摘要:

The relationships between the entropy production per cycle and the thermal efficiency are investigated for a class of irreversible cyclic processes. Examples are given that pinpoint specific sources of irreversibility and their thermodynamic consequences. It is found that an increase (decrease) in an irreversible cycle’s thermal efficiency does not necessarily lead to a decrease (increase) in its entropy production even if the work done per cycle is held constant. Only for the case of a reversible Carnot cycle is it guaranteed that a change (negative for this case) in the efficiency is met by an entropy production change of opposite algebraic sign. Sufficiency conditions are found for which the entropy production and the efficiency η are inversely related for more general cyclic processes. For a given set of heat reservoirs and specified values of the work output W, the absolute minimum and maximum entropy productions are determined and are shown to be monotonically decreasing functions of η for fixed W. It is shown also that, for an irreversible cycle with maximum and minimum temperatures T+and T−, respectively, η? (1−T−/T+)(1+T−ΔS/W)−1, where ΔS is the entropy production per cycle. The equality holds only for a cycle employing two reservoirs. The potential relevance of these results to environmental and technological problems is mentioned.

ISSN:0002-9505    

DOI:10.1119/1.10032

出版商:American Association of Physics Teachers    

年代:1975

数据来源: AIP

摘要:

A laboratory experiment that demonstrates the simultaneous conservation of angular and linear momentum is described. A disk is positioned on an air table, and pellets from an air pistol are fired at a target on the disk. The resulting motion of the disk validates the conservation of momentum laws but violates the expectations of the observer.

ISSN:0002-9505    

DOI:10.1119/1.9935

出版商:American Association of Physics Teachers    

年代:1975

数据来源: AIP

摘要:

A method is described for the measurement of the velocity of sound in various liquids, based on the Raman–Nath theory of light–sound interaction. An analog computer program which calculates the intensity of the light scattered into the various diffraction orders has been adapted for use on a digital computer. The equipment used in this experiment is available in most undergraduate physics laboratories.

ISSN:0002-9505    

DOI:10.1119/1.9936

出版商:American Association of Physics Teachers    

年代:1975

数据来源: AIP

摘要:

We describe a pair of interactive computer–student dialogs developed for use primarily with elementary and high school teachers. The dialogs use graphic facilities for teaching about the sky as seen from the earth and about the phases of the moon. The primary aim is for the teachers to understand the nature of a scientific model, in this case the model of the phases of the moon, through a Socratic interaction with the computer. The dialogs are also being used by other university students.

ISSN:0002-9505    

DOI:10.1119/1.9937

出版商:American Association of Physics Teachers    

年代:1975

数据来源: AIP

摘要:

Negative absolute temperature, defined in terms of the Boltzmann factor, corresponds to an upside‐down energy pyramid. The example of the laser shows population inversion to be accompanied by the switch from absorption to emission, that is, from positive to negative dissipation, from increasing disorder throughout a system to increasing order in a subsystem. Early treatments of the negative‐temperature concept are by Onsager and by Ramsey. Ramsey’s ’’essential requirements’’ are shown to be too strong. A TS diagram for a Carnot cycle operating between two negative temperatures clarifies the origin of amplification. Some biological negative‐resistance amplifiers (nerve, muscle), in particular those involving ordered arrays of water molecules, are conjectured to have negative‐temperature subsystems.

ISSN:0002-9505    

DOI:10.1119/1.9938

出版商:American Association of Physics Teachers    

年代:1975

数据来源: AIP

摘要:

The effect of temperature changes on the pitch of a stretched string of constant length is presented for both the flexible and stiff string. It is shown that the main contribution is through a tension change, but temperature variations of Young’s modulus, coefficient of linear expansion, and radius also produce secondary effects. Numerical examples are presented for a steel wire in a piano. A relaxation of constant length is treated for the flexible string, and the relative importance of the temperature‐dependent parameters is discussed, including an entropy analysis.

ISSN:0002-9505    

DOI:10.1119/1.9939

出版商:American Association of Physics Teachers    

年代:1975

数据来源: AIP

摘要:

Blackbody radiation emitted or absorbed by a single microwave cavity mode is detected and the signal displayed on an oscilloscope using a relatively straightforward technique.

ISSN:0002-9505    

DOI:10.1119/1.9949

出版商:American Association of Physics Teachers    

年代:1975

数据来源: AIP

摘要:

A course in elementary mechanics especially designed for students of athletics and dance is described. The course has been taught for two years at the University of Minnesota. Innovative features include a gymnasium laboratory utilizing an inexpensive photographic technique for analysis of human motion. The order of presentation and the selection of topics, as well as the illustrative examples, are all unconventional in an elementary mechanics course. Student response is described.

ISSN:0002-9505    

DOI:10.1119/1.9950

出版商:American Association of Physics Teachers    

年代:1975

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.9951

出版商:American Association of Physics Teachers    

年代:1975

数据来源: AIP

ISSN:0002-9505    

DOI:10.1119/1.9952

出版商:American Association of Physics Teachers    

年代:1975

数据来源: AIP