Vol. 51 No. 2 Jul-Dec (2005): Revista Mexicana de Física E
Published:
2005-01-01
Artículos
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Non-endoreversible Carnot refrigerator at maximum cooling power
Abstract:Within the context of the so-called finite time thermodynamics a Carnot's refrigerator is studied. It is used the non-endoreversibility concept and it is found an expression for the coefficient of performance, $w$, that permits to obtain values near to the experimental values reported in the literature.⬇️ Scroll down to see the full summary -
An auxiliary vector space that simplifies some calculations of the Kovalevskaya top
Abstract:We discuss here Kovalevskaya's integrable case of a rigid body, with a symmetric inertia moment, with half the value for the different of inertia moment, and the position of the center of mass and fixed point both placed on a plane orthogonal to the axis of symmetry. We introduce an auxiliary vector space that is a function of two complex conjugate variables and enables ys to simplify many of the calculations necessary to separate the variables in the explicit solution of the Kovalevskaya top. This vector space plays an important role in the study of the elliptic integrals and in particular, in the use Kovalevskaya made of the theory of elliptic integrals.⬇️ Scroll down to see the full summary -
Modelo de predicción basado en análisis multivariante para la determinación de concentración de azúcar en solución
Abstract:Polarization principle is a rutinary method to measure the optical activity of solutions with sugar. In this work we use a prediction model based in multivariate analysis to show the applicability of statistical method in analysis of mixtures. We measure near infrared absorption of solution with different sugar concentration for the determination of unknown concentration of sugar. We apply the model to estimate the concentration of sugar in sodas.⬇️ Scroll down to see the full summary -
Evaluación de esquemas de diferencia finita para la construcción de las cartas de Gurney-Lurie
Abstract:Gurney-Lurie Charts are plots of the mathematical model solution for non-steady state heat conduction in prototype geometries like flat plate, infinite length cylinder and sphere. In this work, we report a didactic exercise which consists in the evaluation of three finite difference schemes for the solution of the heat conduction mathematical model and the numerical construction of the Gurney-Lurie Charts, using a particular set of numerical parameters. This work can be applied in advanced courses of science and engineering programs related with the numerical solution of partial differential equations or heat transfer.⬇️ Scroll down to see the full summary -
Observing nanostructures with the Bohr-Heisenberg microscope: a subject for introductory modern physics courses
Abstract:In this work we obtain the ground state energy of the infinite potential well using a quantum concept such as the Bohr-Heisenberg microscope prevailing before the advent of the formalization of quantum mechanics and the uncertainty principle. Such energy value is equal to that obtained using formal quantum mechanics. We used this result to estimate the size of novel quantum structures such as the so called nanostructures or quantum wells, currently under study in solid state physics. This idea could be useful in teaching undergraduate introductory modern physics courses.⬇️ Scroll down to see the full summary -
Por qué y cómo encontramos funciones de matrices: entropía en mecánica cuántica
Abstract:Finding matrix functions is a common occurrence in modern day research. The basic analytic method to calculate matrix functions is based in the existance of the generalized Jordan --- or normal --- form of the argument matrix; usually, this method is tedious or hard to follow. In this report we present the typical generalized Jordan form method and an alternative method, which is simpler and usable for smooth functions that accept a Taylor series expansion. As extra-value, we apply the method finding the entropy of a three-level system interacting with a quantized single-mode radiation field.⬇️ Scroll down to see the full summary -
The relation between momentum conservation and Newton's third law revisited
Abstract:Under certain conditions usually fulfilled in classical mechanics, the principle of conservation of linear momentum and Newton's third law are equivalent. However, the demonstration of this fact is usually incomplete in textbooks. We shall show here that to demonstrate the equivalence, we require the explicit use of the principle of superposition contained in Newton's second law. On the other hand, under some additional conditions the combined laws of conservation of linear and angular momentum, are equivalent to Newton's third law with central forces. The conditions for such equivalence apply in many scenarios of classical mechanics; once again the principle of superposition contained in Newton's second law is the clue.⬇️ Scroll down to see the full summary -
Computational study of forced oscillations in a membrane
Abstract:The solution of the model for small forced oscillations in membranes is described. The cases of rectangular, circular and also elliptical membranes are discussed. A simple computer animation example is provided using the MAPLE software package. The evolution of the first vibrational mode for a circular membrane is presented. The results obtained are useful for the understanding of membrane oscillations in different applications. Also, the method could be used as a help in teaching.⬇️ Scroll down to see the full summary -
