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Apr 2006

Volume 74, Issue 4, pp. 253-367

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Theme Issue Editorial 2006

Kerry Browne, Stamatis Vokos, Brad Ambrose, and Wolfgang Christian

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 253

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Abstract Unavailable
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01.10.-m Announcements, news, and organizational activities
41.00.00 Electromagnetism; electron and ion optics
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Quantitative electric field measurements in an intermediate laboratory

Bruce R. Thomas

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 255 | Cited 2 times

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We describe an apparatus that enables students to study the electric fields and the associated surface charges for several different electrode structures including parallel plates, cylindrical rods over plates, and nested cylinders. The study involves the relations among potential difference, electric field, surface charge density, and total charge. These relations are confirmed to within a few percent.
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01.50.Pa Laboratory experiments and apparatus
41.00.00 Electromagnetism; electron and ion optics

Effect of electrically insulating materials on magnetically induced electrical currents in a tissue-like medium

Brent K. Hoffmeister, Andrew R. Shores, Shubho Banerjee, and Robert A. Malkin

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 260

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Time varying magnetic fields can induce eddy currents in a conductor. Electrical currents also can be induced magnetically in the human body, and there has been considerable interest in the bioeffects of this phenomenon. We present an analytical model derived from Faraday’s law and Coulomb’s law that provides physical insight into how electrically insulating materials in a tissue-like medium redirect magnetically induced currents. The model shows that charge accumulates at the tissue-insulator interface to produce a secondary electric field. This field combines with the magnetically induced electric field to alter the net electric field in the vicinity of the insulator, causing the electric current to flow around the insulator. The model is supplemented by measurements of magnetically induced electric fields in a volume of physiologic saline solution. Good agreement is found among the model, the measurements, and a finite element analysis model of the experiment.
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41.00.00 Electromagnetism; electron and ion optics
87.00.00 Biological and medical physics

Force characterization of eddy currents

J. Pellicer-Porres, R. Lacomba-Perales, J. Ruiz-Fuertes, D. Martínez-García, and M. V. Andrés

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 267 | Cited 1 time

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We discuss an experiment for an undergraduate laboratory designed to characterize eddy currents from the forces associated with them. Eddy currents also can be associated with the magnetization and described by a complex magnetic susceptibility. We establish an iterative method for calculating the magnetic susceptibility that is applicable to different geometries without using advanced mathematics. We apply our results to experiments with massive and hollow cylinders for which the susceptibility depends on the geometry. By measuring the force we obtain the cylinders’ conductivity with a contactless technique.
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41.00.00 Electromagnetism; electron and ion optics

Dancing paperclips and the geometric influence on magnetization: A surprising result

David P. Jackson

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 272

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An impressive demonstration of magnetism can be accomplished by placing some metal paperclips on a horizontal surface and then applying a vertical magnetic field. As the field is increased, the paperclips suddenly jump up, do a little dance, and then stand at attention. This behavior is related to the more common demonstration of paramagnetic and diamagnetic materials, which consists of hanging a small aluminum (paramagnetic) or glass (diamagnetic) cylinder horizontally in a strong horizontal magnetic field. A paramagnetic cylinder aligns its axis parallel to the applied field while a diamagnetic cylinder aligns its axis perpendicular to the field. This paper investigates these demonstrations by analyzing a magnetic spheroid in a uniform external field. Although this analysis explains the behavior of the paperclips, it predicts that both paramagnetic and diamagnetic cylinders will align themselves parallel to a uniform external field, in contrast to the common demonstration experiment.
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41.00.00 Electromagnetism; electron and ion optics
01.50.My Demonstration experiments and apparatus
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How an antenna launches its input power into radiation: The pattern of the Poynting vector at and near an antenna

