ISSN:0002-9505    

DOI:10.1119/1.1807236

出版商:American Association of Physics Teachers    

年代:1981

数据来源: AIP

摘要:

In recent decades great strides have been made in advancing the analytical level and content of introductory college physics for scientists and engineers. In order to accomodate these advances, ’’nonessential’’ or ’’less essential’’ materials may be sidetracked, de‐emphasized, or omitted. Perhaps the most common candidates for characterization as nonessential are the elementary examples and applications of physics from the world of everyday experience. Although mature physicists see great beauty and understanding in analysis, a heavy classroom emphasis on mathematical analysis may give students the impression that the sole goal of elementary physics is the development of the skills of abstract analysis. Analysis is essential! But if our students are to become more than mere analysts, we must share with them some of the fun, fascination, andunderstandingthat can be found in the physics of everyday phenomena, in the applications of physics in other disciplines, and in the interface between science and society. Examples the author has found to be useful are given.

ISSN:0002-9505    

DOI:10.1119/1.12558

出版商:American Association of Physics Teachers    

年代:1981

数据来源: AIP

摘要:

This is the last of a three‐part series. The first essay in the series described Dirac’s generalizations of quantum mechanics leading to his theory of the electron. The second essay surveyed evaluations of Dirac’s theory by other physicists, especially by Bohr who used Dirac’s speculations about negative energy electrons as evidence for the failure of quantum mechanics at nuclear dimensions. This essay shows how the material reality of positrons vindicated quantum mechanics and opened new paths for physics.

ISSN:0002-9505    

DOI:10.1119/1.12559

出版商:American Association of Physics Teachers    

年代:1981

数据来源: AIP

摘要:

Skilled discus throwers claim that a properly thrown discus will travel several meters farther if it is thrown against the wind, than if it is thrown along the direction of the wind. Numerical calculations confirm these claims for winds of up to about 20 m/sec and show that the extra distance is caused by the higher lift and drag forces acting on a discus that is thrown against the wind. Aerodynamic considerations influence numerous aspects of discus throwing, but these have not been dicussed in the scientific literature. In addition to reviewing the available literature, the present article calculates the effect on distance thrown caused by changes in wind velocity, altitude, air temperature, gravity, and release velocity. Some sample results are that a discus can travel: (i) 8.2 m farther against a 10‐m/sec wind than with such a wind; (ii) 0.13 m farther at 0 °C than at +40 °C; (iii) 0.19 m farther with no wind at the elevation of Rome, Italy than at the elevation of Mexico City, Mexico; and (v) 0.34 m farther at the equator than at the poles.

ISSN:0002-9505    

DOI:10.1119/1.12560

出版商:American Association of Physics Teachers    

年代:1981

数据来源: AIP

摘要:

This paper presents the theoretical background for and the detailed analysis of Kaufmann’s 1901–1905 experiments to determine thee/mratio for fast electrons. Far from providing the first experimental confirmation of Einstein’s special theory of relativity, as is often claimed in physics textbooks today, these data were initially interpreted as confirming Abraham’s classical model of a rigid spherical electron and as providing evidence against special relativity. Only in 1906–1907, upon Planck’s subsequent reanalysis of Kaufmann’s 1905 data, did these experiments become evidence marginally in favor of relativity over classical models of the electron. This particular issue, of the superiority of special realtivity over classical theory in providing a fit toe/mdeterminations, was not definitely settled until 1914 with new extensive and accurate data obtained by Neumann. The entire episode provides another example that science does not proceed by a strict falsificationist methodology. It shows rather that a great scientist such as Einstein at times gives more weight to a theory that has a certain beauty and produces equations simple in form than he does to experimental results that apparently conflict with such a theory.

ISSN:0002-9505    

DOI:10.1119/1.12561

出版商:American Association of Physics Teachers    

年代:1981

数据来源: AIP

摘要:

Microwave optics experiments operating at a 3.33‐cm wavelength (9 GHz) are described that have an overall signal gain of 58 dB, which is achieved by using a 1000‐Hz square‐wave modulated reflex klystron and a high‐gain, narrowband amplifier following a point‐contact detector. A metal‐plate electromagnetic lens is employed that has a gain of 22.5 dB relative to an isotropic radiator, produces a beam collimated to within 12 deg between half‐power points, and has a 3.7% bandwidth around a center frequency of 9 GHz. This basic system was used to developed microwave versions of the Michelson interferometer, Bragg reflection, Brewter’s law and total internal reflection, and Young’s interference experiment. Measured radiation intensities for four signal sources are shown to lie well within Federal performance regulations established in 1971.

ISSN:0002-9505    

DOI:10.1119/1.12562

出版商:American Association of Physics Teachers    

年代:1981

数据来源: AIP

摘要:

It is shown that Dirac’s ’’instant form’’ dynamics provides a theoretical framework in which models of relativistic quantum mechanics can be constructed. The convariant harmonic oscillator formalism discussed in previous papers is shown to be such a model. Dirac’s ’’point’’ and ’’front’’ forms are shown to generate a space‐time geometry convenient for describing Lorentz deformation properties of relativistic extended hadrons.

ISSN:0002-9505    

DOI:10.1119/1.12563

出版商:American Association of Physics Teachers    

年代:1981

数据来源: AIP

摘要:

Advanced undergraduate experiments utilizing a commerically available, thin252Cf spontaneous fission (SF) source are described. These include studies of the energy and mass distribution of the fission fragments and their energy and angular correlation. The energy loss of heavy ions traversing solid and gas absorbers is measured and their effective charges deduced. The SF rate relative to the alpha decay rate is used to infer the SF half‐life. These experiments provide a useful introduction to fission, nuclear mass equations, heavy‐ion physics, and related topics.

ISSN:0002-9505    

DOI:10.1119/1.12564

出版商:American Association of Physics Teachers    

年代:1981

数据来源: AIP

摘要:

It is shown how both the qualitative and the quantitative features of the asymptotic freedom of quantum chromodynamics can be understood in a rather intuitive way. The starting point is the spin of the gluon, which because of the gluon self‐coupling makes the vacuum behave like a paramagnetic substance. Combining this result with Lorentz invariance, we conclude that the vacuum exhibits dielectric antiscreening and hence asymptotic freedom. The calculational techniques are with some minor modifications those of the Landau theory on the diamagnetic properties of a free‐electron gas.

ISSN:0002-9505    

DOI:10.1119/1.12565

出版商:American Association of Physics Teachers    

年代:1981

数据来源: AIP

摘要:

A simple apparatus has been constructed to measure the atmospheric electrostatic‐potential gradient at the surface of the Earth. On 10 September 1979 in an open field in South Hadley, Massachusetts (elevation 250 ft) a potential gradient of 197 V/m was obtained, in reasonable agreement with previous measurements in comparable locations and under similar weather conditions.

ISSN:0002-9505    

DOI:10.1119/1.12566

出版商:American Association of Physics Teachers    

年代:1981

数据来源: AIP