Vol. 57 No. 1 Jan-Jun (2011): Revista Mexicana de Física E
Published:
2011-01-01
Artículos
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Design of 2$^{nd}$ order low-pass active filters by preserving the physical meaning of design variables
Abstract:The purpose of this paper is, by one hand, offer to students basics on active filter design by introducing the Butterworth approach as well as some practical examples not only to show the proposed design flow (DF), but also to show that the design flow's stages have physical meaning mainly supported on physical laws. With the help of these laws, further, it is shown how additional filter design specifications can be translated to the physical design without affect neither the design approach nor DF. On the other hand, because any physical implementation suffer of the non-idealities of electronic components, the modeling of some of them based on both experimental results and spice simulations is presented in order to show how unwanted effects may be added to the DF. An advantage of this proposal is that DF preserves the physical meaning of the design variables. The laboratory-based learning adopted in this work has allowed to students be able to understand physical concepts, capture and analyze experimental data, and use design tools in a correct way mainly to avoid ``trial and error'' approaches.⬇️ Scroll down to see the full summary -
On the Sackur-Tetrode equation in an expanding universe
Abstract:In this work we investigate the thermodynamic properties satisfied by an expanding universe filled with a monoatomic ideal gas. We show that the equations for the energy density, entropy density and chemical potential remain the same of an ideal gas confined to a constant volume $V$. In particular the Sackur-Tetrode equation for the entropy of the ideal gas is also valid in the case of an expanding universe, provided that the constant value that represents the current entropy of the universe is appropriately chosen.⬇️ Scroll down to see the full summary -
How can acoustic resonance reduce the average velocity in a falling body?
Abstract:In this article, a simple experiment is described to overcome the misconception that acoustic pressure and levitation effects are difficult to observe in school laboratories. Analysis of the free fall velocity of a toy parachute inside a vertical tube, driven by sound in a range of frequencies around the resonant condition, exhibits the resonance frequency, the node pressure zones, and the optimal conditions to obtain acoustical levitation of a light body.⬇️ Scroll down to see the full summary -
Approaching to nanostructures using basic concepts of quantum mechanics
Abstract:In this work we discuss some concepts of quantum mechanics showing the result for the ground state energy of the infinite potential well that, together with elementary thermal physics concepts applied to semiconductors, help us to estimate the size of nanostructures. The energy value of the infinite potential well is compared with the finite potential well, some results were obtained with numerical calculations using basic quantum mechanics, particularly we used the BenDaniel-Duke model used for semiconductor junctions to analyze the structure GaAs-Al$_{1 - x}$Ga$_{x}$As, the energy levels were obtained of the confined states in the quantum well of the nanostructure in function of Al percentage. This system is representative of nanostructures quantum devices, currently under study in electronic solid state physics. This presentation could be very useful to teach in undergraduate applied physics courses.⬇️ Scroll down to see the full summary -
A novel set of reduced equations to model perfect layer matched (PML) in FDTD
Abstract:We propose a new set of reduced equations describing the Perfectly Matched Layer (PML) boundary condition for the Finite Difference Time Domain Method (FDTD) algorithm. These expressions take into account the main properties of the electromagnetic wave propagation in continuos medias: absorbing, free space and conductive, simplifying the solution of electromagnetic problems as such as the FDTD lattice. A two-dimensional (2-D) transversal electric TE mode Gaussian pulse propagating along free-space is presented as a vehicle of study. The efficiency of this model is validated by a new way to compute the power reflection coefficient of the electromagnetic field arriving at the PML interface at several points. Also a detailed description of the rounding up process to obtain integer values for FDTD equations indexes is discussed.⬇️ Scroll down to see the full summary -
On brillouin zones and related constructions
Abstract:In this paper we discuss the physical and geometrical content of the various equivalent definitions that have been given so far in the literature of a crystal's Brillouin zones. This serves as a motivation to introduce a computationally and conceptually simpler definition. Calculation of Brillouin-zone related properties in two-dimensional lattices is carried out as an illustration of the versatility of this new approach, particularly a count of the number of Landsberg subzones in these Bravais lattices is given, which could be of interest for theoretical physics and number theory.⬇️ Scroll down to see the full summary -
