摘要:

The survival probabilities of a particle diffusing within an expanding ‘‘cage’’ and near the edge of a receding ‘‘cliff,’’ with death occurring when the diffuser reaches a boundary of the system, are investigated. Especially interesting behavior arises when the position of the boundary recedes from the diffuser as √At. In this case, the recession matches the rms displacement √Dtwith which diffusion tends to bring the diffuser to its demise. For both the cage and cliff problems, the survival probabilityS(t) exhibits a nonuniversal power‐law decay in time,S(t)∼t−β, in which the value of β is dependent on the detailed properties of the boundary motion. Heuristic approaches are applied for the cases of ‘‘slow’’ (A/D≪1) and ‘‘fast’’ (A/D≫1) boundary motion which yield approximate expressions for β. An asymptotic analysis of the survival probability for the cage and cliff problems is also performed. The approximate expressions for β are in good agreement with the exact results for nearly the entire range of possible boundary motions.

ISSN:0002-9505    

DOI:10.1119/1.18152

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

The role of the Thomas precession in the dynamic formation of the electric field lines of a moving charged particle is demonstrated. A simple derivation of the Thomas precession formula is given, based only on the Lorentz contraction of moving bodies. A simple and physically appealing construction is developed for determining points on a field line. Field line diagrams are generated and discussed. These diagrams vividly reveal the existence and magnitude of the Thomas precession.

ISSN:0002-9505    

DOI:10.1119/1.18153

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

This paper describes the use of the computer‐based interview as a research technique for investigating how students think about physics. Two computer programs provide the context: one intended for instruction, the other for research. The one designed for use as an instructional aid displays the motion of a ball rolling along a track that has level and inclined segments. The associated motion graphs are also shown. The other program, which was expressly designed for use in research, is based on the simulated motion of a modified Atwood’s machine. The programs require students to predict the effect of the initial conditions and system parameters on the motion or on a graph of the motion. The motion that would actually occur is then displayed. The investigation focuses on the reasoning used by the students as they try to resolve discrepancies between their predictions and observations.

ISSN:0002-9505    

DOI:10.1119/1.18154

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

This paper describes a simple direct technique for obtaining experimentally the shape of the polarization ellipse of an elliptically polarized wave. A simple construction which recognizes and corrects for the distortion introduced by the analyzer is the key to the extraction of the ellipse from the experimentally obtained polar diagrams.

ISSN:0002-9505    

DOI:10.1119/1.18155

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

On the basis of the solution of the Fourier transformed inhomogeneous wave equation expressed in terms of both the charge and current density source, two basic field equations are derived, from which the time‐dependent generalized Coulomb and Biot–Savart laws are readily obtained. For the specific case of an arbitrarily moving point charge, the use of the Fourier‐transform approach makes the derivation of two intermediate field equations, required to get the fields in both the Heaviside–Feynman and Liénard–Wiechert form, straightforward.

ISSN:0002-9505    

DOI:10.1119/1.18265

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

Christiaan Huygens was the first person to discover the general formula for the center of oscillation length of a physical pendulum. He was motivated to do this work by his interest in constructing accurate pendulum clocks. It is remarkable that Huygens arrived at his brilliant solution about a quarter of a century before the publication of the first edition of Newton’sPrincipia. This paper analyzes Huygens’ little‐known derivation in detail. We take particular note of Huygens’ ingenious idea of having the moving physical pendulum ‘‘theoretically explode’’ into constituent pieces which then rise until their kinetic energy is exhausted.

ISSN:0002-9505    

DOI:10.1119/1.18156

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

This article describes a simple nonlinear mechanical oscillator suitable for an undergraduate physics laboratory. The oscillating system is a Plexiglas blade, with nonlinearity introduced by means of contact with stretched rubber bands. Using a variable driving frequency, students can make amplitude and phase lag measurements with sufficient accuracy for quantitative work. It is possible to study classical resonance with the linear system and then, with the modified system, produce nonlinear effects such as hysteresis.

ISSN:0002-9505    

DOI:10.1119/1.18157

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

After the concept of entropy has been introduced in the classical approach to thermodynamics, the definition of a reversible process may be refined in a mathematical way. If a given process can be modified by appropriate adjustment of the thermodynamic force involved so that it approaches a limit of zero entropy production, defining a limit process, and if the reverse process can be similarly modified, defining the same limit process in reverse, then we say that the process is reversible in this limit. Two examples are given, one dealing with the heating of a system and the second involving the adiabatic expansion of a viscous ideal gas.

ISSN:0002-9505    

DOI:10.1119/1.18158

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

A soap film model is described that demonstrates catastrophic changes of the film pattern caused by continuous changes in configuration. The scope of the model is extended by the use of computer graphics both to animate the pattern changes of the films and to plot the corresponding movement on energy‐configuration and other curves. Such energy‐configuration curves are identical to those of systems that experience first‐order and second‐order phase changes.

ISSN:0002-9505    

DOI:10.1119/1.18159

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

The color wave functions of the quark model for colorless multiquark configurations (color singlets) are constructed by an elementary method without an explicit use of group theory. It is shown that no normalizable color singlet wave functions exists for free quarks, free diquarks and free diquark–antiquarks. The color wave functions of the mesons and the baryons are derived.

ISSN:0002-9505    

DOI:10.1119/1.18459

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP