ISSN:0002-9505    

DOI:10.1119/1.15247

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.15249

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.15253

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

Student understanding of the real images produced by converging lenses and concave mirrors was investigated both before and after instruction in geometrical optics. The primary data were gathered through interviews in which undergraduates taking introductory physics were asked to perform a set of prescribed tasks based on a simple demonstration. The criterion used to assess understanding was the ability to apply appropriate concepts and principles, including ray diagrams, to predict and explain image formation by an actual lens or mirror. Performance on the tasks, especially by students who had not had college instruction in geometrical optics, suggested the presence of certain naive conceptions. Students who had just completed the study of geometrical optics in their physics courses were frequently unable to relate the concepts, principles, and ray‐tracing techniques that had been taught in class to an actual physical system consisting of an object, a lens or a mirror, and a screen. Many students did not seem to understand the function of the lens, mirror, or screen, nor the uniqueness of the relationship among the components of the optical system. Difficulties in drawing and interpreting ray diagrams indicated inadequate understanding of the concept of a light ray and its graphical representation.

ISSN:0002-9505    

DOI:10.1119/1.15254

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

This paper traces an important chapter in the evolution of the temperature concept in classical thermodynamics. The centerpiece in the story is the temperature function discovered by Carnot, and gradually developed over a period of 30 yr by Clapeyron, Holtzmann, Helmholtz, Joule, Rankine, Thomson (Kelvin), and Clausius. In Thomson’s final resolution of the problem, Carnot’s function simply determined the thermodynamic temperature scale.

ISSN:0002-9505    

DOI:10.1119/1.15255

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

Delightful sounds, produced in simple experiments, agree with an elementary theoretical analysis.

ISSN:0002-9505    

DOI:10.1119/1.15229

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

Using Wick’s normal‐ordered expression for the quantum‐mechanical energy of a radiation field, so that in absence of matter the zeropoint energy of a radiation field would vanish, the Casimir effect of attraction between uncharged condenser plates in absence of photons will still follow, due to a nonvanishing but finite zeropoint radiation energy in the presence of these plates. If we assume the plates to be transparent for electromagnetic waves of sufficiently high frequency, the remaining zeropoint energy density itself will be finite (and not merely its variation under a change of the distance between the plates). These results are obtained by a more careful consideration of where in sums over modes of vibration one should use sums over running waves with a periodicity condition, where one should sum over standing waves with nodes at conducting boundaries, and where these sums may be replaced by integrals.

ISSN:0002-9505    

DOI:10.1119/1.15230

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

For conservative central forces the constancy of angular momentum provides a more natural substitution than is normally used to transform the force law into a differential equation. This is a transverse velocity substitution, which emphasizes angular rather than radial dependence for dynamical variables, implying a functional rotation. The method gives the standard results in new forms and they are expressed in more natural units of length, time, velocity, and energy than appears in other approaches. For the inverse square force the transverse velocity explicitly depends on the two invariant velocities, and the total energy is the difference of the kinetic energies for the two velocities. The invariant angular momentum is the sum of two variable angular momenta for the two velocities. The Laplace–Runge–Lenz vector is one of a triplet of orthogonal invariant vectors, the cross product of the invariant momentum and angular momentum. For noncircular trajectories this triplet and the scalar total energy are constants of the motion.

ISSN:0002-9505    

DOI:10.1119/1.15231

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

Starting from the standard current density of probability for a quantum particle in a potentialV(r), thetotalprobability current in a stationary state is considered and investigated. Its physical interpretation leads to a natural definition of the time period for such a quantum stationary state. A simple and elegant formula relates the total current to the energy level of the state. This formula is shown to be consistent with the Correspondence Principle. The results are applied to the hydrogen atom and the harmonic oscillator.

ISSN:0002-9505    

DOI:10.1119/1.15232

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

The airflow around a simple automobile shape is calculated using a distributed source–sink technique. This is then used to determine the trajectories of snowflakes, and it is shown that, for a given snowflake size, there is a range of travel speeds that minimizes the impaction of snowflakes on the windshield.

ISSN:0002-9505    

DOI:10.1119/1.15233

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP