Showing papers in "Revista Mexicana De Fisica E in 2013"

Alternative method to calculate the magnetic field of permanent magnets with azimuthal symmetry

Abstract: The magnetic field of a permanent magnet is calculated analytically for different geometries. The cases of a sphere, cone, cylinder, ring and rectangular prism are studied. The calculation on the axis of symmetry is presented in every case. For magnets with cylindrical symmetry, we propose an approach based on an expansion in Legendre polynomials to obtain the field at points off the axis. The case of a cylinder magnet was analyzed with this method by calculating the force between two magnets of this shape. Experimental results are presented too, showing a nice agreement with theory.

Exact solution of the 1D riemann problem in Newtonian and relativistic hydrodynamics

Abstract: Some of the most interesting scenarios that can be studied in astrophysics, contain fluids and plasma moving under the influence of strong gravitational fields. To study these problems it is required to implement numerical algorithms robust enough to deal with the equations describing such scenarios, which usually involve hydrodynamical shocks. It is traditional that the first problem a student willing to develop research in this area is to numerically solve the one dimensional Riemann problem, both Newtonian and relativistic. Even a more basic requirement is the construction of the exact solution to this problem in order to verify that the numerical implementations are correct. We describe in this paper the construction of the exact solution and a detailed procedure of its implementation

A handy exact solution for flow due to a stretching boundary with partial slip

Abstract: Received 27 November 2012; accepted 21 March 2013In this article we provide an exact solution to the nonlinear differential equation that describes the behaviour of a flow due to a stretching flatboundary due to partial slip. For this, we take as a guide the search for an asymptotic solution of the aforementioned equation.Keywords: Nonlinear differential equations; partial slip; stretching boundary; non-Newtonian fluids.PACS: 47.50.-d; 47.15.-x

Probing students' critical thinking processes by presenting ill-defined physics problems

Abstract: Hemos investigado las habilidades estudiantiles de revisar la definicion de un problema fisico y la significatividad de la solucion. La muestra constaba de 276 estudiantes de secundaria y universitarios croatas. Los estudiantes, que estaban en niveles diferentes de educacion, estaban familiarizados con los conceptos fisicos correspondientes. Los procesos del pensamiento critico de estudiantes eran investigados basandose en su respuesta a dos problemas mal definidos abiertos. Tambien era investigada la habilidad de profesores de prever el enfoque estudiantil tipico. Para esta ocasion 48 profesores respondieron a una encuesta de tipo cerrado compuesta de respuestas verdaderas de los estudiantes a dos problemas mal definidos. Los resultados muestran que los problemas mal definidos pretenden estimular a los estudiantes a expresar una gama amplia de sus ideas muy arraigadas sobre la significatividad del planteamiento y la solucion del problema. Los resultados indican tambien el nivel bajo del pensamiento critico estudiantil, independientemente del nivel de educacion y del curriculo. La enserianza tradicional obviamente no desarrolla suficientemente el pensamiento critico. Los profesores estiman correctamente la habilidad estudiantil de revisar la realidad del resultado, pero por otra parte, sobrestiman considerablemente su criticismo a la hora de definir el problema. Creemos que este tipo de problemas podrian facilitar el esfuerzo de profesores dirigido hacia el desarrollo sistematico de los procesos del pensamiento critico estudiantil. Por consiguiente, el afrontamiento de estudiantes a los problemas mal definidos podria ayudarles en mejorar sus competencias en la vida real, tanto como desarrollar la costumbre de tomar una postura critica hacia la definicion y la solucion de problemas fisicos

Variational symmetries of Lagrangians

Abstract: G.F. Torres del CastilloDepartamento de F´isica Matem´atica, Instituto de CienciasUniversidad Autonoma de Puebla, 72570 Puebla, Pue., M´ ´exico.C. Andrade Miron, and R.I. Bravo Rojas´Facultad de Ciencias F´isico Matem aticas Universidad Aut´ onoma de Puebla,´Apartado postal 165, 72001 Puebla, Pue., Mexico.´

Harmonic oscillator position eigenstates via application of an operator on the vacuum

