摘要:

The fact that metallic gadolinium is ferromagnetic with a Curie temperature of 294 K is used in the constuction of an inexpensive and simple thermomagnetic engine, which can be driven by an electric lamp or by sunlight. The apparatus can be used as a simple demonstration of the Carnot principle.

ISSN:0002-9505    

DOI:10.1119/1.14959

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

The frequently used explanation of the aerodynamic lifting force using Bernoulli’s law is shown to have fundamental drawbacks and incomplete reasoning. On the other hand, an explanation based on repulsion forces is shown to be complete, easier to assimilate, more powerful, and better related to the basic principles of mechanics. Furthermore, it is shown that the high streaming velocity at the upper side of the aerofoil is not the reason for the low pressure. To the contrary, the low pressure generated by the aerofoil is the reason for the high streaming velocity.

ISSN:0002-9505    

DOI:10.1119/1.14960

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

In one‐dimensional quantum mechanics, there is one bound state near zero energy in a weak attractive potential. A series is derived for the bound state energy in powers of the potential. Normally the leading term isO[V2], but when the potential has a long attractive tail, −V0‖a/x‖P, 0

ISSN:0002-9505    

DOI:10.1119/1.14961

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

Symbolic proof of the Helmholtz theorem on the unique determination of a vector field from its divergence and curl is given which requires only an elementary knowledge of electrostatics and vector analysis. The conditions for uniqueness emerge as natural consequences of the symbolic method employed.

ISSN:0002-9505    

DOI:10.1119/1.14970

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

A rubber ball, with a 4.9‐cm diameter and 64‐g mass, is dropped from a certain heighthto bounce onto the floor, and the time between the first bounce and thenth bounce is measured. In the first version this time is taken by a digital stopwatch with one centisecond resolution, and in the second version, by an integrated‐circuit timer with 1‐ms resolution, triggered by a microphone. Considering the formulah=(1/2)gt2and the coefficient of restitution of energy in the collision of the ball against the floor, the acceleration of gravitygmay be determined to 2%–3% in the first version and to better than 1% in the second one. The experiment opens many problems, i.e., about the time of contact, the internal vibrational energy of the ball, and the dependence of the elastic properties of the rubber on the temperature.

ISSN:0002-9505    

DOI:10.1119/1.14971

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

The standard Newton’s equation of motion for variable masses is set up and solved to describe the time‐dependent rise of a liquid in a capillary tube. The findings in the nonviscous case are also supported by quantitative estimates of the potential energy changes suffered by the interfacial films. The theoretical oscillations in the viscous case are confirmed experimentally for tubes having a radius larger than a critical value.

ISSN:0002-9505    

DOI:10.1119/1.15312

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

A geometrical model of the Foucault pendulum is presented, which corrects some common misconceptions concerning the ‘‘fixed’’ plane in which the pendulum oscillates. It is shown visually, and by the use of metric geometry, that the direction of oscillation of the pendulum undergoes a parallel displacement as it moves along the Earth’s spherical surface. However, this direction of oscillation is not fixed relative to the stars.

ISSN:0002-9505    

DOI:10.1119/1.14972

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

We derive a relativistic mass tensor (dyadic or matrix) whose origin and properties have a direct geometric interpretation in terms of projection operators related to the particle’s world line and local inertial frame in Minkowski space, yet whose eigenvalues are simply the longitudinal (ml) and the transverse (mt) mass. Writing the noncovariant equations of motion (EOM) for a point particle in terms of this mass tensor bridges the gap between the compact but sterile form of the Lorentz covariant EOM and the usual (‘‘unwieldy’’) noncovariant EOM in whichmlandmtappear. General expressions for 3‐ and 4‐space mass (inverse mass) tensors are presented in terms of the system Lagrangian (Hamiltonian).

ISSN:0002-9505    

DOI:10.1119/1.14973

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

A simple argument is presented to make plausible the theorem that a homogeneous dielectric placed in a uniform external field is uniformly polarized only if its shape is ellipsoidal. Expressions for the depolarization factors are displayed in a form which makes immediately apparent the well‐known sum rule. An additional symmetry relation between the depolarization factors is used to write integral expressions for them. It is shown that these factors reduce to the standard form. Neither Dirichlet’s integral representation nor ellipsoidal coordinates are used in this derivation.

ISSN:0002-9505    

DOI:10.1119/1.14974

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP

摘要:

Classical thermodynamics as usually formulated is remarkable for the total absence of any variational principles. It is shown here that as far as its implications for quasistatic transitions are concerned the Second Law can be formulated as a variational principle which guarantees the existence of an empirical entropy function.

ISSN:0002-9505    

DOI:10.1119/1.14975

出版商:American Association of Physics Teachers    

年代:1987

数据来源: AIP