Simulation of Michelson and Young experiments using moiré fringes
Abstract:The ``visualization'' of classical physics experiments for a classroom demonstration is highly stimulating and encouraging for students in a General Physics course. Optics is unique in this matter because it offers the opportunity to awe the class with fun and beauty. The use of moiré fringes is a motivating, easy way to fulfill these purposes. This paper shows how moiré fringes can be used to simulate wave interference by means of the Michelson and Young experiments. A computer code in Postscript is added to help in tracing a set of concentric circles as an example of spherical wavefronts. The validity of the ``experimental measurements'' are checked with the Young and Michelson theories.⬇️ Scroll down to see the full summary -
On the transformation of torques between the laboratory and center of mass reference frames
Abstract:It is commonly stated in Newtonian Mechanics that the torque with respect to the laboratory frame is equal to the torque with respect to the center of mass frame plus an $\mathbf{R}\times \mathbf{F}$ factor, with $\mathbf{R}$ being the position of the center of mass and $\mathbf{F}$ denoting the total external force. Although this assertion is true, there is a subtlety in the demonstration that is overlooked in the textbooks. In addition, it is necessary to point out that if the reference frame attached to the center of mass rotates with respect to certain inertial frame, the assertion is no longer true.⬇️ Scroll down to see the full summary -
Addendum to ``On the vector solutions of Maxwell equations in spherical coordinate systems''
Abstract:The discussion of our previous work concerning the vector solution of boundary-value problems in electromagnetism is extended to the case of no azimuthal symmetry by means of the spin-weighted spherical harmonics.⬇️ Scroll down to see the full summary -
Statistical properties and linguistic coherence in noncoding DNA sequences
Abstract:It has generally been thought that the vast majority of the DNA of living organisms (about 95%) was constituted of what is now called non-coding DNA (NC-DNA). No mechanisms of the genetic expression were known for this NC-DNA, as opposed to the protein expression for coding DNA (C-DNA). So NC-DNA was traditionally assigned a role as a cover-up (with no biological function of its own) against the random attack of mutagenic elements on the C-DNA. Nevertheless (and in some sense motivated by the discovery of the tertiary structure of the genetic code), studies into the nature and biological function of NC-DNA began. Some of the tools of multifractal theory and statistical linguistics were recently applied to the analysis of coherence and correlation in non-coding DNA fragments. As a result, the presence of long-range correlations, coherent patterns, and even some well defined structural features, showed up. This structure and correlation would be impossible to find in a random nucleotide sequence (as NC-DNA was originally thought to be constituted).⬇️ Scroll down to see the full summary -
The confined two-dimensional hydrogen atom in the linear variational approach
Abstract:In this work we analyze the problem of a two-dimensional hydrogen atom confined in a circular region of impenetrable walls. We study the size effect on the energy eigenvalues as a function of the radius of the circle $\rho_0$. We used the linear variational method with the basis set of a free particle in a circle. We compare our results with those obtained by perturbation theory and the Rayleigh-Ritz variational method reported previously.⬇️ Scroll down to see the full summary -
Discharge curves for a Ni-Cd secondary battery monitored by LabVIEW
Abstract:We present an experimental project aimed at understanding how batteries work by analyzing discharge curves for a secondary battery. Our proposed experiment uses simple electronic components and takes advantage of the outstanding features provided by the commercial software LabVIEW in order to acquire, plot and save data on a disk in conjunction with an analog-digital board and a personal computer. We report discharge curves ($E$ vs $t)$, power, charge stored and energy delivered by a commercial secondary Ni-Cd battery as a function of time, using two different discharging methods: constant-load resistance and constant-current.⬇️ Scroll down to see the full summary -
De la física de carácter ingenieril a la creación de la primera profesión de física en México
Abstract:During the XVIII and XIX centuries, the development of the engineering and physics took parallel trajectories in Mexico. However, it was until the first decades of the XX century when a small group of intellectuals and engineers struggled, for more than two decades, to get a space where physics could become independent from engineering and the mainstream pragmatic approach that had characterized it for more than a century. The endeavor of these engineers paid off in the decade of 30's, when the first profession of physics and the first physics institute were created.⬇️ Scroll down to see the full summary