Abstract: Harmonic oscillator squeezed states are states of minimum uncertainty, but unlike coherent states, in which the uncertainty in position and momentum are equal, squeezed states have the uncertainty reduced, either in position or in momentum, while still minimizing the uncertainty principle. It seems that this property of squeezed states would allow to obtain the position eigenstates as a limiting case, by doing null the uncertainty in position and infinite in momentum. However, there are two equivalent ways to define squeezed states, that lead to different expressions for the limiting states. In this work, we analyze both definitions and show the advantages and disadvantages of using them in order to find position eigenstates. With this in mind, but leaving aside the definitions of squeezed states, we find an operator that applied to the vacuum gives position eigenstates. We also analyze some properties of the squeezed states, based on the new expressions obtained for the eigenstates of the position

La red internacional de rayos cósmicos, Manuel Sandoval Vallarta y la física en México

Abstract: En 1932 Arthur Holly Compton organizo una gran expedicion por el norte de Canada, Michigan, Illinois, Hawaii, Nueva Zelanda, Australia, Peru y Mexico con el proposito de establecer una red internacional de estaciones de rayos cosmicos para estudiar la distribucion geografica de los rayos cosmicos. En esta expedicion se coordino a un grupo de especialistas distribuidos por distintas rutas de viaje. Para organizar la red de rayos cosmicos fue necesario establecer contactos con personas e instituciones en los lugares donde se iban a tomar las medidas. Ademas, se requirio de la construccion y estandarizacion de los instrumentos cientificos y las tecnicas para tomar medidas. La circulacion de instrumentos cientificos, personas y practicas fueron esenciales para llevar a cabo esta expedicion. Mexico fue uno de los lugares donde habrian de tomarse medidas de intensidad de rayos cosmicos. Esto ocurrio con la intervencion de Manuel Sandoval Vallarta, quien en ese momento era profesor asociado del Massachusetts Institute of Technology (MIT), y con la colaboracion de un grupo de ingenieros de la Universidad Nacional Autonoma de Mexico. A finales de la decada de los 30s esta colaboracion se convirtio en un factor clave para la creacion del primer instituto de fisica en Mexico

Ninth grade students' mental representations of the refraction of light: didactic implications

Abstract: The study of students’ mental representations of physics concepts and phenomena constitutes a central part of physics education research, as they play a decisive role in teaching. In the study presented here, we investigate 213 ninth grade students’ mental representations of the phenomenon of refraction, after they were taught about it in school. The empirical data was gathered through an interview using 3 tasks which involved the evaluation of hypothetical situations. The research data included mental representations that cause difficulty in the comprehension of refraction.

On average forces and the Ehrenfest theorem for a particle in a semi-infinite interval

Abstract: Received 18 April 2013; accepted 20 June 2013We study the issues of average forces and the Ehrenfest theorem for a particle restricted to a semi-infinite interval by an impenetrable wall.We consider and discuss two specific cases: (i) a free particle in an infinite step potential, and (ii) a free particle on a half-line. In eachsituation, we show that the mean values of the position, momentum and force, as functions of time, verify the Ehrenfest theorem (the stateof the particle being a general wave packet that is a continuous superposition of the energy eigenstates for the Hamiltonian). However, theinvolved force is not the same in each case. In fact, we have the usual external classical force in the first case and a type of nonlocal boundaryquantum force in the second case. In spite of these different forces, the corresponding mean values of these quantities give the same results.Accordingly, the Ehrenfest equations in the two situations are equivalent. We believe that a careful and clear consideration of how the twocases differ but, in the end, agree, is pertinent, and has not been included in the literature.Keywords: Quantum mechanics; Schrodinger equation; Ehrenfest theorem; average forces.¨PACS: 03.65.-w; 03.65.Ca

Fourier description of lock-in

Abstract: In this study, a new interpretation about the operation of a traditional lock-in and dual lock-in is presented from the viewpoint of Fourier analysis. Once the mathematical principles under which these devices operate are understood, we could take full advantage of the magnitude and phase of the Fourier coefficients to measure the physical variables, as shown in the final example of this study. Also, a comparison between signal-to-noise ratio (SNR) of a square reference lock-in and a pure sinusoid lock-in is also presented

