrodeposition system. The performance of the system is tested depositing the biopolymer chitosan, the second most abundant natural biopolymer in nature, on stainless steel 316L. Deposited films were characterized with common techniques employed in non-biological materials studies. The vibrational, structural and chemical characteristics were obtained by infrared spectroscopy (IRS), scanning electron microscopy (SEM), and X-ray energy dispersive spectroscopy (EDS). Chitosan films were deposited on the negative electrode with their morphology depending on the pH and solution molalility.⬇️ Scroll down to see the full summary -
Semblanza del profesor Juan Bautista de Oyarzabal Orueta
Abstract:This work describes the life of the Spanish Marine and Professor of Physics, Juan Bautista de Oyarzabal Orueta (1913-1977). It provides interesting information about his family, standing out his well-known labor as a Spanish Marine (1931-1939), as a foreign student in Mexico and as a scientific researcher and Mexican professor (he naturalized Mexican in 1941), especially at the Universidad Nacional Autónoma de México (1944-1977), and the Instituto Politécnico Nacional (1945-1962).⬇️ Scroll down to see the full summary
