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Nov 2002

Volume 70, Issue 11, pp. 1081-1167

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The training, careers, and work of Ph.D. physical scientists: Not simply academic

Steven J. Smith, Liane Pedersen-Gallegos, and Catherine Riegle-Crumb

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1081 | Cited 2 times

Online Publication Date: Oct 2002

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We present an in-depth portrait of the training, careers, and work of recent Ph.D. physical scientists. Use of specialized training varies widely, with about half often using knowledge of their Ph.D. specialty area in their jobs. The use of specialized training does not, however, correlate with job satisfaction. In this and other important measures, there are relatively few differences between “academics” and “nonacademics.” Important job skills for all employment sectors include writing, oral presentation, management, data analysis, designing projects, critical thinking, and working in an interdisciplinary context. Rankings given by respondents of graduate training in some of these skill areas were significantly lower than the importance of these skills in the workplace. We also found that the rated quality of graduate training varies relatively little by department or advisor. Finally, although nonacademic aspirations among graduate students are fairly common, these do not appear to be well supported while in graduate school. © 2002 American Association of Physics Teachers.
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01.40.-d Education

Grip-slip behavior of a bouncing ball

Rod Cross

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1093 | Cited 29 times

Online Publication Date: Oct 2002

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Measurements of the normal reaction force and the friction force acting on an obliquely bouncing ball were made to determine whether the friction force acting on the ball is due to sliding, rolling, or static friction. At low angles of incidence to the horizontal, a ball incident without spin will slide throughout the bounce. At higher angles of incidence, elementary bounce models predict that the ball will start to slide, but will then commence to roll if the point of contact on the circumference of the ball momentarily comes to rest on the surface. Measurements of the friction force and ball spin show that real balls do not roll when they bounce. Instead, the deformation of the contact region allows a ball to grip the surface when the bottom of the ball comes to rest on the surface. As a result the ball vibrates in the horizontal direction causing the friction force to reverse direction during the bounce. The spin of the ball was found to be larger than that due to the friction force alone, a result that can be explained if the normal reaction force acts vertically through a point behind the center of the ball. © 2002 American Association of Physics Teachers.
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01.55.+b General physics
46.55.+d Tribology and mechanical contacts

When physical intuition fails

Chandralekha Singh

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1103 | Cited 13 times

Online Publication Date: Oct 2002

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We analyze the problem-solving strategies of physics professors in a case where their physical intuition fails. A nonintuitive introductory-level problem was identified and posed to twenty physics professors. The problem placed the professors in a situation often encountered by students, and their response highlights the importance of intuition and experience in problem solving. Although professors had difficulty in solving the problem under the time constraint, they initially employed a systematic approach, for example, visualizing the problem, considering various conservation laws, and examining limiting cases. After finding that familiar techniques were not fruitful, they made incorrect predictions based on one of two equally important factors. In contrast, other more familiar problems that require the consideration of two important principles (for example, conservation of both energy and momentum for a ballistic pendulum) were quickly solved by the same professors. The responses of students who were given the same problem reflected no overarching strategies or systematic approaches, and a much wider variety of incorrect responses were given. This investigation highlights the importance of teaching effective problem-solving heuristics, and suggests that instructors assess the difficulty of a problem from the perspective of beginning students. © 2002 American Association of Physics Teachers.
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01.40.G- Curricula and evaluation
01.40.Fk Research in physics education

Tunneling through arbitrary potential barriers and the apparent barrier height

Oliver M. Probst

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1110 | Cited 4 times

Online Publication Date: Oct 2002

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A simple matrix formalism is presented that allows the tunneling current between two planar electrodes with an arbitrary barrier potential to be calculated conveniently. This formalism is a straightforward extension of the usual textbook example for tunneling through a rectangular barrier and can be easily implemented on a personal computer. A first principles derivation of the tunneling current based on free-electron electrodes is given that is useful for the teaching of solid-state physics. Examples showing the utility of the method are the transmission through a tunnel barrier modified by the image potential and tunneling via intermediate states (resonances). These issues are interesting topics in scanning tunneling microscopy where both phenomena are believed to contribute to the image formation. © 2002 American Association of Physics Teachers.
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01.50.-i Educational aids
73.40.Gk Tunneling

Demonstration of negative group delays in a simple electronic circuit

T. Nakanishi, K. Sugiyama, and M. Kitano

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1117 | Cited 12 times

Online Publication Date: Oct 2002

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We present a simple electronic circuit that produces negative delays. When a pulse is sent to the circuit as input, the output is a pulse with a similar wave form that is shifted forward in time. The advance time or negative delay can be increased to the order of seconds so that we can observe the advance with the naked eye by observing two light emitting diodes that are connected to the input and the output. The negative group delay in the electronic circuit shares the same mechanism with superluminal light propagation, where the group velocity exceeds the speed of light or even becomes negative. © 2002 American Association of Physics Teachers.
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01.50.My Demonstration experiments and apparatus
84.30.Sk Pulse and digital circuits

Series capacitors and the inverse sum rule

G. T. Carlson and B. L. Illman

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1122 | Cited 2 times

Online Publication Date: Oct 2002

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An exact solution for the capacitance of a three plate system is obtained and is used to quantify the error made by the inverse sum rule. Three finite, parallel disks are arranged as two capacitors in series. By treating the system as an electrostatic boundary value problem, exact solutions for the potential and capacitance are derived for the total system and for each pair of plates. The inverse sum rule is then applied to the two constituent capacitors and compared to the exact solution for the total system. Also provided are analyses of the equal plate charge hypothesis typically assumed for series capacitors, relations for the coefficients of capacitance for the system, a capacitive maximum, repulsive plate forces, charge migration on the inner plate, and plots of the field lines for the three plate system. © 2002 American Association of Physics Teachers.
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84.32.Tt Capacitors
02.30.Zz Inverse problems
02.60.Lj Ordinary and partial differential equations; boundary value problems

Electromagnetic conic sections

Tevian Dray and Corinne A. Manogue

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1129 | Cited 1 time

Online Publication Date: Oct 2002

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Certain orthogonal coordinate systems naturally correspond to basis vectors which are both curl-free and divergence-free, and hence solve Maxwell’s equations. After first comparing several different traditional approaches to computing div, grad, and curl in curvilinear coordinates, we present a new approach, based on these “electromagnetic” basis vectors, which combines geometry and physics. Not only is our approach tied to a physical interpretation in terms of the electromagnetic field, it is also a useful way to remember the formulas themselves. We give several important examples of coordinate systems in which this approach is valid, in each case discussing the electromagnetic interpretation of the basis. We also give a general condition for when an electromagnetic interpretation is possible. © 2002 American Association of Physics Teachers.
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01.50.-i Educational aids
03.50.De Classical electromagnetism, Maxwell equations
41.20.-q Applied classical electromagnetism

The exponential of the curl: Application to Maxwell fields

E. A. Power and T. Thirunamachandran

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1136 | Cited 2 times

Online Publication Date: Oct 2002

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The operation of the exponential of the curl on a general vector field is studied. It is nonlocal, and it is shown that the resultant field may be written as the sum of integrals over all space. The formal solutions of the time-dependent Maxwell’s equations for an arbitrary current density are first written in terms of the curl, and explicit expressions for the electric and magnetic fields are given in terms of the source current densities loaded with these kernels. This method of deriving the fields obviates the introduction of electromagnetic potentials. The well-known expressions for the fields are derived for a fixed oscillating dipole and a charge in arbitrary motion. For a moving dipole the source current includes, in addition to the polarization current, a coupling of the polarization to the velocity of the dipole known as the Röntgen current. The magnetic field due to this current is given. © 2002 American Association of Physics Teachers.
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01.50.-i Educational aids
03.50.De Classical electromagnetism, Maxwell equations
41.20.-q Applied classical electromagnetism

Efficiency of nonideal Carnot engines with friction and heat losses

E. Rebhan

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1143 | Cited 1 time

Online Publication Date: Oct 2002

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In nonideal thermodynamic engines the efficiency is well below the Carnot efficiency η=1−T1/T2. In 1975 an expression for the efficiency of a nonideal Carnot engine with heat losses was derived, yielding η=1−math at maximum power output. In this paper, a corresponding relation is obtained for more general nonideal Carnot engines. If there are friction losses only, the result is η=(1−T1/T2)/2. If friction and heat losses are both included, the efficiency at maximum power depends on a dimensionless parameter λ∗ that takes into account the effects of friction and heat conduction, and can vary between the values obtained for friction and heat losses separately, (1−T1/T2)/2<ηpmax<1−math. A general relation between efficiency and power output is established, and it is shown that an appreciable gain in efficiency can be obtained at a power output only slightly below its maximum. © 2002 American Association of Physics Teachers.
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01.50.-i Educational aids
07.20.Pe Heat engines; heat pumps; heat pipes
05.70.-a Thermodynamics