Variational approximation for wave propagation in continuum and discrete media

Abstract: We develop a variational approximation for wave propagation in continuum and discrete media based on the modulation of wave profiles described by appropriate trial functions. We illustrate the method by considering an application to the theory of dislocation of materials. We first consider the continuum approximation of the model and reproduce the exact traveling known solution. We then consider the fully discrete non integrable model and obtain an approximate solution based on trial functions with functional form similar to the exact solution of the continuum. The description of this discrete approximate solution is in terms of a discrete nonlinear dispersion relation between the wave parameters. In this last situation we compare the numerical and variational solutions at the stationary case. We thus illustrate the usage of a variational asymptotic approximation to study nonlinear problems and we contrast the differences and difficulties between continuum and discrete problems

Poincaré, la mecánica clásica y el teorema de la recurrencia

Abstract: En conmemoracion de los 101 anos de la muerte de Henri Poincare hacemos un recuento de algunas de sus aportaciones a la mecanica clasica aderezandolo con un esbozo de su biografia academica. Usamos de un pendulo para ilustrar la tecnica cualitativa para analizar las soluciones a una ecuacion dinamica; lo empleamos tambien, pero suponiendolo extensible, para ilustrar el uso de los mapeos de Poincare para diferenciar las soluciones regulares de las caoticas, un tipo de soluciones que el mismo descubrio al estudiar el famoso problema de los tres cuerpos. Demostramos su resultado sobre la recurrencia de las soluciones en un sistema dinamico segun el cual toda solucion a la que se le exija ser tanto confinada como que conserve la energia debera de regresar despues de un tiempo, Tr, a estar tan cerca como se quiera de sus condiciones iniciales. Este es un resultado fundamental que debiera ser mas conocido por los estudiantes de fisica

Numerical evaluation of Bessel function integrals for functions with exponential dependence

Abstract: A numerical method for the calculation of Bessel function integrals is proposed for trial functions with exponential type behavior and evaluated for functions with and without explicit exponential dependence This method utilizes the integral representation of the Bessel function to recast the problem as a double integral; one of which is calculated with Gauss-Chebyshev quadrature while the other uses a parameter-dependent Gauss-Laguerre quadrature in the complex plane Accurate results can be obtained with relatively small orders of quadratures for the studied classes of functions

Introduction of the concepts of hole and effective mass using an alternative to the E-k diagram

Abstract: Received 26 February 2013; accepted 9 October 2013An alternative to the E-kdiagram is proposed as a mean of teaching the concept of hole to undergraduate students who have never studiedQuantum Mechanics. Some background on the application of Quantum Mechanics to crystalline solids is given in order to discuss theconcept of effective mass and the concept of hole. Then, the so-called

En busca de los efectos gravitacionales de Birkhoff: la ultracentrífuga transelástica

Abstract: This work reviews the origin, development, completion, and outcome of a trans-elastic ultracentrifuge project of Mexico’s Nuclear Center through 1971 to 1986. The project had its origin in the search for an effect that supposedly would validate Birkhoff’s gravity theory over Einstein’s General Relativity. For this purpose an extraordinary ultracentrifuge was built which deserved the 1973 National Award for Instruments (Mexico). The ultracentrifuge was also used to investigate the feasibility of uranium enrichment by solid state centrifugation. Highly enriched uranium was obtained, but in small quantities.

Spatial location in 360 - of reference points over an object by using stereo vision

Abstract: scan of an object in order to obtain topography details or getting the spatial position of agiven set of points of interest. However, the required computational time could be slow to perform real-time measurements. In this work wepresent an alternative approach based on Hering’s coordinate system and the use of high reflective markers, used as reference points to trackan object movement. The advantage of using these markers is that their detection is faster than a full scene correlation since it is performedby matching the position of the centroids of each marker without using any pixel-by-pixel analysis.Keywords: Hering’s coordinates; stereo vision; track markers.PACS: 42.30.Tz; 42.40.My; 42.30.Sy.

Interface para esqueletizado y suavizado de interferogramas

Abstract: It is developed a GUI (graphical user interface) in order to provide a fast and easy to use computational tool, which performs smoothing and/or skeletonizing of interferometric images. The code used in the interface is based in smoothing algorithms for interferograms, where the intensity distribution along rows or columns of the image is fitted by means of least squares and using Bessel functions of the first kind as basis functions, allowing the suppression of high frequency noise components. Additionally, the dilation method is used using a Strehl structural element in order to perform skeletonizing of the interferograms.