ISSN:0002-9505    

DOI:10.1119/1.17142

出版商:American Association of Physics Teachers    

年代:1992

数据来源: AIP

摘要:

This Resource Letter provides a guide to the literature on medical imaging, with emphasis on magnetic resonance imaging. The letter E after an item indicates elementary level of material of general interest to persons becoming informed in the field. The letter I, for intermediate level, indicates material of somewhat more specialized nature; and the letter A indicates rather specialized or advanced material.

ISSN:0002-9505    

DOI:10.1119/1.17100

出版商:American Association of Physics Teachers    

年代:1992

数据来源: AIP

摘要:

Using the example of a monopole that is spontaneously generated above a thin conducting sheet, the simplicity and power of Maxwell’s 1872 theory of eddy currents in thin conducting sheets is illustrated. This theory employs a receding image construction, with a characteristic recession velocityv0=2/(μ0σd), where the sheet has conductivity σ and thicknessd. A modern derivation of the theory, employing the magnetic scalar potential, is also presented, with explicit use of the uniqueness theorem. Also discussed are limitations on the theory of which Maxwell, living in a time before the discovery of the electron, could not have been aware. Previous derivations either have not appealed explicitly to the uniqueness theorem, or have employed the now unfamiliar current function, and are therefore either incomplete or inaccessible to the modern reader. After the derivation, two important examples considered by Maxwell are presented−a monopole moving above a thin conducting sheet, and a monopole above a rotating thin conducting sheet (Arago’s disk)−and it is argued that the lift force thus obtained makes Maxwell the grandfather, if not the father, of eddy current MAGLEV transportation systems. An energy conservation argument is given to derive Davis’s result that, for a magnet of arbitrary size and shape moving parallel to a thin conducting sheet at a characteristic heighth, with velocityv, the ratio of drag force to lift force is equal tov0/v, provided thatd≪δc, where δc=√2h/(μ0σv).Ifd≫δc, the eddy currents are confined to a thickness δc, leading to an increase in the dissipation and the drag by a factor ofd/δc, so that the ratio of drag to lift force becomes proportional to √v’0/v, wherev’0= 2/(μ0σh). The case of a monopole fixed in position, but oscillating in strength (such as can be simulated by one end of a long, narrow, ac solenoid), is also treated. This is employed to obtain the results for an oscillating magnetic dipole whose moment is normal to the sheet. A general discussion of electromagnetic induction and electrical conductors, both thick and thin, is given, emphasizing the difference between the high‐frequency limit, where flux expulsion occurs and the self‐inductance dominates, and the low‐frequency limit, where the flux penetrates and the electrical resistance dominates. A discussion of Lenz’s law, as a statement about motion, is given. It is argued that the most general form of such a statement of Lenz’s law is that induced currents tend to accelerate a conductor in the direction that most effectively decreases the rate of Joule heating. A calculation, in the low‐frequency limit, of the drag force on a magnetic dipole falling down a long conducting tube, is also given. This last case can be given a striking demonstration with the newly available neodymium–iron–boron magnets.

ISSN:0002-9505    

DOI:10.1119/1.17101

出版商:American Association of Physics Teachers    

年代:1992

数据来源: AIP

摘要:

According to the ‘‘standard model’’ of particles and fields, hadrons can be formed from a triplet of quarks (baryons), each of which may have any one offflavors (d, u, s, c, b, etc.), or from a quark–antiquark pair (mesons) each one of which may have any flavor. Some numerics of flavor and electric charge content of baryons and mesons are presented. Multiplicity of symmetry types of baryons based on these simple numerical results is calculated without direct appeal to group theory in the hope that the dimensionality of hadron multiplets that contain the building blocks of normal matter will become more widely understood.

ISSN:0002-9505    

DOI:10.1119/1.17141

出版商:American Association of Physics Teachers    

年代:1992

数据来源: AIP

摘要:

After three years of teaching an integrated calculus and physics course, a number of observations on student learning have been made which suggest how teachers of the two subjects might better support and take advantage of the efforts of each other to improve the educational experience for the students. Physics teachers, for example, may well overestimate the depth of understanding their students have of the concepts of calculus, while underestimating their computational skills. Calculus teachers, on the other hand may be unaware of what their students actually need to know to succeed in physics and other quantitative disciplines. In the following, several of the areas of mismatch between calculus and physics will be discussed, as evidenced by student performance in our course, and recommendations will be made as to how the physics teacher might better take advantage of the knowledge of calculus studentsactuallyhave while striving to improve the compatability of the two subjects.

ISSN:0002-9505    

DOI:10.1119/1.17077

出版商:American Association of Physics Teachers    

年代:1992

数据来源: AIP

摘要:

A review of the simple oscillator model of the Helmholtz resonator is presented and the nonlinear behavior of the elastic force is analyzed in terms of the thermodynamic process occurring in the air enclosed in the resonator. This nonlinearity is shown to explain the appearance of an excess average pressure inside the resonator and a net reaction force acting on the resonator itself when the resonator is driven at high sound pressure amplitudes. A direct measurement of the net reaction force on a resonator confirms quantitatively the predictions from the model. Finally, a simple demonstration is presented in which four identical resonators are mounted in an anemometerlike rotor and set into motion solely by the action of a strong periodic sound field at the resonance frequency of the resonators.

ISSN:0002-9505    

DOI:10.1119/1.17078

出版商:American Association of Physics Teachers    

年代:1992

数据来源: AIP

摘要:

The dynamics of relativistic strings is studied with emphasis on the close analogy with the transverse motion of the conventional stretched string of freshman physics. A constraint‐free parametrization of the world sheet in terms of arbitrary wave packets moving along the string is given. In particular, it is found that for the initial condition of right and left moving wave packets in a collision arrangement, there is no scattering−each packet only experiences a delay upon passing through the other. Also discussed is the geometry of the spontaneous singularities generically arising for strings in two spatial dimensions.

ISSN:0002-9505    

DOI:10.1119/1.17079

出版商:American Association of Physics Teachers    

年代:1992

数据来源: AIP

摘要:

The basic principles of Newtonian mechanics can be interpreted as a system of rules defining a medley of modeling games. The common objective of these games is to develop validated models of physical phenomena. This is the starting point for a promising new approach to physics instruction in which students are taught from the beginning that in science ‘‘modeling is the name of the game.’’ The main idea is to teach a system of explicit modeling principles and techniques, to familiarize the students with a basic set of physical models, and to give them plenty of practice in model building, model validation by experiment, and model deployment to explain, to predict, and to describe physical phenomena. Unfortunately, a complete implementation to this approach will require a major overhaul of standard instructional materials which is yet to be accomplished. This article lays down physical, epistemological, historical, and pedagogical rationale for the approach.

ISSN:0002-9505    

DOI:10.1119/1.17080

出版商:American Association of Physics Teachers    

年代:1992

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.17081

出版商:American Association of Physics Teachers    

年代:1992

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.17082

出版商:American Association of Physics Teachers    

年代:1992

数据来源: AIP