Experimental estimation of the band gap in silicon and germanium from the temperature–voltage curve of diode thermometers

Jürgen W. Precker and Marcílio A. da Silva

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1150 | Cited 10 times

Online Publication Date: Oct 2002

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Semiconductor diodes, in conjunction with a constant current source, are sometimes used as thermometers. It has been observed experimentally that, within a certain temperature range, the relation between temperature and voltage is almost linear. We show that this linearity is a direct consequence of the constancy of the current flowing through the diode, and that the parameters resulting from a least-squares fit to the experimental data can be used to determine the band gap energy of the semiconductor. We test the validity of our model by comparing our results to measurements on diodes made of germanium and silicon. If we take into account the simplifications used in our model, the results agree well with known values of the energy gaps. © 2002 American Association of Physics Teachers.
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71.20.Mq Elemental semiconductors
07.20.Dt Thermometers
85.30.Kk Junction diodes

Sonic band structure and localized modes in a density-modulated system: Experiment and theory

Christopher Carr and Roger Yu

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1154 | Cited 3 times

Online Publication Date: Oct 2002

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The sonic passing bands and stopping gaps of a quasi-one-dimensional air tube with modulated mass density were studied experimentally and theoretically. Some gap modes whose wave functions are strongly localized near the ends of the air tube were also found. The simple experiment can be used as a demonstration of band structure in an upper-division physics course. © 2002 American Association of Physics Teachers.
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01.50.My Demonstration experiments and apparatus
71.20.-b Electron density of states and band structure of crystalline solids

Measuring the angular solar diameter using two pinholes

Costantino Sigismondi

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1157 | Cited 1 time

Online Publication Date: Oct 2002

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At the turn of the sixteenth century Tycho Brahe and Johannes Kepler made single pinhole measurements of the solar diameter. Their accuracy was limited by diffraction (unknown to them) and the motion of the image on the screen. We discuss how two pinholes built on the same mask can be used to bypass all the problems inherent in the single pinhole approach. The distance at which the two images of the Sun are in contact is the only measurement needed, and the experimental accuracy is much better than measuring the diameter of a single moving image. We obtained 0.5% accuracy, sufficient to follow the angular variations of the solar diameter due to the motion of the Earth in its orbit. © 2002 American Association of Physics Teachers.
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96.60.Bn Diameter, rotation, and mass
01.50.My Demonstration experiments and apparatus
01.50.Pa Laboratory experiments and apparatus
01.65.+g History of science
95.90.+v Historical astronomy and archaeoastronomy; and other topics in fundamental astronomy and astrophysics; instrumentation, techniques, and astronomical observations
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Comment on “Galileo’s discovery of scaling laws,” by Mark A. Peterson [Am. J. Phys. 70 (6), 575–580 (2002)]–Galileo and the existence of hell

Peter Pesic

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1160

Online Publication Date: Oct 2002

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© 2002 American Association of Physics Teachers.
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01.50.-i Educational aids
01.78.+p Science and government (funding, politics, etc.)
98.80.-k Cosmology
01.70.+w Philosophy of science

A remarkable mathematical property of the Landé factor in quantum mechanics

Albrecht Pfister and Herbert Pfister

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1162 | Cited 1 time

Online Publication Date: Oct 2002

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We show that the Landé factor can, in principle, attain any positive or negative rational number. © 2002 American Association of Physics Teachers.
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01.50.-i Educational aids
32.30.Rj X-ray spectra
32.60.+i Zeeman and Stark effects

Comment on “The Thomas rotation,” by John P. Costella et al. [Am. J. Phys. 69 (8), 837–847 (2001)]

Krzysztof Rębilas

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1163 | Cited 2 times

Online Publication Date: Oct 2002

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© 2002 American Association of Physics Teachers.
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01.50.-i Educational aids
03.30.+p Special relativity
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A Guide to First-Passage Processes

Sidney Redner, Author and J. R. Dorfman, Reviewer

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1166

Online Publication Date: Oct 2002

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Abstract Unavailable
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01.30.Vv Book reviews
05.20.Dd Kinetic theory

The Atmospheric Environment: Effects of Human Activity

Michael B. McElroy, Author and Richard C. J. Somerville, Reviewer

American Journal of Physics -- November 2002 -- Volume 70, Issue 11, pp. 1166

Online Publication Date: Oct 2002

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Abstract Unavailable
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01.30.Vv Book reviews
82.33.Tb Atmospheric chemistry
92.60.Sz Air quality and air pollution
92.60.-e Properties and dynamics of the atmosphere; meteorology
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