ISSN:0002-9505    

DOI:10.1119/1.16698

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.16699

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

It is shown how an infinite superconducting slab supports a permanent (‘‘hard’’) magnet against gravity. The equilibrium position is found, using the method of images and the property that superconductors expel magnetic flux, and then the supercurrents (in the approximation that they are localized at the surface) are explicitly determined, using the boundary condition associated with Ampere’s law. This calculation, which does not require the London equation (although a brief discussion of its effects is given), can be done by motivated freshmen. (Measurements by Heller on high‐Tcsamples yield a flotation height about half the predicted value; this is attributed to incomplete screening of the applied field, due to penetration by magnetic flux lines.) From this example two subjects can then be discussed that normally precede a discussion of superconductivity. First, these supercurrents are equal and opposite to the Amperian surface currents that attract a permanent magnet to an infinitely permeable (‘‘soft’’) magnetic slab (the image method yields equal and opposite images if the superconductor supporting the permanent magnet is replaced by an infinitely permeable material). Moreover, these supercurrents are the same as the initial eddy currents generated if a solenoid is suddenly energized or if a permanent magnet suddenly materializes above a conducting slab. Following a discussion of Maxwell’s theory of eddy currents in thin conducting sheets, the time‐development of this initial eddy current distribution is obtained.

ISSN:0002-9505    

DOI:10.1119/1.16700

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

In the standard model of particle physics, the force laws changed discontinuously as the Universe cooled below the critical temperature for electroweak gauge symmetry breaking. In a recently proposed model, the gauge symmetry of electrodynamics, which is responsible for the photon being massless, will also be broken as the Universe cools below some critical temperature. As a result, the photon will suddenly acquire a mass that is of the order of 10−4eV/c2, corresponding to a Compton wavelength of a centimeter. The effects of such a transition on our civilization are examined. All long‐range electric and magnetic fields would vanish, as would all low‐frequency electromagnetic waves. Atomic structure and solar radiation would not be appreciably affected. Effects on neural transmission might be serious, depending on the precise value of the photon mass. It is concluded that the impact on society would be severe, but probably not fatal. Life would continue, and our civilization could be restructured to accommodate the new laws of electromagnetism.

ISSN:0002-9505    

DOI:10.1119/1.16701

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

This article summarizes our calculation of the normal modes and the normal coordinates for a commercially available Wilberforce pendulum. A procedure is presented by which the normal coordinates may be produced experimentally, so that the frequencies of the normal modes can be obtained both theoretically and experimentally. Where possible, results have been compared with those from previous papers. Finally, PCbasicprograms have been written in which the behavior of the Wilberforce pendulum has been theoretically reproduced.

ISSN:0002-9505    

DOI:10.1119/1.16702

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

A scanning tunneling microscope (STM) has been constructed as an undergraduate project. Its mechanical and electronic design are described and results obtained using it are presented. At the same time, the requirements to be met in STM design are noted. Design developments which have paved the STM’s way into the teaching laboratory are identified.

ISSN:0002-9505    

DOI:10.1119/1.16703

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

Isaac Newton introduced his theory of gravitation in Book III of thePrincipia. To demonstrate the power of his new theory, Newton for the first time calculated the mass, surface gravity, and density of Jupiter, Saturn, and the Earth (relative to the Sun). In the final printing of thePrincipia, an error appears in Newton’s calculation of the mass of the Earth and its origin is traced. In the course of reconstructing the source of the error, several other minor inconsistencies are resolved.

ISSN:0002-9505    

DOI:10.1119/1.16704

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

An alternative approach to the one‐dimensional scattering problem is presented in which the potential is replaced by a sequence of flat barriers or wells. The resulting problem is solved exactly and the transmission coefficient obtained via multiplication of a string of 2×2 matrices.

ISSN:0002-9505    

DOI:10.1119/1.16705

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

The spectrum and eigenfunctions of a particle moving in two dimensions under the influence of an external uniform magnetic field and in the presence of a ‘‘point interaction’’ is determined. This is done after an elementary discussion of how to construct a ‘‘point interaction’’ in two dimensions circumventing the well‐known difficulties with the Dirac δ2(r) interaction.

ISSN:0002-9505    

DOI:10.1119/1.16714

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP

摘要:

A hybrid system that consists of an oscilloscope and a photodiode is used to demonstrate optical bistability. The details of the experimental setup and its graphical solution will be presented.

ISSN:0002-9505    

DOI:10.1119/1.16715

出版商:American Association of Physics Teachers    

年代:1991

数据来源: AIP