Vol. 61 No. 2 Jul-Dec (2015): Revista Mexicana de Física E

Published: 2015-01-01

Artículos

  • Ecuación de estado cúbica para substancia pura usando los datos del segundo coeficiente del virial

    F. de J
    58-64
    Abstract:
    In this work, a methodology to determine the cubic equation of state of a pure substance using experimental second virial coefficient data, the critical point, and the acentric factor, is described. For every cubic equation of state, the critical compressibility and the second virial coefficient are related through a restriction, which is shown and discussed. It was found that the restriction has important effects on the predictions of the thermodynamic properties (phase diagram) and the second virial coefiente of substance. From a cubic equation of state, the resulting predictions of thermodynamic properties and second virial coefficient cannot simultaneously describe the experimental data. This conclusion is derived from the numerical evidences.
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  • Construcción de un generador electromecánico para la enseñanza de ondas en cuerdas, utilizando generador virtual para tablet

    J.E. Molina-Coronell, B.P. Rodríguez-Villanueva
    65-68
    Abstract:
    In this work, it is proposed the construction of a simple electromechanical system with a low cost to produce mechanical waves in ropes for the wave physics applications laboratory. This Device consist in a $5''$ speaker transformed with a removed cone, a membrane is added and aluminum axes used to tie a rope and a counterweight at the end of the string, to produce stationary waves. In other hand a 10 W amplifier with a TDA2050 integrated circuit to feed the electromechanical system is constructed. As a wave generator, an App of Android for Tablets is used, it is called ``Frequency Maker Pro''. As a result of the electromechanical oscillator and the Tablet a constant value of the mass per length unit is founded $\mu$ corresponding to $1.185 \times 10^{-3}$ kg/m that is so closed to the average value $1.2 \times 10^{-3}$ kg/m with an 1.25% error.
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  • The concept of entropy, from its origins to teachers

    F. Flores Camacho, N. Ulloa Lugo, H. Covarrubias Martínez
    69-80
    Abstract:
    Research on alternative conceptions shows that the entropy concept is among the most difficult for students to understand. Entropy is a concept with a complex history and has been the subject of diverse reconstructions and interpretations. This paper presents a brief review of the origin of the concept (Clausius and Boltzmann), a description of some influential textbooks (Planck, Fermi and Gibbs) and a comparative review on textbooks and how the concept of entropy is conceived and taught by university teachers. The results show that there is a great variety in the descriptions and meanings of entropy provided in textbooks, confusing students and teachers. A brief discussion focuses on new conceptual approaches to entropy. We conclude by suggesting how history can contribute to teachers and students' awareness about changes in the meaning of entropy and their acceptance of a more relevant commitment in their understanding of physical concepts.
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  • Equation of motion for describing a massive pulsating system with spherical symmetry

    H. Dehnen, M.G. Corona-Galindo
    81-85
    Abstract:
    An equation of motion for describing the motion of a massive pulsating system with spherical symmetry has been deduced. Such equation has applications in astrophysics and cosmology as long as the physical and the symmetry conditions are satisfied.
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  • Generating bipolar magnetic fields by using unipolar power sources

    A. Hernández, E.E. Mazón, E. Mena, M.E. Cano
    86-92
    Abstract:
    A low cost current inversion system is presented as solution to achieve bipolar magnetic fields through a series array of analog programmable unipolar power sources. The system is based on a digital signal controller to provide an accurate zero-crossing detection of the voltage supplied to a commercial electromagnet, and a polarity inversion system composed of a set of switches in H-bridge configuration and optical isolators. Furthermore a stage of magnetic uncoupling is performed to generate a pair of independent signals to drive each power supply. In general the system have shown be able to generate fine and coarse magnetic field sweeps, which can be used to conduct studies of basic science or academic works in a physics laboratory, bypassing the absence of an expensive bipolar power source.
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  • Figuras y entidades pioneras de la física en México

    M. de la Paz Ramos Lara
    93-103
    Abstract:
    This paper examines the adverse conditions faced by some intellectuals to professionalize and institutionalize the scientific research in the field of physics in Mexico. The pioneering institutions were the Faculty of Sciences and the Institute of Physics at the National Autonomous University of Mexico. The work is especially dedicated to the beginning of the second one, which marked the canons of the university scientific research and positioned itself as a fundamental institution in the development of physical sciences nationwide.
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  • Espinores de Weyl y el formalismo de helicidad

    J.L. Díaz Cruz, B. Larios, O. Meza Aldama, J. Reyes Pérez
    104-112
    Abstract:
    In this work we give a review of the original formulation of the relativistic wave equation for particles with spin one-half. Traditionally (Ã la Dirac), it's proposed that the ``square root'' of the Klein-Gordon (K-G) equation involves a 4 component (Dirac) spinor and in the non-relativistic limit it can be written as 2 equations for two 2 component spinors. On the other hand, there exists Weyl's formalism, in which one works from the beginning with 2 component Weyl spinors, which are the fundamental objects of the helicity formalism. In this work we rederive Weyl's equations directly, starting from K-G equation We also introduce the electromagnetic interaction with the Weyl spinors. As an example of the use of that formalism, we calculate Compton scattering using the helicity methods.
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  • Symmetry projection, geometry and choice of the basis

    R. Lemus, A.O.Hernández-Castillo.
    113-128
    Abstract:
    A geometrical point of view of symmetry adapted projection to irreducible subspaces is presented. The projection is applied in two stages. The first step consists in projecting over subspaces spanning irreducible representations (irreps) of the symmetry group, while the second projection is carried out over the irreps of a subgroup defined through a suitable group chain. It is shown that choosing different chains is equivalent to propose alternative bases (passive point of view), while changing the projected function corresponds to the active point of view where the vector to be projected is rotated. Because of the importance of choosing the appropriate basis, an approach based on the concept of invariant operators to obtain the basis for discrete groups is presented. We show that this approach is analogue to the case of continuum groups and it is closely related to the definition of quantum numbers. The importance of these concepts is illustrated through the effect of symmetry breaking. Because of the deep insight into the group theory concepts, we believe the presented viewpoint helps to understand the main ingredients involved in group representation theory using the latest advances in the subject for discrete groups.
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