J. D. Jackson

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 280 | Cited 3 times

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I address the question of whether the seat of the power radiated by an antenna made of conducting members is distributed over the arms of the antenna according to −J⋅E, where J is the current density and E is the electric field produced by the source. Poynting’s theorem permits only a global identification of the total input power, usually from a localized generator, with the total power radiated to infinity, not a local correspondence of −J⋅Ed3x with a specific radiated power, r2S⋅mathdΩ. I describe a model antenna consisting of two perfectly conducting hemispheres of radius a separated by a small equatorial gap across which occurs the driving oscillatory electric field. The fields and surface current are determined by solution of the boundary value problem. In contrast to the first approach (not a boundary value problem), the tangential electric field vanishes on the metallic surface. There is no radial Poynting vector at the surface. Numerical examples are shown to illustrate how the energy flows from the input region at the gap and is guided near the antenna by its arms until it is launched at larger r∕a into the radiation pattern determined by the value of ka.
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41.00.00 Electromagnetism; electron and ion optics

Enhanced electromagnetic wave transmission through narrow conducting channels

G. F. Brand

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 289

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The transmission of an electromagnetic wave through a thick conducting grating, where the conductors are separated by a small fraction of a wavelength, is examined. The conditions for which almost all of the incident radiation can pass through are found. This theoretical result is confirmed by a microwave experiment at 35 GHz.
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01.50.Pa Laboratory experiments and apparatus
41.00.00 Electromagnetism; electron and ion optics
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Off-axis electric field of a ring of charge

Fredy R. Zypman

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 295 | Cited 4 times

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We consider the electric field produced by a charged ring and develop analytical expressions for the electric field based on intuition developed from numerical experiments. Our solution involves the approximation of elliptic integrals. Problems are suggested for an arbitrarily charged ring.
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41.00.00 Electromagnetism; electron and ion optics

Surprises in the multipole description of macroscopic electrodynamics

O. L. de Lange and R. E. Raab

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 301 | Cited 2 times

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The theory of macroscopic electrodynamics (in its multipole form) for harmonic plane-wave electric and magnetic fields in linear anisotropic media yields constitutive relations and expressions for the material constants and other macroscopic observables. It is shown that the calculated dynamic material constants are unphysical: of the 36 elements of the constitutive tensor, 27 (comprising the inverse permeability and two magnetoelectric tensors) depend on the choice of coordinate origin, while the remaining 9 (the permittivity tensor) are physical at electric dipole order, but not beyond. Thus quantities such as the time average of the instantaneous Poynting vector have origin-dependent values. We use properties of the macroscopic Maxwell equations and the equation of wave propagation to show how physically acceptable results can be obtained in a relatively simple manner for both nonmagnetic and magnetic dissipative media. In doing so, it is essential to adhere to a certain hierarchy for the pairing of electric and magnetic multipoles∕polarizabilities. We also comment on the Post constraint in relation to this theory, and use the Buckingham effect to illustrate origin-dependent and origin-independent properties.
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41.00.00 Electromagnetism; electron and ion optics

Time in classical electrodynamics

Fritz Rohrlich

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 313 | Cited 3 times

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There is considerable misunderstanding in the literature concerning time in classical electrodynamics. The following conceptual exposition focuses on three points: advanced radiation exists mathematically but not physically; there exists an asymmetry of time due to radiation; and the equations of motion of charged particles are invariant under time reversal. These three points are mutually consistent.
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41.00.00 Electromagnetism; electron and ion optics
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Using visualizations to teach electrostatics

Janet M. Casperson and Marcia C. Linn

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 316 | Cited 8 times

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We investigate the effect of visualizations of electrostatics phenomena on students’ ability to integrate microscopic and macroscopic views of electrostatics in the context of a technology-enhanced inquiry project that features particle interactions. We used knowledge integration instructional design patterns to determine activity sequences. A pretest/post-test design was used to assess the students’ overall gains. The results from the implementation of the project in two classes demonstrate that it helped students form a more integrated understanding of electrostatics. An analysis of student responses to prompts embedded with the visualizations reveals that interactions with the visualizations played a significant role in increasing understanding.
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41.00.00 Electromagnetism; electron and ion optics
01.40.G- Curricula and evaluation

Teaching about circuits at the introductory level: An emphasis on potential difference

A. S. Rosenthal and C. Henderson

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 324 | Cited 2 times

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Introductory physics students often fail to develop a coherent conceptual model of electric circuits. In part, this failure occurs because the students did not develop a good understanding of the concept of electric potential. We describe an instructional approach that emphasizes the electric potential and the electric potential difference. Examples are given to illustrate this approach and how it differs from traditional treatments of these concepts. Assessment data is presented to suggest that this approach is successful in improving student understanding of electric potential and electric circuits.
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84.30.-r Electronic circuits

