Authors:M. A. Carrillo-Bernal; P. E. Mancera-Piña, H. H. Cerecedo-Núñez, P. Padilla-Sosa, H. N. Núñez-Yépez A. L. Salas-Brito Abstract: PubDate: 2014-06-23T18:29:21Z

Authors:Joshua Von Korff; N. Sanjay Rebello Abstract: From the perspective of an introductory calculus course, an integral is simply a Riemann sum: a particular limit of a sum of small quantities. However, students connect those mathematical quantities to physical representations in different ways. For example, integrals that add up mass and integrals that add up displacement use infinitesimals differently. Students who are not cognizant of these differences may not understand what they are doing when they integrate. Further, they may not understand how to set up an integral. We propose a framework for scaffolding students' knowledge of integrals using a distinction between “change” and “amount” infinitesimals. In support of the framework, we present results from two qualitative studies about student understanding of integration. PubDate: 2014-06-23T18:29:05Z

Authors:Benjamin D. Gamari; Dianwen Zhang, Richard E. Buckman, Peker Milas, John S. Denker, Hui Chen, Hongmin Li Lori S. Goldner Abstract: Single-molecule-sensitive microscopy and spectroscopy are transforming biophysics and materials science laboratories. Techniques such as fluorescence correlation spectroscopy (FCS) and single-molecule sensitive fluorescence resonance energy transfer (FRET) are now commonly available in research laboratories but are as yet infrequently available in teaching laboratories. We describe inexpensive electronics and open-source software that bridges this gap, making state-of-the-art research capabilities accessible to undergraduates interested in biophysics. We include a discussion of the intensity correlation function relevant to FCS and how it can be determined from photon arrival times. We demonstrate the system with a measurement of the hydrodynamic radius of a protein using FCS that is suitable for the undergraduate teaching laboratory. The FPGA-based electronics, which are easy to construct, are suitable for more advanced measurements as well, and several applications are described. As implemented, the system has 8 ns timing resolution, can control up to four laser sources, and can collect information from as many as four photon-counting detectors. PubDate: 2014-06-23T18:28:53Z

Authors:Francisco Solis Abstract: This article reviews Soft Matter Physics. by Masao Doi 268 pp. , Oxford, UK. 2013. Price $74.95 (hardcover) ISBN 978-0-19-965295-2. PubDate: 2014-06-23T18:28:25Z

Authors:Declan Mulhall; Matthew J. Moelter Abstract: We present a graphical approach to understanding the degeneracy, density of states, and cumulative state number for some simple quantum systems. By taking advantage of basic computing operations, we define a straightforward procedure for determining the relationship between discrete quantum energy levels and the corresponding density of states and cumulative level number. The density of states for a particle in a rigid box of various shapes and dimensions is examined and graphed. It is seen that the dimension of the box, rather than its shape, is the most important feature. In addition, we look at the density of states for a multi-particle system of identical bosons built on the single-particle spectra of those boxes. A simple model is used to explain how the N-particle density of states arises from the single particle system it is based on. PubDate: 2014-06-23T18:27:55Z

Authors:José Arnaldo Redinz Abstract: Why can a bird safely rest on a high-voltage power line' We discuss three effects that can lead to the development of voltages and currents in the bird's body. To explain the absence of electric shocks, we give numerical estimates of these voltages and currents obtained from the standard solution for the voltage along a two-wire transmission line. PubDate: 2014-06-23T18:27:34Z

Authors:Thomas D. Gutierrez Abstract: A technique to reconstruct one-dimensional, reflectionless potentials and the associated quantum wave functions starting from a finite number of known energy spectra is discussed. The method is demonstrated using spectra that scale like the lowest energy states of standard problems encountered in the undergraduate curriculum, such as the infinite square well and the simple harmonic oscillator. PubDate: 2014-06-23T18:27:16Z

Authors:Kumiko Kotera; Angela Olinto Abstract: This Resource Letter provides a guide to the literature on high energy cosmic rays and their associated messengers. Journal articles and books are cited for the following topics: cosmic rays from high to ultrahigh energies, gamma-rays, neutrinos, high energy astrophysical sources, particle acceleration mechanisms, cosmic ray interactions and propagation. PubDate: 2014-06-23T18:27:06Z

