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Oct 2004

Volume 72, Issue 10, pp. 1271-1360

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Every word should count

Jan Tobochnik and Harvey Gould

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1271

Online Publication Date: Sep 2004

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Abstract Unavailable
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01.30.-y Physics literature and publications
01.40.-d Education
01.50.-i Educational aids
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Discovery-based Gauss’s law

Mark B. Schneider

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1272 | Cited 1 time

Online Publication Date: Sep 2004

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The mathematical parallels between electrostatics and laminar fluid flow are exploited to develop a set of problems that lead introductory physics students to discover Gauss’s law. The primary fluid experiment involves examining the flow between two closely spaced plates and the continuity equation in two dimensions. The extrapolation of the two-dimensional velocity field to three dimensions yields a radial dependence that is analogous to Coulomb’s law. This analogy allows the student to predict an electrical version of the continuity equation equivalent to Gauss’s law. © 2004 American Association of Physics Teachers.
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01.50.-i Educational aids
47.15.-x Laminar flows
41.20.Cv Electrostatics; Poisson and Laplace equations, boundary-value problems
47.60.-i Flow phenomena in quasi-one-dimensional systems

An elementary treatment of the reverse sprinkler

Alejandro Jenkins

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1276 | Cited 6 times

Online Publication Date: Sep 2004

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We discuss the reverse sprinkler problem: How does a sprinkler turn when submerged and made to suck in water? We propose a solution that requires only a knowledge of mechanics and fluid dynamics at the introductory university level. We argue that as the flow of water starts, the sprinkler briefly experiences a torque that would make it turn toward the incoming water, while as the flow of water ceases it briefly experiences a torque in the opposite direction. No torque is expected when water is flowing steadily into it unless dissipative effects, such as viscosity, are considered. Dissipative effects result in a small torque that would cause the sprinkler arm to accelerate toward the steadily incoming water. Our conclusions are discussed in light of an analysis of forces, conservation of angular momentum, and the experimental results reported by others. We review the conflicting published treatments of this problem, some of which have been incorrect and many of which have introduced complications that obscure the basic physics involved. © 2004 American Association of Physics Teachers.
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01.50.-i Educational aids
47.10.-g General theory in fluid dynamics
47.27.-i Turbulent flows

On negative streamers: A deterministic approach

Manuel Arrayás

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1283 | Cited 5 times

Online Publication Date: Sep 2004

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The formation of streamers and spontaneous branching is commonly observed in dielectric breakdown phenomena. We discuss a deterministic streamer model that explains branching phenomena in terms of a Laplacian instability similar to that found in viscous fingering. Particular attention is paid to the physical interpretation of the model. © 2004 American Association of Physics Teachers.
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01.50.-i Educational aids
52.80.Wq Discharge in liquids and solids
02.30.Jr Partial differential equations

The physical limits of communication or Why any sufficiently advanced technology is indistinguishable from noise

Michael Lachmann, M. E. J. Newman, and Cristopher Moore

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1290 | Cited 1 time

Online Publication Date: Sep 2004

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It has been well known since the pioneering work of Claude Shannon in the 1940s that a message transmitted with optimal efficiency over a channel of limited bandwidth is indistinguishable from random noise to a receiver who is unfamiliar with the language in which the message is written. We derive some similar results about electromagnetic transmissions. In particular, we show that if electromagnetic radiation is used as a transmission medium, the most information-efficient format for a given message is indistinguishable from blackbody radiation. The characteristic temperature of the radiation is set by the amount of energy used to make the transmission. If information is not encoded in the direction of the radiation, but only in its timing, energy, and polarization, then the most efficient format has the form of a one-dimensional blackbody spectrum. © 2004 American Association of Physics Teachers.
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01.40.-d Education
41.20.Jb Electromagnetic wave propagation; radiowave propagation
44.40.+a Thermal radiation
05.40.Ca Noise

A new approach to Monte Carlo simulations in statistical physics: Wang-Landau sampling

D. P. Landau, Shan-Ho Tsai, and M. Exler

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1294 | Cited 30 times

Online Publication Date: Sep 2004

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We describe a Monte Carlo algorithm for doing simulations in classical statistical physics in a different way. Instead of sampling the probability distribution at a fixed temperature, a random walk is performed in energy space to extract an estimate for the density of states. The probability can be computed at any temperature by weighting the density of states by the appropriate Boltzmann factor. Thermodynamic properties can be determined from suitable derivatives of the partition function and, unlike “standard” methods, the free energy and entropy can also be computed directly. To demonstrate the simplicity and power of the algorithm, we apply it to models exhibiting first-order or second-order phase transitions. © 2004 American Association of Physics Teachers.
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05.10.Ln Monte Carlo methods
05.40.Fb Random walks and Levy flights
05.70.Fh Phase transitions: general studies
05.20.-y Classical statistical mechanics
05.70.Ce Thermodynamic functions and equations of state
02.50.Ng Distribution theory and Monte Carlo studies

