Thermodynamics working substance

As the magnirnde of the heat exchanged in an isothermal step of a Carnot cycle is proportional to a function of an empirical temperature scale, the magnitude of the heat exchanged can be used as a thermometric property. An important advantage of this approach is that the measurement is independent of the properties of any particular material, because the efficiency of a Carnot cycle is independent of the working substance in the engine. Thus we define a thermodynamic temperature scale (symbol T) such that... 

The relationship between the thermodynamic temperature scale and the ideal gas temperature scale can be derived by calculating the thermodynamic quantities for a Camot cycle with an ideal gas as the working substance. Eor this purpose, we shall use 0 to represent the ideal gas temperamre. 

In Chapter 2, we have analyzed one particular type of heat engine, the reversible Carnot cycle engine with an ideal gas as the working substance, and found that its efficiency is e = 1 — Tc/Th. For both practical and theoretical reasons, we ask if it is possible, with the same two heat reservoirs, to design an engine that achieves a higher efficiency than the reversible Carnot cycle, ideal gas engine. What can thermodynamics tell us about this possibility ... 

Students are reminded of the upper thermodynamic limit set on the efficiency of a heat engine, for example the internal combustion and gas-turbine engines. The ideal and totally unrealistic engine would operate on the so-called Carnot cycle where the working substance (e.g. the gas) is taken in at the high temperature (Th) and pressure and after doing external work is exhausted at the lower temperature (Tc) and lower pressure. The Carnot efficiency, /, is given by... 

This is a useful thermodynamic quantity to characterize changes at constant volume of the working substance.) For a change at constant temperature, from Eq. (4-26),... 

The existence of a finite heat transfer in the isothermal processes is affected with the assumption of a non-endoreversible cycle with ideal gas as working substance. Power output and ecological function have also an issue that shows direct dependence on the temperature of the working substance. Expressions obtained with the changes of variables have the virtue of leading directly to the shape of the efficiency through Z, function. Thus, in classical equilibrium thermodynamics, the Stirling cycle has its efficiency like the Carnot cycle efficiency in finite time thermodynamics, this cycle has an efficiency in their limit cases as the Curzon-Ahlborn cycle efficiency. 

What has been accomplished by this cycle is the absorption of an amount of heat Q1 at the higher temperature T and the rejection of heat 02 at the lower temperature 7V Since the working substance returns to its original state ( U = 0) the net amount of work, Wt accomplished during the cycle must, according to the first law of thermodynamics, be equal to the difference between the amounts of heat absorbed and rejected, %e,... 

We have seen in Chapters 4 and 5 that in thermodynamic work it is customary to obtain values of standard enthalpies and Gibbs energies of species. It will be remembered that these quantities relate to the formation of 1 mol of substance from elements in their standard states, usually at 25.0" C. In the case of entropies it is common to obtain absolute values, based on the third law of thermodynamics. 

The first conclusion is that a Thermodynamic temperature scale exists which has fixed ratios of temperature between any two equilibrium states. Fixing the temperature of any one equilibrium state then fixes the temperature of all others. This follows from the fact that q /92 is a fixed number for any two equilibrium states, being independent of the size, shape, or working substance of the (hypothetical) Carnot engine used. These conclusions follow from the derivations we have omitted. 

The specific thermodynamic work of adhesion, W, of two substances is equal to the sum of the two surface tensions less the interfacial tension... 

Increase of the surfactant content above a certain value results in a drop in the thermodynamic work of adhesion due to the decrease of the surface tension. It can be assiuned that the drop in adhesion strength for IS substances is related not only to the change of the thermodynamic work of adhesion but also to the formation of micelles that form a weak zone at the boundary between the adhesive and the substrate. The accumulation of such micelles occurs gradually with increasing surfactant concentration, as a consequence of which a comparatively smooth decrease of the adhesion strength is observed. 

As Fig. 2.22 shows, the adhesion strength increases as o-adj,Hg fTadhair decreases. RS substances were used to control the interphase tension. The thermodynamic work of adhesion is related to the surface tension at the bormdaries between adhesive and air, between adhesive and sohd, and between sohd and air by the relationship... 

A somewhat different dependence is observed for RS substances (see Fig. 2.21, curve 1). For small quantities of surfactant this dependence for RS substances coincides with the analogous dependence for IS substances at greater quantities of surfactant this dependence does not indicate any drop in the adhesion strength in that it becomes independent of the surfactant content and the change in the adhesion strength shows the same dependence as the polymer surface tension. This correlation is not random insofar as the adhesive spreads well on the steel surface, the thermodynamic work of adhesion of the cured adhesive to a solid surface is essentially twice the value of the surface tension, and consequently the dependence of the adhesion strength on the surfactant content must coincide with the change in the siuTace... 

It was suggested by Fowkes that the only significant interactions across an interface are those that are common to both phases in contact. Based on the additivity approach [37], the total thermodynamic work of adhesion between two substances in contact comprises contributions arising from various types... 

Aleshin U. P., Arefieva L. H., Lugovtsev V. V, Speed of sound and isentropic index for Freon-12, -13, -12B1, -22, and ammonia.—In Thermodynamic Properties of the Most Important Working Substances for Refrigeration Equipment.—Tr. VNI (Kh), Moscow, 1976, p. 7—12. 

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