ISSN:0002-9505    

DOI:10.1119/1.17660

出版商:American Association of Physics Teachers    

年代:1994

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.17662

出版商:American Association of Physics Teachers    

年代:1994

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.17663

出版商:American Association of Physics Teachers    

年代:1994

数据来源: AIP

摘要:

Discovered in 1951, the 21 cm hyperfine line of atomic hydrogen has become one of the best‐studied spectral features in radioastronomy. Used to trace the distribution and velocity of atomic gas in our own and in other galaxies, the line has made an enormous contribution to our understanding of galactic structure. Now, 40 years after the line’s discovery, a low‐cost receiving system can be assembled from items of consumer electronics to enable study of the line in an undergraduate laboratory.

ISSN:0002-9505    

DOI:10.1119/1.17664

出版商:American Association of Physics Teachers    

年代:1994

数据来源: AIP

摘要:

A zero‐order rainbow angle may be defined as the relative minimum angle of deviation of geometrical light rays transmitted without internal reflections through a transparent particle. If the incident rays are parallel and the particle is a sphere, such a minimum does not exist. But if the incident rays are not parallel or if the particle has an elliptical rather than circular cross section, an angle of minimum deviation, hence a zero‐order rainbow, can occur. For a spherical water droplet, the zero‐order rainbow will occur when a point source is placed less than a droplet radius away from its surface. If a column of water with an elliptical cross section is illuminated by a plane wave, a zero‐order rainbow will occur if the length of the major axis of the cross section is more than twice the length of the minor axis.

ISSN:0002-9505    

DOI:10.1119/1.17665

出版商:American Association of Physics Teachers    

年代:1994

数据来源: AIP

摘要:

1994 is the hundredth anniversary of what Max Planck described in 1935 as the ‘‘black year’’ of German physics. In the eight months between January 1st and September 8th 1894, Heinrich Hertz, August Kundt, and Hermann von Helmholtz died. This article reviews the lives of these three important physicists, their research contributions, and their unique positions in the German physics community. In conclusion, the relationships of these three physicists to Planck are discussed, and Planck’s evaluation of the impact of 1894 on physics in Germany is appraised from our perspective of one hundred years.

ISSN:0002-9505    

DOI:10.1119/1.17666

出版商:American Association of Physics Teachers    

年代:1994

数据来源: AIP

摘要:

Examination of two alternative models of a bead sliding on a rotating circular wire reveals the meaning of a conserved Hamiltonian where it is not the total energy. Analogies to other such dual treatments are discussed.

ISSN:0002-9505    

DOI:10.1119/1.17667

出版商:American Association of Physics Teachers    

年代:1994

数据来源: AIP

摘要:

One of the more familiar systems in electrostatics is the parallel plate capacitor (PPC). While this system has received considerable attention in the close plate approximation, little is known about the exact solution for arbitrary plate separations. Although the solution was first given, in cylindrical coordinates by Sneddon, it was part of a more general treatise on mixed boundary value problems and appears to be unknown to much of the physics community. We present here a dedicated derivation of the solution to the boundary value problem for parallel disks, in cylindrical coordinates. The resulting expressions for potential and capacitance are in closed form, but depend on a functionf(t) which is determined from an integral equation of the Fredholm type, known as Love’s equation. By adopting an orthogonal series approach to the solution of Love’s equation, we have calculated the capacitance for a number of plate separation to plate radius ratios. A quantitative measure of the close plate approximation is then presented by comparing these values to those one would obtain using the elementary capacitance equation ε0A/d.

ISSN:0002-9505    

DOI:10.1119/1.17668

出版商:American Association of Physics Teachers    

年代:1994

数据来源: AIP

摘要:

Many papers begin with the statement that Kamerlingh Onnes discovered superconductivity in 1911; one wonders what urged him to do the experiment that led to this discovery. Superconductivity was definitely not foreseen at that moment, and for many years the theory could not even predict it. Hence what drove Kamerlingh Onnes to measure the electrical resistivity at a temperature range where one could not expect to find anything radically new? What were the prevailing theories at that time? This is discussed after a general description of the man, his laboratory, and the state of physics in the preceding years. Conclusion: Although the driving factor in the low temperature resistance work was most likely the need for a reliable and reproducible thermometer, the speculations about what would happen in these regions were based on Kelvin’s theory, at least in the initial phase of the work. This theory predicted exactly the opposite of what was found eventually.

ISSN:0002-9505    

DOI:10.1119/1.17669

出版商:American Association of Physics Teachers    

年代:1994

数据来源: AIP

摘要:

We present a version of Jefimenko’s formulas for electric and magnetic fields in material media, and we demonstrate how this version of the formulas may be used for deriving the electric and magnetic fields produced by an arbitrarily moving dipole containing both electric and magnetic moments. Like the fields produced by a charge in arbitrary motion, the fields of an arbitrarily moving dipole can also be written in two forms: The Heaviside–Feynman form and the Liénard–Wiechert form. We derive here the first form and the radiation fields associated with the second form. These radiation fields are expressed by means of concise, symmetric, and illuminating formulas.

ISSN:0002-9505    

DOI:10.1119/1.17759

出版商:American Association of Physics Teachers    

年代:1994

数据来源: AIP