ISSN:0002-9505    

DOI:10.1119/1.15161

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.15163

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.15164

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.15143

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

The Schrödinger, Klein–Gordon, and Dirac equations for the hydrogen atom are solved inDdimensions, whereDmay be noninteger. ForD=1 the nondegenerate ground state with infinite binding energy is obtained as a limit asDtends to 1 for the Schrödinger equation. The corresponding solution of the Klein–Gordon equation is shown to be unacceptable, contradicting previous work. Unexpectedly, no bound state solutions of the Dirac equation withD=1 are found at all. A few comments are made on theD=3 and 2 cases.

ISSN:0002-9505    

DOI:10.1119/1.15144

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

Cosmic strings are very long, thin structures which might stretch over vast reaches of the universe. If they exist, they would have been formed during phase transitions in the very early universe. The space‐time surrounding a straight cosmic string is flat but nontrivial: A two‐dimensional spatial section is a cone rather than a plane. This feature leads to unique gravitational effects. The flatness of the cone means that many of the gravitational effects can be understood with no mathematics beyond trigonometry. This includes the observational predictions of the double imaging of quasars and the truncation of the images of galaxies.

ISSN:0002-9505    

DOI:10.1119/1.15145

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

Results from research on student understanding of velocity and acceleration have been used to guide the development of a conceptual approach to teaching kinematics. This paper describes how instruction based on the observation of actual motions can help students: (1) develop a qualitative understanding of velocity as a continuously varying quantity, of instantaneous velocity as a limit, and of uniform acceleration as the ratio of the change in instantaneous velocity to the elapsed time; (2) distinguish the concepts of position, velocity, change of velocity, and acceleration from one another; and (3) make connections among the various kinematical concepts, their graphical representations, and the motions of real objects. Instructional strategies designed to address specific difficulties identified in the investigation are illustrated by example.

ISSN:0002-9505    

DOI:10.1119/1.15122

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

The motion of a spinning football provides interesting illustrations of some of the principles of rigid‐body motion. The familiar ‘‘wobbly spiral’’ is an example of torque‐free precession; it is shown that the ‘‘wobble‐to‐spin’’ ratio depends only on the principal moments of inertia of the football. The angular momentum of a spinning football is not always conserved in flight. The response of a football to aerodynamic forces can best be understood by comparing its motion to that of a spinning top or gyroscope.

ISSN:0002-9505    

DOI:10.1119/1.15123

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

A straightforward investigation at an introductory level of the interconnection between electricity and magnetism initially leads to the paradoxical result that a charge at rest with respect to a current‐carrying wire feels a magnetic force due to that current. Students may benefit from a presentation of this paradox and its resolution.

ISSN:0002-9505    

DOI:10.1119/1.15124

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

The average power outputP, efficiency η’, and entropy productionPshave been determined as function of cycle frequency ν for a nonideal Carnot engine where the ratiorof the heat absorption time and heat rejection time can be varied and the heat is transferred by conduction into the working fluid. The power output has an optimum at the valuer=(β/α)1/2, where α and β are the thermal conductances of the upper and lower isotherm, respectively. The efficiency decreases and the conduction losses increase drastically with cycle frequency. The power output goes through a maximum, peaked at a frequency ν*. The efficiency can be increased without much sacrifice in power output if the engine is run below the frequency ν* (12% efficiency increase at 3% loss in power for ν=0.85ν*). The entropy production grows approximately asPs∼ν2.

ISSN:0002-9505    

DOI:10.1119/1.15125

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP