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5. app.solid new

# 5. app.solid new - T Y Tan 5 APPLYING LAWS OF...

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T. Y. Tan 1 5. APPLYING LAWS OF THERMODYNAMICS TO CONDENSED MATTER While the three thermodynamic laws were formulated from studying gaseous systems, they do apply to the condensed matter systems of liquids and solids. This can be true because all thermodynamic properties or state functions and variables as defined in arriving at the laws ap- pear to be independent of the nature of the system being gaseous or not. The internal energy U defined in the first law, Δ U = Q + W (5.1a) or dU = δ Q + δ W (5.1b) is obviously independent of whether the system involved is gaseous or not. Thus, we may expect the concept to be also applicable to solids and liquids. The definition of the entropy S, dS = dQ rev T , (5.2) is also independent of whether the system involved is gaseous or not. Thus, we expect that the concept of entropy may also be applicable to condensed matter. Since all other state functions or properties of a system are extensions of U and S, it is obvious that the thermodynamic laws may be applicable to condensed matter just as they are applicable to gases. The fact that the thermodynamic laws should indeed be applicable to condensed matter is evidenced by the phase equilibrium phenomena, the details of which will be discussed in chap- ters 6-8. It is a common experience that, in thermal equilibrium, condensed maters are always coexisting with some appropriate gaseous phases of its constituent species. For example, water and/or ice are/is always coexisting with water vapor, the GaAs crystal is always coexisting with four gaseous phases: Ga 1 , As 1 , As 2 , and As 4 . Since thermodynamic laws hold for the gaseous phases under thermal equilibrium conditions, the same laws must also hold for the condensed matter when expressed in appropriate forms. To apply the thermodynamic laws to condensed matter, the fact that the matter is in a con- densed state must be accounted for. The result of this effort must be in concert with the thermo-

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T. Y. Tan 2 dynamic laws, i.e., a contradiction with thermodynamic laws should not arise. This is indeed the case, which constitutes an additional necessary support of the fact that thermodynamic laws do apply to solids. The fundamental difference between gaseous and condensed matter states is that in the condensed state the matter possesses a cohesive energy, discussed in the following section. A cohesive energy does not exist among molecules constituting a gas. However, for a molecular gas, its molecule does possess an internal cohesive energy among the constituent atoms. 5.1 Cohesive Energy of Atoms Condensed mater, i.e., liquids and solids, are characterized by the fact that the system atoms possess a cohesive or binding energy. This cohesive energy constitutes part of the internal energy of the matter. The cohesive or binding energy of an atom in condensed matter is the difference of the atom's energy between existing in vacuum and in the condensed state. An atom in vacuum is an atom in a gas with P=0 and T=0. The energy of an atom (whatever species) in vacuum consti- tutes the only possible
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5. app.solid new - T Y Tan 5 APPLYING LAWS OF...

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