Authors:W. B. Hodge; S. V. Migirditch W. C. Kerr Abstract: This paper analyzes how the existence of electron spin changes the equation for the probability current density in the quantum-mechanical continuity equation. A spinful electron moving in a potential energy field experiences the spin-orbit interaction, and that additional term in the time-dependent Schrödinger equation places an additional spin-dependent term in the probability current density. Further, making an analogy with classical magnetostatics hints that there may be an additional magnetization current contribution. This contribution seems not to be derivable from a non-relativistic time-dependent Schrödinger equation, but there is a procedure described in the quantum mechanics textbook by Landau and Lifschitz to obtain it. We utilize and extend their procedure to obtain this magnetization term, which also gives a second derivation of the spin-orbit term. We conclude with an evaluation of these terms for the ground state of the hydrogen atom with spin-orbit interaction. The magnetization contribution is generally the larger one, except very near an atomic nucleus. PubDate: 2014-06-23T18:26:47Z

Authors:R. E. Urbina; K. K. Díaz, W. Bramer-Escamilla I. Sánchez Abstract: We present a fascinating experiment, with great potential for entry-level educational and public outreach applications: a granular fountain. It is an example of the rich and sometimes counterintuitive phenomena that takes place when fine sand is vertically vibrated at low frequencies. We offer guidelines for reproducing the experiment and also a short discussion on the physics behind the effect. PubDate: 2014-06-20T11:00:00Z

Authors:Giorgio Volpe; Sylvain Gigan Giovanni Volpe Abstract: Unlike passive Brownian particles, active Brownian particles, also known as microswimmers, propel themselves with directed motion and thus drive themselves out of equilibrium. Understanding their motion can provide insight into out-of-equilibrium phenomena associated with biological examples such as bacteria, as well as with artificial microswimmers. We discuss how to mathematically model their motion using a set of stochastic differential equations and how to numerically simulate it using the corresponding set of finite difference equations both in homogenous and complex environments. In particular, we show how active Brownian particles do not follow the Maxwell-Boltzmann distribution—a clear signature of their out-of-equilibrium nature—and how, unlike passive Brownian particles, microswimmers can be funneled, trapped, and sorted. PubDate: 2014-06-20T10:40:00Z

Authors:William Case; Sahar Jalal Abstract: A rattleback is a canoe-shaped body that, when spun on a smooth surface, rotates stably in one direction only; when spun in the reverse direction it oscillates violently (i.e., it “rattles”) and reverses its direction of spin. This behavior can be traced to the misalignment of the principal axes of the body with respect to the symmetry axis of its bottom surface. Although analyses of the phenomenon exist in the literature, there is not a clear, direct presentation of the basic mechanism responsible for the reversal of direction. The goal of this paper is to present, as clearly as possible, a treatment of the phenomenon by focusing on the geometry of the usual rattleback. Two initial conditions are considered: rotation about the vertical axis with no oscillation, and oscillation about a horizontal axis with no rotation. For the first initial state, oscillatory motion about the two horizontal axes is analyzed using a combination of linearization and reasonable assumptions. The reversal is then analyzed using energy considerations. The analysis for the second initial state is more direct and elementary. In combination, these analyses explain the transitions from rotation to oscillation to rotation in the opposite direction. The non-reversal for the rotation opposite initial rotation is also accounted for. We also comment on how the rattleback might be modified allowing it to reverse in both directions and thus to repeatedly reverse its direction of rotation. PubDate: 2014-06-20T10:35:00Z

Authors:M. A. Illarramendi; R. Hueso, J. Zubia, G. Aldabaldetreku, G. Durana A. Sánchez-Lavega Abstract: We discuss the design of a simple experiment that reproduces the operation of the Michelson stellar interferometer. The emission of stellar sources has been simulated using light emerging from circular end-faces of step-index polymer optical fibers and from diffuse reflections of laser beams. Interference fringes have been acquired using a digital camera, coupled to a telescope obscured by a double aperture lid. The experiment is analogous to the classical determination of stellar sizes by Michelson and can be used during the day. Using this experimental set-up, we can determine the size of extended sources, located at a distance of about 75 m from our telescope, with errors less than 25%. PubDate: 2014-06-20T10:35:00Z

Authors:Neil F. Comins Abstract: This article reviews In Search of the True Universe: The Tools, Shaping, and Cost of Cosmological Thought. by Martin Harwit 410 pp. , 2013. Price $50 (hardcover) ISBN 978-1-107-04406-7. PubDate: 2014-05-22T19:56:15Z

Authors:Cameron Reed Abstract: This article reviews Renewable Energy: A First Course. by Robert Ehrlich 461 pp. , 2013. Price $79.96 (paper) ISBN 978-1-4398-6115-8. PubDate: 2014-05-22T19:56:05Z