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Oct 2008

Volume 76, Issue 10, pp. 893-983

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The American Association of Physics Teachers: Melba Newell Phillips Medal for 2008—Judy Franz

Ken Heller, AAPT Past PresidentAwards Committee Chair, 2007

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 893

Online Publication Date: Sep 2008

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Abstract Unavailable
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01.10.-m Announcements, news, and organizational activities

The American Association of Physics Teachers: Oersted Medal for 2008—Mildred Dresselhaus

Ken Heller, AAPT Past PresidentAwards Committee Chair, 2007

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 893

Online Publication Date: Sep 2008

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Abstract Unavailable
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01.10.-m Announcements, news, and organizational activities

The American Association of Physics Teachers: Richtmyer Memorial Award for 2008—Vera Rubin

Ken Heller, AAPT Past PresidentAwards Committee Chair, 2007

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 894

Online Publication Date: Sep 2008

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01.10.-m Announcements, news, and organizational activities
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Robots in the introductory physics laboratory

Mary Lowe, Howard Moore, Edwin Langrall, and Chuck Gehrman

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 895

Online Publication Date: Sep 2008

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We have incorporated robots into the introductory undergraduate physics laboratory to introduce a creative component and to show some applications of physics principles in a way that strongly engages student interest. We have developed low-cost mobile platforms and navigation methods suitable for large numbers of introductory level students. Radio remote control was the dominant navigation method implemented in the classroom, but autonomous navigation was also achieved. We discuss robot construction, the organization of the lab, student reactions, and our experiences in sustaining the program.
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07.00.00 Instruments, apparatus, and components common to several branches of physics and astronomy

Analysis of a model race car

Vincent P. Coletta and Jonathan Evans

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 903

Online Publication Date: Sep 2008

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We analyze the motion of a gravity powered model race car on a downhill track of variable slope. Using a simple algebraic function to approximate the height of the track as a function of the distance along the track, and taking account of the rotational energy of the wheels, rolling friction, and air resistance, we obtain analytic expressions for the velocity and time of the car as functions of the distance traveled along the track. Photogates are used to measure the time at selected points along the track, and the measured values are in excellent agreement with the values predicted from theory. The design and analysis of model race cars provides a good application of principles of mechanics and suggests interesting projects for classes in introductory and intermediate mechanics.
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47.00.00 Fluid dynamics

Differences between bouncing balls, springs, and rods

Rod Cross

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 908 | Cited 2 times

Online Publication Date: Sep 2008

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When one hard steel ball collides with another, kinetic energy is conserved, even if the balls have different diameters. Why is kinetic energy conserved in such a collision, given that kinetic energy is not conserved when two unequal length steel springs or rods collide? Experimental results with bouncing balls, springs, and rods are presented, which reveal the answer. For colliding springs and rods a significant fraction of the initial kinetic energy is retained after the collision as vibrational energy in the longer spring and rod. When two hard balls collide, a negligible fraction of the initial energy is converted to vibrational energy because the collision time is much longer than the transit time of an acoustic wave across each ball due to the fact that the contact region of a hard spherical ball is much softer than the rest of the ball.
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47.00.00 Fluid dynamics

Making optical vortices with computer-generated holograms

Alicia V. Carpentier, Humberto Michinel, José R. Salgueiro, and David Olivieri

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 916 | Cited 2 times

Online Publication Date: Sep 2008

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An optical vortex is a screw dislocation in a light field that carries quantized orbital angular momentum and, due to cancellations of the twisting along the propagation axis, experiences zero intensity at its center. When viewed in a perpendicular plane along the propagation axis, the vortex appears as a dark region in the center surrounded by a bright concentric ring of light. We give detailed instructions for generating optical vortices and optical vortex structures by computer-generated holograms and describe various methods for manipulating the resulting structures.
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42.00.00 Optics

The conversion of phase to amplitude fluctuations of a light beam by an optical cavity

Alessandro S. Villar

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 922 | Cited 3 times

Online Publication Date: Sep 2008

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Very low intensity and phase fluctuations are present in a bright light field such as a laser beam. These subtle quantum fluctuations may be used to encode quantum information. Although intensity is easily measured with common photodetectors, accessing the phase information requires interference experiments. We introduce one such technique, the rotation of the noise ellipse of light, which employs an optical cavity to achieve the conversion of phase to intensity fluctuations. We describe the quantum noise of light and how it can be manipulated by employing an optical resonance technique and compare it to similar techniques, such as Pound–Drever–Hall laser stabilization and homodyne detection.
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42.00.00 Optics

The transverse traceless gauge and quadrupole sources

Richard H. Price and Yan Wang

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 930

Online Publication Date: Sep 2008

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A gravitational wave propagating in the +z direction is usually described by the transverse spatial components hxx, hyy, hxy in the transverse traceless gauge in which hxx+hyy=0. The transverse components are not changed by a gauge transformation, which implies that the traceless condition must be automatically met. We analyze how this comes about in the calculation of the waves in an explicit example, a circular binary orbit of equal mass points. This calculation is a useful exercise in linearized general relativity and gravitational waves, and a model of an important astrophysical source of detectable waves. We also demonstrate automatic transverse tracelessness for the general case of a slow motion source.
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04.00.00 General relativity and gravitation

The trajectory of an electron in a plasma

Arnaud Beck and Nicole Meyer-Vernet

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 934 | Cited 1 time

Online Publication Date: Sep 2008

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Collisions in gaseous plasmas are fundamentally different from collisions in neutral gases because a charged particle interacts simultaneously with many others via the Coulomb potential. This difference is not as intuitive for students as the billiard ball like collisions of neutral particles. We present trajectories of electrons in a weakly coupled plasma obtained from particle simulations. The trajectories illustrate the concept of distant collisions and the variation of the free path with energy, which has major consequences in plasma physics.
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52.00.00 Physics of plasmas and electric discharges

