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Aug 2009

Volume 77, Issue 8, pp. 677-768

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NEWTON AND NEWTON’S THIRD LAW

Dewey I. Dykstra, Jr.

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 677

Online Publication Date: Jul 2009

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Abstract Unavailable

DIET SODA AND LIQUID NITROGEN

Bart H. McGuyer, Justin M. Brown, and Hoan B. Dang

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 677

Online Publication Date: Jul 2009

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Abstract Unavailable
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01.50.My Demonstration experiments and apparatus
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Analysis of standing sound waves using holographic interferometry

Daniel A. Russell, David E. Parker, and Russell S. Hughes

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 678

Online Publication Date: Jul 2009

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Optical holographic interferometry was used to study standing sound waves in air inside a resonance tube driven by a small loudspeaker at one end. The front face of the resonance tube was constructed with plexiglass, allowing optical interrogation of the tube interior. The object beam of the holographic setup was directed through the plexiglass and reflected off the back wall of the resonator. When driven at resonance, the fluctuations in the air density at the antinodes altered the refractive index of the air in the tube, causing interference patterns in the resulting holographic images. Real-time holography was used to determine resonance frequencies and to measure the wavelengths of the standing waves. Time-average holography was used to observe the effect of increasing the sound pressure level on the resulting fringe pattern. A simple theory was developed to successfully predict the fringe pattern.
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42.00.00 Optics
43.00.00 Acoustics

The Planck mass and the Chandrasekhar limit

David Garfinkle

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 683 | Cited 2 times

Online Publication Date: Jul 2009

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The Planck mass is often assumed to play a role only at the extremely high energy scales where quantum gravity becomes important. However, this mass plays a role in any physical system that involves gravity, quantum mechanics, and relativity. We examine the role of the Planck mass in determining the maximum mass of white dwarf stars.
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95.00.00 Fundamental astronomy and astrophysics; instrumentation, techniques, and astronomical observations

The kinematic origin of the cosmological redshift

Emory F. Bunn and David W. Hogg

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 688 | Cited 7 times

Online Publication Date: Jul 2009

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A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity) but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon’s path. In the context of the expanding universe, the kinematic interpretation corresponds to a family of comoving observers and hence is more natural.
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04.00.00 General relativity and gravitation

Nonconvergence of the Ehrenfest thought experiment

C. R. MacCluer

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 695

Online Publication Date: Jul 2009

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The classical Ehrenfest thought experiment does not converge. A slight modification yields a more appealing and convergent experiment.
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02.50.-r Probability theory, stochastic processes, and statistics
05.00.00 Statistical physics, thermodynamics, and nonlinear dynamical systems

Spectroscopy applied to observations of terrestrial light sources of uncertain origin

Karl D. Stephan, Sagar Ghimire, William A. Stapleton, and James Bunnell

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 697 | Cited 1 time

Online Publication Date: Jul 2009

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Sensitive spectroscopic instrumentation is now sufficiently portable to be used in field experiments involving terrestrial light sources as well as for astronomical observations. We report the results of a 20 night investigation of a phenomenon known as “Marfa lights” with the aid of a Schmidt-Cassegrain telescope and a CCD-array spectrometer. We show that the combination of computer azimuth and altitude logging, video recording, and continuous spectroscopy provides enough data for unequivocal identification of false positives such as distant streetlamps, automotive headlamps, and fires. We demonstrate that spectroscopic analysis of atmospheric molecular oxygen absorption can be used to determine distances of continuum-spectrum sources with an accuracy of 6% or better for distances of 4 km or greater. We also used astronomical objects for both directional references and approximate estimation of the system noise level in terms of minimum usable light flux.
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42.00.00 Optics

Optical tweezers for undergraduates: Theoretical analysis and experiments

M. S. Rocha

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 704 | Cited 1 time

Online Publication Date: Jul 2009

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A theoretical treatment of optical tweezers is presented at a level suitable for undergraduates. We explore the Rayleigh and the geometrical optics regimes with an emphasis on the latter. We discuss a model for the geometrical optics regime, including spherical aberration effects, and show that the model can easily be implemented numerically. A comparison of the model with experimental data yields excellent agreement between theory and experiment. We also briefly discuss a theory of optical tweezers valid for microspheres of any size.
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01.50.Pa Laboratory experiments and apparatus
42.00.00 Optics

First- and second-order Poisson spots

William R. Kelly, Eric L. Shirley, Alan L. Migdall, Sergey V. Polyakov, and Kurt Hendrix

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 713 | Cited 4 times

Online Publication Date: Jul 2009

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Although Thomas Young is generally given credit for being the first to provide evidence against Newton’s corpuscular theory of light, it was Augustin Fresnel who first stated the modern theory of diffraction. We review the history surrounding Fresnel’s 1818 paper and the role of the Poisson spot in the associated controversy. We next discuss the boundary-diffraction-wave approach to calculating diffraction effects and show how it can reduce the complexity of calculating diffraction patterns. We briefly discuss a generalization of this approach that reduces the dimensionality of integrals needed to calculate the complete diffraction pattern of any order diffraction effect. We repeat earlier demonstrations of the conventional Poisson spot and discuss an experimental setup for demonstrating an analogous phenomenon that we call a “second-order Poisson spot.” Several features of the diffraction pattern can be explained simply by considering the path lengths of singly and doubly bent paths and distinguishing between first- and second-order diffraction effects related to such paths, respectively.
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01.50.Pa Laboratory experiments and apparatus
42.00.00 Optics

The field concept in Ampère’s magnetostatics

Artice M. Davis

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 721

Online Publication Date: Jul 2009

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We update Ampère’s theory using vector notation and derive his expression for the force between two current elements. We assume that the two elements are in different current loops and integrate over one to obtain the force on a differential element in the second. This procedure allows us to define the magnetic field in a natural manner and to derive the Lorentz force for a current segment. We equate the magnetic moments of current and permanent magnet dipoles and show that Biot and Savart could have performed their experiment using a small current loop, thus establishing the Biot-Savart law as a consequence of Ampère’s theory.
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41.00.00 Electromagnetism; electron and ion optics

A brief primer on tamped fission-bomb cores

B. Cameron Reed

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 730 | Cited 3 times

Online Publication Date: Jul 2009

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A previously published discussion of the physics of fission-bomb cores is extended to incorporate the effects of a surrounding tamper/reflector. I examine the effect of a tamper of finite thickness on the calculation of critical mass in contrast to the infinite-thickness approximation usually invoked. An infinite-thickness tamper reduces the critical mass by a factor of eight; a more realistic estimate for a tamper of nonzero mass is a factor of about two. An approximate model of the Hiroshima Little Boy bomb results in a predicted explosive yield of about 13 kilotons, in good agreement with the actual value of approximately 12 kilotons, although this agreement might be fortuitous. A spreadsheet for estimating the critical masses of tamped and untamped cores and expected bomb yields and efficiencies is available.
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20.00.00 NUCLEAR PHYSICS

Particle in a box with a δ-function potential: Strong and weak coupling limits

Yogesh N. Joglekar

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 734 | Cited 1 time

Online Publication Date: Jul 2009

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A particle in a box with the δ-function potential Vλ(x)=λδ(x−x0) has been recently explored. We show that this example is solvable in the weak (λ→0±) and the strong (1∕λ→0±) coupling limits. In either limit the attractive and repulsive potentials lead to identical spectra, with the possible exception of a single negative-energy state that is present when 1∕λ→0−. We numerically obtain the spectra near the strong-coupling limit and discuss the consequences of the degeneracy that arises when 1∕λ→0±.
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03.65.-w Quantum mechanics

Understanding the focusing of charged particle beams in a solenoid magnetic field

Vinit Kumar

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 737 | Cited 2 times

Online Publication Date: Jul 2009

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The focusing of a charged particle beam in a solenoid is typically explained by invoking the concept of a Larmour frame and using Busch’s theorem. Often, there is some confusion about how a uniform magnetic field of a long solenoid focuses the electron beam because it is generally understood that a uniform magnetic field can only guide charged particles. We perform a simple analysis of the dynamics of a charged particle beam in a solenoid and emphasize an intuitive understanding of some of the interesting features.
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41.00.00 Electromagnetism; electron and ion optics

A modified time-dependent perturbation approach to quantum dynamics and its application to relaxation phenomena in magnetic molecules

C. A. Dartora, G. G. Cabrera, and K. Z. Nobrega

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 742 | Cited 1 time

Online Publication Date: Jul 2009

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We introduce a modified version of the interaction picture applicable to situations in which the nonperturbed Hamiltonian is time-dependent but can be diagonalized by the same basis of states at all times. We probe the method using a simple model Hamiltonian to study the spin dynamics of molecular magnets with time-dependent fields. Fully quantum mechanical calculations show the superiority of our method compared to the usual time-dependent perturbation theory. Our approach takes into account time-dependent fields and converges rapidly to the exact solution.
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03.65.-w Quantum mechanics
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What course elements correlate with improvement on tests in introductory Newtonian mechanics?

Elsa-Sofia Morote and David E. Pritchard

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 746 | Cited 1 time

Online Publication Date: Jul 2009

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In an MIT calculus-based introductory Newtonian mechanics course, we study the effectiveness of various instructional course elements: electronic and written homeworks, collaborative group problems, and class participation. We measure effectiveness by the slope of the regression line between a student’s score (used as a proxy for participation) on a particular course element and his normalized gain on various assessment instruments. These instruments were the MIT final exam comprised mainly of multipart problems demanding analytic responses and two widely used standard physics tests that emphasize conceptual knowledge: the Force Concept Inventory and the Mechanics Baseline Test. The results show that interactive course elements are associated with higher gains on assessment instruments: doing interactive electronic homework administered by myCyberTutor correlated with large gains on the final exam producing a learning effect of 1.8±0.4 standard deviations on the final examination score. myCyberTutor and collaborative group problem solving correlated with gains on the more conceptual tests. We also report surveys that demonstrate that students have had an increasingly favorable opinion of myCyberTutor over the four terms of its use.
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01.40.Fk Research in physics education
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Comment on “Can Maxwell’s equations be obtained from the continuity equation?” by J. A. Heras [Am. J. Phys. 75, 652–657 (2007)]

Edward Kapuscik

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 754 | Cited 1 time

Online Publication Date: Jul 2009

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Heras’ derivation of Maxwell’s equations from the continuity equation is criticized.
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41.00.00 Electromagnetism; electron and ion optics

Reply to “Comment on ‘Can Maxwell’s equations be obtained from the continuity equation?,’” by E. Kapuscik [Am. J. Phys. 77, 754 (2009)]

José A. Heras

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 755

Online Publication Date: Jul 2009

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Abstract Unavailable
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41.00.00 Electromagnetism; electron and ion optics
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Experimental special relativity with a meter stick and a clock

M. Lund and U. I. Uggerhøj

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 757 | Cited 1 time

Online Publication Date: Jul 2009

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We present measurements of the velocity of electrons, performed with an apparatus using the time-of-flight method, to demonstrate that the speed of light c is the limiting speed for electrons. The results obtained show the applicability of relativity in a regime where nonrelativistic physics predicts velocities exceeding c. The main advantages of the apparatus are the simplicity of its operation and the data analysis. By using electrons from weak radioactive sources the experiment is suitable for the undergraduate laboratory. Furthermore, we show that with electrons from a linear accelerator and the same apparatus, excellent agreement between data and the relativistic expression for the speed of electrons in the keV-MeV energy range is observed.
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01.50.Pa Laboratory experiments and apparatus
03.30.+p Special relativity

Observation of standing light wave by using fluorescence from a polymer thin film and diffuse reflection from a glass surface: Revisiting Wiener’s experiment

Min Sung Kim, Byeong Joo Kim, Hwan Hong Lim, and Myoungsik Cha

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 761

Online Publication Date: Jul 2009

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In 1892 Drude and Nernst reported the first observation of a standing light wave by using a fluorescent thin film, which is a real-time version of Wiener’s first experiment on standing light wave. We reproduce the experiment by Drude and Nernst with improved efficiency using modern laser technology and a highly fluorescent π-conjugated polymer. We also used a glass plate scratched slightly on one surface to visualize the same phenomenon using diffuse reflection instead of fluorescence. This method has the advantage of being a wavelength-independent observation compared to the fluorescence method.
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01.50.Pa Laboratory experiments and apparatus
42.00.00 Optics

Construction of a simple spherical air bearing

M. F. Masters, C. Reynolds, H. Suedhoff, and F. M. DeArmond

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 764

Online Publication Date: Jul 2009

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A simple method requiring minimal tools and mechanical ability, is presented for making a spherical air bearing. The air bearings are used for an exploration of precession. Results of test measurements are presented.
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01.50.Pa Laboratory experiments and apparatus
47.00.00 Fluid dynamics
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Worlds of Flow: A History of Hydrodynamics from the Bernoullis to Prandtl

Olivier Darrigol and David C. Montgomery, Reviewer, Reviewer

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 767

Online Publication Date: Jul 2009

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Abstract Unavailable
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01.30.Vv Book reviews
47.00.00 Fluid dynamics

BOOKS RECEIVED

American Journal of Physics -- August 2009 -- Volume 77, Issue 8, pp. 768

Online Publication Date: Jul 2009

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