Restructuring the introductory electricity and magnetism course

Ruth Chabay and Bruce Sherwood

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 329 | Cited 14 times

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In the electricity and magnetism (E&M) segment of the traditional introductory calculus-based physics course, many new and increasingly abstract concepts, embodied in complex formal relations, are introduced at a rapid pace. As a result, many students find E&M significantly more difficult than classical mechanics. We describe a different intellectual structure for the E&M course that stresses conceptual coherence, connects the abstract field concept to concrete microscopic models of matter, and follows a clear story line, culminating in the classical model of the interaction of electromagnetic radiation and matter. This sequence has proven to be effective in teaching the basic concepts of E&M.
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01.40.Di Course design and evaluation
41.00.00 Electromagnetism; electron and ion optics

Teaching Faraday’s law of electromagnetic induction in an introductory physics course

Igal Galili, Dov Kaplan, and Yaron Lehavi

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 337 | Cited 5 times

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Teaching Faraday’s law of electromagnetic induction in introductory physics courses is challenging. We discuss some inaccuracies in describing a moving conductor in the context of electromagnetic induction. Among them is the use of the ambiguous term “area change” and the unclear relation between Faraday’s law and Maxwell’s equation for the electric field circulation. We advocate the use of an expression for Faraday’s law that shows explicitly the contribution of the time variation of the magnetic field and the action of the Lorentz force, which are usually taught separately. This expression may help students’ understanding of Faraday’s law and lead to improved problem solving skills.
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41.00.00 Electromagnetism; electron and ion optics

Why is Ampère’s law so hard? A look at middle-division physics

Corinne A. Manogue, Kerry Browne, Tevian Dray, and Barbara Edwards

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 344 | Cited 4 times

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Because mathematicians and physicists think differently about mathematics, they have different goals for their courses and teach different ways of thinking about the material. As a consequence, there are a number of capabilities that physics majors need in order to be successful that might not be addressed by any traditional course. The result is that the total cognitive load is too high for many students at the transition from the calculus and introductory physics sequences to upper-division courses for physics majors. We illustrate typical student difficulties in the context of an Ampère’s law problem.
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01.40.G- Curricula and evaluation
41.00.00 Electromagnetism; electron and ion optics
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Research-inspired problems for electricity and magnetism

Beth Parks

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 351 | Cited 1 time

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See Also: Erratum

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I present several problems related to current research problems and techniques that are suitable for introductory and intermediate electricity and magnetism.
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41.00.00 Electromagnetism; electron and ion optics
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Gaussian beams in the optics course

Enrique J. Galvez

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 355 | Cited 2 times

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We introduce the physics of high-order Gaussian beams to the treatment of Gaussian beams in the undergraduate optics course. Of particular interest are Laguerre-Gauss beams, which provide the basis for discussing the new and increasingly important concept of the orbital angular momentum of light. We also describe laboratory exercises that complement the class material.
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01.50.Pa Laboratory experiments and apparatus
42.00.00 Optics
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Post-Use Review. Matter and Interactions (two volumes)

Ruth Chabay, Bruce Sherwood, and Aaron Titus, Reviewer

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 362

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Abstract Unavailable
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01.30.Vv Book reviews
01.55.+b General physics

The Man Who Changed Everything: The Life of James Clerk Maxwell

Basil Mahon and Jay M. Pasachoff, Reviewer

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 364

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Abstract Unavailable
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01.60.+q Biographies, tributes, personal notes, and obituaries
41.00.00 Electromagnetism; electron and ion optics
01.30.Vv Book reviews

Electricity, Magnetism and Light

Wayne Saslow and Lane H. Seeley, Reviewer

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 365

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Abstract Unavailable
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41.00.00 Electromagnetism; electron and ion optics
01.30.Vv Book reviews

Post-Use Review. Modern Problems in Classical Electrodynamics

Charles A. Brau and Joel D. Rauber, Reviewer

American Journal of Physics -- April 2006 -- Volume 74, Issue 4, pp. 366

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Abstract Unavailable
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01.30.Vv Book reviews
41.00.00 Electromagnetism; electron and ion optics
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