Wigner functions and Weyl transforms for pedestrians

William B. Case

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 937 | Cited 3 times

Online Publication Date: Sep 2008

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Wigner functions and Weyl transforms of operators offer a formulation of quantum mechanics that is equivalent to the standard approach given by the Schrödinger equation. We give a short introduction and emphasize features that give insight into the nature of quantum mechanics and its relation to classical physics. A careful discussion of the classical limit and its difficulties is also given. The discussion is self-contained and includes complete derivations of the results presented.
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03.65.-w Quantum mechanics

Statistical consequences of the zero-point energy of the harmonic oscillator

Luis de la Peña, Andrea Valdés-Hernández, and Ana María Cetto

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 947 | Cited 5 times

Online Publication Date: Sep 2008

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In a recent thermodynamic analysis of the harmonic oscillator Boyer has shown, using an interpolation procedure, that the existence of a zero-point energy leads to Planck’s law. We avoid the interpolation procedure by adding a statistical argument to arrive at Planck’s law as a consequence of the existence of the zero-point energy. As in Boyer’s argument, no explicit assumption of quantum mechanics is introduced. We discuss the relation of our results to the analysis of Planck and Einstein which led to the notion of the quantized radiation field. We then inquire into the discrete or continuous behavior of the energy and pinpoint the origin and meaning of the discontinuities. To include zero-point fluctuations (which are neglected in the thermodynamic analysis), we discuss the statistical (in contrast to the purely thermodynamic) description of the oscillator, which accounts for both the thermal and temperature-independent contributions to the dispersion of the energy.
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05.00.00 Statistical physics, thermodynamics, and nonlinear dynamical systems

Period-speed analysis of a pendulum

Yavor Kostov, Ragib Morshed, Barbara Höling, Ray Chen, and P. B. Siegel

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 956

Online Publication Date: Sep 2008

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We analyze a simple pendulum by measuring the period and the maximum speed of the bob. Both quantities are measured to high precision using a laser diode and an infrared photodetector located at the bottom of the pendulum. Expressing the period in terms of the maximum speed enables students to examine the large angle dependence of the period, and provides a method to calibrate the speed and do a detailed analysis of the effect of air friction on a sphere. We find that the force due to air friction is well described by a linear and quadratic term in the speed. We investigate the dependence of each term on the sphere’s diameter for Reynolds numbers from 250 to 104.
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01.50.Pa Laboratory experiments and apparatus
47.00.00 Fluid dynamics

A laboratory experiment to demonstrate Gauss’s law for electric fields

J. Patrick Donohoe

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 963

Online Publication Date: Sep 2008

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This paper describes an experiment that demonstrates Gauss’s law for electric fields. The experiment utilizes easily obtained equipment and requires simple electrostatic field measurements. The field measurements are obtained for azimuthally symmetric electrodes located over a conducting ground plane. The measurements are shown to be consistent with the image theory solution for a spherical electrode over a ground plane. The relation between Gauss’s law and the concept of capacitance is also demonstrated.
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01.50.Pa Laboratory experiments and apparatus
41.00.00 Electromagnetism; electron and ion optics
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Analysis of non-Newtonian power-law liquids with a weight-controlled capillary viscometer

Rafael M. Digilov, B. Gurfinkel, and M. Reiner

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 968

Online Publication Date: Sep 2008

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We analyze the non-Newtonian behavior of a liquid draining under gravity from a tank through a capillary discharge tube and obtain an expression for the time variation of the weight of the liquid in the tank. As a result, a previously proposed weight-controlled capillary viscometer can be applied to both Newtonian and non-Newtonian liquids.
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01.50.Pa Laboratory experiments and apparatus
47.00.00 Fluid dynamics

Realistic problems involving thermal conductivity

Gregory Bee, Katherine Ballentine, and Marshall Thomsen

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 970 | Cited 1 time

Online Publication Date: Sep 2008

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Abstract Unavailable
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44.00.00 Heat transfer
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An audible demonstration of the speed of sound in bubbly liquids

Preston S. Wilson and Ronald A. Roy

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 975 | Cited 3 times

Online Publication Date: Sep 2008

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The speed of sound in a bubbly liquid is strongly dependent upon the volume fraction of the gas phase, the bubble size distribution, and the frequency of the acoustic excitation. At sufficiently low frequencies, the speed of sound depends primarily on the gas volume fraction. This effect can be audibly demonstrated using a one-dimensional acoustic waveguide, in which the flow rate of air bubbles injected into a water-filled tube is varied by the user. The normal modes of the waveguide are excited by the sound of the bubbles being injected into the tube. As the flow rate is varied, the speed of sound varies as well, and hence, the resonance frequencies shift. This can be clearly heard through the use of an amplified hydrophone and the user can create aesthetically pleasing and even musical sounds. In addition, the apparatus can be used to verify a simple mathematical model known as Wood’s equation that relates the speed of sound of a bubbly liquid to its void fraction.
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01.50.Pa Laboratory experiments and apparatus
43.00.00 Acoustics
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A Passion for Discovery

Peter Freund and Helge Kragh, Reviewer, Reviewer

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 982

Online Publication Date: Sep 2008

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Abstract Unavailable
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01.30.Vv Book reviews
01.60.+q Biographies, tributes, personal notes, and obituaries

Four Laws that Drive the Universe

Peter Atkins and Harvey S. Leff, Reviewer

American Journal of Physics -- October 2008 -- Volume 76, Issue 10, pp. 982

Online Publication Date: Sep 2008

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Abstract Unavailable
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01.30.Vv Book reviews
05.70.-a Thermodynamics
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