摘要:

Fundamentals of thermodynamics are introduced and illuminated using a model ‘‘gas’’ consisting of a single particle of massmand kinetic energyEin a one‐dimensional box of lengthL. Constancy of the classical action,A=2√2mELunder slow, mechanically reversible changes inLreflects the symmetry of reversibility. Heat is defined in terms of work using adiabatic paths to connect an arbitrary pair of states. The ensemble average ofAincreases for many irreversible mechanical processes that break the reversibility symmetry, and entropy can be linked to ln(A). Although not strictly ‘‘thermodynamic,’’ this model helps clarify the statistical nature of the second law of thermodynamics and illuminates the very essence of thermodynamics, namely, the invisible mixing of energy within systems.

ISSN:0002-9505    

DOI:10.1119/1.18030

出版商:American Association of Physics Teachers    

年代:1995

数据来源: AIP

摘要:

We describe a simple modification to a grating‐stabilized diode laser intended for use in neutral atom cooling and trapping. Modulation of the laser diode’s driving current at microwave frequencies induces sidebands in the laser’s spectrum that eliminate the need for a second laser in a trapping apparatus, yielding a considerable savings in effort and cost. A detailed description of the process for selection and assembly of microwave components is given. In an appendix we list the hardware used, with vendor and pricing information.

ISSN:0002-9505    

DOI:10.1119/1.18031

出版商:American Association of Physics Teachers    

年代:1995

数据来源: AIP

摘要:

A now forgotten figure, Emil Cohn was one of the leading experts in electrodynamics at the turn of the century. Unsatisfied with Lorentz’s electron theory, he proposed an alternative electrodynamics of moving bodies based on a modification of Maxwell’s macroscopic field equations that managed to account for all relevant electrodynamic and optical experiments, including that of Michelson and Morley. Some of his insights foreshadow aspects of Einstein’s relativity theory, especially the elimination of the ether and the criticism of Lorentz’s implicit distinction between true and measured coordinates of space and time. However, Cohn did not believe in the general validity of the relativity principle, and did not require a revision of the usual concepts of space and time.

ISSN:0002-9505    

DOI:10.1119/1.18032

出版商:American Association of Physics Teachers    

年代:1995

数据来源: AIP

摘要:

The calculation of the induced electromotive force in a circular coil placed around the outside of an infinitely long, ideal solenoid is found in many textbooks. This calculation has led to some confusion since we appear to have a changing magnetic field producing an electric field in a region where there is no magnetic field. An exact solution of Maxwell’s equations is given when the source is a steady alternating current. The results of the two methods are compared. In addition, this solution makes it possible to give an exact calculation of the radiation reaction and compare it with the radiated power.

ISSN:0002-9505    

DOI:10.1119/1.18033

出版商:American Association of Physics Teachers    

年代:1995

数据来源: AIP

摘要:

We present a method for determining the bound and quasibound state eigenvalues of an arbitrary multiple quantum‐well potential based on the poles of the reflection coefficient at the first boundary of the potential. The method has the advantage of computational simplicity and can be used to approximate the eigenvalues of a coordinate‐dependent potential. As examples, the method is used to determine the bound and tunneling state eigenvalues of a three‐boundary potential and the bound state eigenvalues of a five quantum‐well potential. The method is compared with other methods of analysis of quantum‐well potentials.

ISSN:0002-9505    

DOI:10.1119/1.18034

出版商:American Association of Physics Teachers    

年代:1995

数据来源: AIP

摘要:

A formal derivation of Jefimenko’s formulas for the time‐dependent generalizations of the Biot–Savart and Coulomb laws is presented. Maxwell’s equations and the free‐space retarded Green function for the scalar wave equation are the basic ingredients of this derivation.

ISSN:0002-9505    

DOI:10.1119/1.18086

出版商:American Association of Physics Teachers    

年代:1995

数据来源: AIP

摘要:

Variants of student experiments on electrical fluctuations are described. Thermal noise is measured over a wide temperature range using incandescent lamps. A common resonant circuit serves as the load of a thermionic diode when measuring shot noise. The quality factor of this resonant circuit must be measured while the diode current passes through it. Boltzmann’s constant and the electron charge are evaluated from the data obtained. Finally, a simple circuit is recommended as a rms voltmeter for measuring noise voltages.

ISSN:0002-9505    

DOI:10.1119/1.18035

出版商:American Association of Physics Teachers    

年代:1995

数据来源: AIP

摘要:

The influence of a reflecting barrier on the eigenstates of a Dirac potential well is analyzed in detail. This simple case provides a good example for discussing in a very simple way several basic features of quantum mechanics. First, it is shown that the presence of the barrier entails a destabilization of the bound state and can even suppress it; this is expected on physical grounds since the barrier acts in a repulsive way. Second, an image method is used to recover the results obtained in the conventional way. As regards the unbound states, it is seen that the presence of the barrier suppresses the twofold degeneracy always present in one dimension.

ISSN:0002-9505    

DOI:10.1119/1.18036

出版商:American Association of Physics Teachers    

年代:1995

数据来源: AIP

摘要:

A simple way to derive Bardeen’s tunneling theory is introduced. With a low rate of tunneling of, for example, electrons from one electrode to another through a wide vacuum gap, the appearance of the second electrode can be regarded as a perturbation. Then Bardeen’s formula for tunneling follows exactly from Fermi’s golden rule, supposing that this perturbation appears suddenly at some instant of time. Bardeen’s theory is illustrated by assuming a smoothly variable barrier shape, including the rectangular one as a limiting case. This exactly solvable example clarifies the sensitivity of the tunneling probability to the barrier shape and the energy of the tunneling particle.

ISSN:0002-9505    

DOI:10.1119/1.18037

出版商:American Association of Physics Teachers    

年代:1995

数据来源: AIP

摘要:

The effectiveness of in‐class student–student interaction on the learning of physics in an algebra‐based college physics course is investigated. The student–student interaction leads to an improvement of attitudes of the students toward the course, improves the academic environment of the class, and makes students feel better about the material they are learning. However, it is also found that these qualitative improvements in the class do not lead to a significant change in the test scores on either the classroom tests or the Halloun–Hestenes’s mechanics diagnostics test. The correlation between the performances of students on the classroom tests with the performance on the Halloun–Hestenes test used as a pretest is also investigated and it is found that this correlation is less for students with cooperative learning than for students without cooperative learning.

ISSN:0002-9505    

DOI:10.1119/1.18038

出版商:American Association of Physics Teachers    

年代:1995

数据来源: AIP