Setting up divisions in transport vehicles as a safety measure to reduce cumulated neighbor inertial forces on passengers
Abstract:Frequently, humans and animals are densely confined during transportation processes in both small and large spaces (containers, subways, wagons, buses). Clearly when sudden braking or collisions unfortunately occur, passengers may suffer physical damages. Then in order to avoid injuries due to unexpected decelerations during transportation, we propose to build simple extra divisions to decrease cumulative pressures due to the inertial force on standing-up stacked passengers due to neighbor passengers. In order to avoid fractures, we measured maximum compression forces on young adult thoraxes and the corresponding thorax compression limit. For situations of high crowding of these passengers, we estimate the size of the compartments as function of the expected decelerations and speeds. Implementation of the proposed divisions is simple and economical.⬇️ Scroll down to see the full summary -
Students' strategies for solving partially specified physics problems
Abstract:In this study we present a pilot-investigation of high-school and university students' abilities to solve partially specified physics problem and ways which they handle the task. Students are asked to answer the question whether the given problem situation is physically possible or not in real-life without an explicit advice on what to calculate and how to judge. We used a combination of individual interview and written test methods. According to the type of the problem-solving approach the respondents were divided into the five categories. We found the majority belong to categories with preferably quantitative approach regardless of curriculum, level of education or research method. Our results indicate that standard numerical exercises, usually used in teaching, do not develop sufficiently critical thinking and real-life problem-solving abilities. We think that students could be given also partially specified problems to help them in preparing for real-life problem-solving situations. To our knowledge, this study is the first one to explore students' reactions to this type of a problem and we are motivated to continue research.⬇️ Scroll down to see the full summary -
La fuerza normal: ¿una fuerza conservativa?
Abstract:The normal force is a constraint force that arises from the contact between an object and a surface. This force can preserve the mechanical energy of a system or not. It is shown that normal force is conservative when the surface does not evolve over time, as well as the rate to which the mechanical energy changes when the normal force is not conservative. For the latter situation, it is proposed the pseudo-potential energy function associated to normal force in order to obtain from energetic considerations the equation of movement of the object under the above mentioned force.⬇️ Scroll down to see the full summary -
Useful ratios between two-body nonleptonic and semileptonic decays of $B$ mesons
Abstract:We compute important ratios between decay widths of some exclusive two-body nonleptonic and semileptonic $B$ decays, which could be test of factorization hypothesis. We also present a summary of the expressions of the decay widths and differential decay rates of these decays, at tree level, including $l=0$ (ground state), $l=1$ (orbitally excited) and $n=2$ (radially excited) mesons in the final state. From a general point of view, we consider eight transitions, namely $H \to P, V, S, A, A^{'}, T, P(2S), V(2S)$. Our analysis is carried out assuming factorization hypothesis and using the WSB, ISGW and CLFA quark models.⬇️ Scroll down to see the full summary -
Conservative nonlinear oscillators in Abel's mechanical problem
Abstract:We study a family of conservative, \emph{truly nonlinear,} oscillators, arising from particular solutions of Abel's mechanical problem. An exact period-to-amplitude relation is produced for each instance. The lagrangian and hamiltonian formulations of such systems are discussed, along with their relations with the harmonic oscillator.⬇️ Scroll down to see the full summary -
Systematic electrical characterization study of a hybrid organic-inorganic semiconductor heterojunction at different illumination conditions
Abstract:Electrical characterization study of hybrid heterojunctions (HHJs) based on CdS and poly(3octylthiophene) (P3OT) is performed in order to know the process repeatability and materials homogeneity that may influence on power conversion efficiency (PCE) of CdS/P3OT photovoltaic (PV) solar cells. Basic statistical and numerical techniques for solving linear equations were used for systematic analysis of PV performance of those HJs. Adjustment curves were calculated from experimental data with the adjustment factor equal to almost 99.9 %, which means that the model has a high confidence level. They also were combined with theoretical models to establish a mathematical model that can describe the electrical performance of the mentioned junctions. PV response was analyzed under different illumination conditions, 23, 40, 124 and 285 mW/cm$^{2}$ of $I_{rr}$ level. The relationships between short circuit current ($J_{SC}$) and open circuit voltage ($V_{OC}$) with irradiance level ($I_{rr}$) were determined with high confidence level too. The dependence of $J_{SC}$ on $I_{rr}$ is linear, whereas $V_{OC}$ depends logarithmically on $I_{rr}$ and on $J_{SC}$. For spectral response a 100 Watts halogen lamp and light filters from 400 nm to 689 nm of wavelength were used. The maximum incident photon converted to electron efficiency (IPCE) was experimentally determined at 2.75 eV of photon energy. This value corresponds to the optical forbidden gap of the inorganic semiconductor material. The obtained results are in agreement with the theoretical concepts of PV devices.⬇️ Scroll down to see the full summary -
