ISSN:0002-9505    

DOI:10.1119/1.18448

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

An electrostatic orrery for studying Keplerian orbits has been constructed in which one or more negatively charged hollow glass microparticles orbit a 9.5‐mm‐diam metal sphere at +8‐kV potential in a vacuum. The device is similar to an earlier cylindrical orrery in which particles orbit a rod [Bieweretal., Am. J. Phys.62(9), 821–827 (1994)]. Electrically biased cylinders covering the rod supporting the sphere give nearly spherical potential surfaces inside the trap. Additional electrodes at the boundary are used to reduce the perturbation of gravity and to prevent motion resulting in collisions with the supporting rod. Orbits last approximately 10 min or about 104revolutions. The orbiters are illuminated with a slide projector and can be seen with the naked eye as well as videotaped. The trap has been used to observe orbital precession, interparticle collisions, and the effects of time‐independent perturbations. This apparatus provides an opportunity for the study and demonstration of orbital motion in a laboratory.

ISSN:0002-9505    

DOI:10.1119/1.18369

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

We collect together several ideas that we have found helpful in teaching the magnetic vector potentialA. We argue that students can be taught to visualizeAfor simple current distributions and to seeAas something with physical significance beyond its bare definition as the ‘‘thing whose curl isB.’’

ISSN:0002-9505    

DOI:10.1119/1.18400

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

An investigation conducted after standard lecture instruction in a sophomore‐level modern physics course revealed that many students were unable to interpret the photoelectric experiment in terms of the photon model for light. Findings from this research were used to guide the development of an interactive computer‐based tutorial to address the conceptual and reasoning difficulties that were identified. The primary instructional strategy used in the tutorial is the drawing and interpretation of graphs of current versus voltage for the circuit in the experiment. The program has been used both as an aid to instruction and as a probe to obtain additional information about the nature, prevalence, and persistence of specific difficulties. Analysis of student performance on examination problems on the photoelectric experiment indicates that those who have worked through the tutorial make fewer errors and give better explanations than those who have not had this experience. This result suggests that the intellectual engagement required by the program helps students improve their understanding of the photoelectric effect.

ISSN:0002-9505    

DOI:10.1119/1.18360

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

It is shown that the radiation resistance for sinusoidal sources can be calculated directly from the retarded potentials by evaluating the electric field at points on the source and that for electric and magnetic dipoles in the long wavelength limit the calculation is fairly simple. The problem is also examined from the point of view of Maxwell’s equations and their solutions, and it is shown that there is a significant portion of the electric and magnetic field which is finite everywhere, even for points on the source.

ISSN:0002-9505    

DOI:10.1119/1.18361

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

We describe a group laboratory exercise in which neutron activation analysis (NAA) is used to illustrate the concepts of nuclear reactions, nuclear energy levels, characteristics of gamma rays, radioactive decay and half‐life, and Compton scattering. Small groups of 10–12 students participate in the 50‐min laboratory period that includes a 15‐min discussion of the basic principles that underlie NAA, followed by the irradiation and gamma‐ray counting of a sample containing ‘‘unknown’’ elements. Students carry away copies of data printouts for out‐of‐class analysis. The exercise has been implemented in an introductory, calculus‐based general physics course with approximately 200 students.

ISSN:0002-9505    

DOI:10.1119/1.18363

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

The electromotive force (emf) in a loop outside an infinite solenoid with changing current is usually calculated using the vector potential because the magnetic field outside an infinite solenoid is supposed to be zero. However, the magnetic field will only be zero for steady currents. A change in the applied voltage will give rise to a change in the current, which will propagate along the solenoid in the same way as a wave on a transmission line. This gives rise to a transient magnetic field outside the solenoid. It is quite possible to calculate this transient magnetic field and use it in Faraday’s law to calculate the emf directly without using the vector potential. In practice, it is usually simpler to use the vector potential. However, care should be taken to ensure that students are not given the impression that there is no magnetic field and that it is the vector potential that acts on charges in the loop. We give examples of the magnetic field configuration outside an infinite solenoid for a steplike change in driving voltage and for an ac driving voltage.

ISSN:0002-9505    

DOI:10.1119/1.18362

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

The quantum harmonic oscillator has the unique property that its energy levels are equally spaced. This property is connected, via the Wigner function, to the circular symmetry of the Hamiltonian inphasespace, hence to the close relation between harmonic oscillations and rotations, very familiar at the classical level. We here have an example of a spectral regularity which is due to a symmetry in phase space, rather than in configuration space, as is more usual.

ISSN:0002-9505    

DOI:10.1119/1.18364

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

A pair of Helmholtz coils is a standard piece of physics apparatus which is used to produce a region of uniform magnetic field. Such a field is used in the experiment to determinee/mfor electrons. The question arises then exactly how uniform is the field along the path of these electrons. We provide an elementary derivation of this midplane field and compare the results with measured values.

ISSN:0002-9505    

DOI:10.1119/1.18365

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP

摘要:

The optical–mechanical analogy involves the expression of geometrical optics and particle mechanics in the same mathematical language. In this paper, an especially simple version of the optical–mechanical analogy is extended to general relativity. A variational principle for the trajectories of photons and particles is obtained which is applicable to a broad class of metrics. This permits us to cast the exact equations of motion for both massive and massless particles into the form of Newtonian mechanics. The new equations of motion are illustrated by applications to the Schwarzschild metric.

ISSN:0002-9505    

DOI:10.1119/1.18366

出版商:American Association of Physics Teachers    

年代:1996

数据来源: AIP