Quantum mysteries revisited again

P. K. Aravind

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1303 | Cited 7 times

Online Publication Date: Sep 2004

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This paper describes a device, consisting of a central source and two widely separated detectors with six switch settings each, that provides a simple gedanken demonstration of the nonclassical correlations that are the subject of Bell’s theorem without relying on either statistical effects or the occurrence of rare events. The mechanism underlying the operation of the device is revealed for readers with a knowledge of quantum mechanics. © 2004 American Association of Physics Teachers.
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01.50.My Demonstration experiments and apparatus
03.65.Ud Entanglement and quantum nonlocality (e.g. EPR paradox, Bell's inequalities, GHZ states, etc.)

Time-reversal breaking of acoustic waves in a cavity

Vincent Bertaix, Julien Garson, Nicolas Quieffin, Stefan Catheline, Julien Derosny, and Mathias Fink

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1308 | Cited 3 times

Online Publication Date: Sep 2004

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Acoustic time-reversal is a well-established technique that focuses an ultrasonic wave on the location of its source. It is based on the time-reversal invariance of the wave equation and is usually implemented using time-reversal mirrors made up of a hundred of piezoelectric transducers. However, a time-reversal experiment can be performed in a closed cavity (a water-filled beaker) with only one transducer as a pulse-echo system. This easy-to-build and low cost experiment involves students in the general concept of the time-reversal invariance of the wave equation. We show that it also can be adapted to become an ultrasonic time-reversal thermometer. A careful study of the focal point shows a dependence of its position as a function of temperature variations. © 2004 American Association of Physics Teachers.
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01.50.Pa Laboratory experiments and apparatus
43.38.Fx Piezoelectric and ferroelectric transducers
43.35.-c Ultrasonics, quantum acoustics, and physical effects of sound

Swimming in Newtonian space–time: Orbital changes by cyclic changes in body shape

Michael J. Longo

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1312 | Cited 3 times

Online Publication Date: Sep 2004

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I investigate whether, in the context of Newtonian physics, a satellite can change its orbit by cyclic shape changes without any external forces other than gravity. This question has been discussed in the context of general relativity, and Wisdom has estimated that a swimmer near the surface of the Earth using meter-sized deformations can achieve a displacement of ∼10−23 m. I show that much larger displacements, ∼10−7 m, can occur for a Newtonian swimmer. Such a swimmer can change its orbit by rotating about its center of mass and trading chemical, solar, or other energy for orbital energy, without the need for any propellant. © 2004 American Association of Physics Teachers.
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01.50.-i Educational aids

Reflection of light from a uniformly moving mirror

Aleksandar Gjurchinovski

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1316 | Cited 11 times

Online Publication Date: Sep 2004

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We derive a formula for the law of reflection of a plane-polarized light beam from an inclined flat mirror in uniform rectilinear motion by applying the Huygens–Fresnel principle. We then use this formula and the postulates of special relativity to show that the moving mirror is contracted along the direction of its motion by the usual Lorentz factor. The result emphasizes the reality of Lorentz contraction by showing that the contraction is a direct consequence of the first and second postulates of special relativity, and is not a consequence of the relativistic measurement of the length. © 2004 American Association of Physics Teachers.
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01.50.-i Educational aids
42.25.Gy Edge and boundary effects; reflection and refraction
03.30.+p Special relativity

Einstein’s mirror and Fermat’s principle of least time

Aleksandar Gjurchinovski

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1325 | Cited 4 times

Online Publication Date: Sep 2004

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We derive a formula for the reflection of light from a uniformly moving mirror based on Fermat’s principle of least time. © 2004 American Association of Physics Teachers.
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01.50.-i Educational aids
42.25.Gy Edge and boundary effects; reflection and refraction

Photolithographic fabrication of diffraction and interference slit patterns for the undergraduate laboratory

Harriet Slogoff, Jeff Mackowiak, Milen Shishkov, and Alan T. Johnson

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1328 | Cited 1 time

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Measuring the interference and diffraction patterns produced by light passing through single and double narrow slits is a classic undergraduate physics experiment that dramatically confirms the wave nature of light. Despite this critical educational role, the experiment often is problematic because of the failings of commercially available slit patterns which produce faint unconvincing projected patterns. We discuss a low cost photolithographic process that produces diffraction slits in a chromium film deposited on a glass substrate. Slit dimensions as small as 4 μm are attainable with this method with complete control over the spacing of double slits. The method also can be used to expand the variety of slit patterns to two-dimensional arrays and circular patterns. © 2004 American Association of Physics Teachers.
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01.50.Pa Laboratory experiments and apparatus
42.25.Fx Diffraction and scattering
42.25.Hz Interference

Exploring factors affecting measurements of the static coefficient of friction: An application of fractional factorial experiment design

Timothy J. Folkerts

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1335

Online Publication Date: Sep 2004

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The common undergraduate experiment for determining the static coefficient of friction between a wooden block and a horizontal tabletop often produces inconsistent results. We show how several factors influence the measured magnitude and consistency of the coefficient of friction. We utilize a fractional factorial experimental design, which is a method that has been specifically developed to effectively and efficiently uncover key factors that most strongly influence experimental results. © 2004 American Association of Physics Teachers.
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01.50.Pa Laboratory experiments and apparatus
46.55.+d Tribology and mechanical contacts

Nibbling at A/D in introductory electronics

Denis Donnelly

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1342 | Cited 1 time

Online Publication Date: Sep 2004

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Circuits for four-bit digital/analog (D/A) and analog/digital (A/D) converters, suitable for the laboratory of an introductory electronics course, are provided. By placing variable resistors in the D/A circuit, students have some control over the step sizes in the conversion process. A component level A/D provides students an opportunity to directly observe the steps in the conversion process. © 2004 American Association of Physics Teachers.
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01.50.Pa Laboratory experiments and apparatus
01.50.Qb Laboratory course design, organization, and evaluation
84.32.Ff Conductors, resistors (including thermistors, varistors, and photoresistors)
84.30.-r Electronic circuits
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Chladni plates revisited

J. R. Comer, M. J. Shepard, P. N. Henriksen, and R. D. Ramsier

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1345 | Cited 1 time

Online Publication Date: Sep 2004

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© 2004 American Association of Physics Teachers.
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01.50.My Demonstration experiments and apparatus
01.30.Rr Surveys and tutorial papers; resource letters
01.50.Pa Laboratory experiments and apparatus
43.75.-z Music and musical instruments

All-reflective automated beam alignment device for ultrafast lasers

Matt Anderson and Catherine Ward

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1347

Online Publication Date: Sep 2004

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We present a novel device for automatically aligning laser beams using two quadrature photodiodes set immediately behind slightly transparent mirrors. The system utilizes feedback electronics and a LABVIEW program to automatically align an ultrafast laser quickly and accurately, even in the presence of gross thermal warping of the optical table. The design and implementation of this device is straightforward, making it an ideal research project for undergraduate students. © 2004 American Association of Physics Teachers.
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01.50.My Demonstration experiments and apparatus
42.60.By Design of specific laser systems
42.79.Bh Lenses, prisms and mirrors
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
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What do students really learn from interactive multimedia? A physics case study

Shelley Yeo, Robert Loss, Marjan Zadnik, Allan Harrison, and David Treagust

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1351 | Cited 7 times

Online Publication Date: Sep 2004

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Interactive multimedia is promoted as an effective and stimulating medium for learning science, but students do not always interact with multimedia as intended by the designers. We discuss students’ interactions with an interactive multimedia program segment about projectile motion in the context of long jumping. Qualitative data were collected using a video camera and split-screen recorder to record each student’s image, voice, and student–program interactions. Left to themselves, students’ interactions were superficial, but when asked to explain their observations of projectile motion illustrations, they were observed to retain common intuitive conceptions. Only following researcher intervention did students develop an awareness of abstract aspects of the program. These results suggest that, despite interactivity and animated graphics, interactive multimedia may not produce the desired outcome for students learning introductory physics concepts. © 2004 American Association of Physics Teachers.
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01.50.ht Instructional computer use
45.40.Gj Ballistics (projectiles; rockets)
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Post-Use Review. Biological Physics: Energy, Information, Life

Philip Nelson, Author and Stephen J. Hagen, Reviewer

American Journal of Physics -- October 2004 -- Volume 72, Issue 10, pp. 1359

Online Publication Date: Sep 2004

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Abstract Unavailable
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01.30.Vv Book reviews
01.40.-d Education
87.10.-e General theory and mathematical aspects
87.14.G- Nucleic acids
87.14.E- Proteins
01.50.-i Educational aids
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