Thermodynamic properties of simple multi-Yukawa fluids: a variational approach
Abstract:The perturbation theory of dense fluids interacting according to an intermolecular potential represented as a linear combinations of m-Yukawa functions, whose reference interaction is the hard-sphere potential, allows setting an equation of state in terms of the ratio $\lambda $ of the corresponding species molecular sizes. In this work we determine $\lambda $ by solving numerically the non-linear equation that results from the minimization of the system Helmholtz free energy. The resulting values of $\lambda $ are density and temperature dependent, and are in quantitative agreement with those from the development of Mansoori and Canfield. The proposed method also provides the compressibility factor of the corresponding Lennard-Jones fluid, represented by the combination of a hard-sphere plus two Yukawa terms, in good agreement with the available values from Monte Carlo simulations.⬇️ Scroll down to see the full summary -
Efecto casimir en anti de Sitter $1+1$
Abstract:In this paper we study the effects of curvature on the Casimir force, we consider an Anti de Sitter background in two dimensions. We use the Green's approaching and we show clearly how the method is carried out. The result obtained agrees with the plane case in the limit in which the Anti de Sitter radius is small, so we can see the corrections due to the curvature of the Casimir force.⬇️ Scroll down to see the full summary -
Surface electric current distributions on spheres and spheroids as sources of pure quadrupole magnetic fields
Abstract:Neutral atom magnetic traps and nuclear magnetic resonance imaging require internal regions with constant gradient magnetic induction fields, which are identified as pure quadrupole fields. This contribution starts from such fields in the interior of spheres and spheroids in cartesian coordinates, identifying immediately their respective scalar magnetic potentials. Next, the corresponding potentials inside and outside are constructed using spherical and spheroidal harmonic functions, respectively, except for a proportionality constant to be determined by the boundary conditions at the surface of spheres $r=a$, prolate $\xi=\xi_0$ and oblate $\zeta=\zeta_0$ spheroids, where the electric current sources are distributed. The negative gradients of the scalar potentials yield the respective magnetic induction fields inside ($r\leq a$, $\xi\leq \xi_0$, $\zeta \leq \zeta_0$) and outside ($r\geq a$, $\xi\geq \xi_0$, $\zeta \geq \zeta_0$). Gauss's law in its boundary condition form determines the normalization constant of the external potentials, while Ampere's law determines the electric current source distributions on the surface of the spheres and spheroids.⬇️ Scroll down to see the full summary -
Origen de la construcción de instrumentos ópticos en México
Abstract:In 1769 there were several important astronomical phenomena which were observed from diverse parts of New Spain. From the study of the work then published by our astronomers, data emerge so far not taken into account by specialists, which show that at that time, Antonio Alzate y Ramirez, remarkable New Spaniard who did much to introduce and popularize modern science in Mexico, was the first manufacturer of optical instruments -in particular telescopes- in this nation. By comparing his activity with that of others noteworthy Americans of the late eighteenth century, we get evidence that he might be the first builder of telescopes in America.⬇️ Scroll down to see the full summary -
Alborada de la física newtoniana en México
Abstract:Through the study of different colonial documents, it can be established which New Spaniards in the eighteenth century read directly texts written by Newton and the influence of their reading had in the process of changing scientific paradigms in New Spain. It also indicates the Mexican libraries where these works are currently stored.⬇️ Scroll down to see the full summary -
Benjamin Olinde Rodrigues, matemático y filántropo, y su influencia en la Física Mexicana
Abstract:We present the little-known facts on the life of the mathematician, banker, socialist Olinde Rodrigues for their importance in the discovery of physical and mathematical knowledge that have had a major impact on the history of mathematics and physics. Also describes the history of the implementation of his ideas in Mexico.⬇️ Scroll down to see the full summary
