ME 311 THERMODYNAMICS S. Masutani FALL 2007 CHAPTER 6 The preceding lectures attempted to provide insight on how entropy can be evaluated using quantum–statistical (microscopic) concepts. As mentioned previously, the difference in the entropy of two thermodynamic states can also be determined from macroscopic properties that can be measured directly. The Gibbs equations are important relationships that are employed to determine entropy from macroscopic properties. Before we proceed with this discussion, we first need to develop thermodynamic definitions of temperature and pressure. RECALL:1.Entropy is a thermodynamic property of matter (at equilibrium). What this means:a.for, say, a SCS: S = S(U,V) for an incompressible liquid: S = S(U) for a SMS: ),(MVUSSr=; Mr= magnetization vector b.Intensive entropy (entropy/unit mass) is defined as MSs≡; units of s are energy/(temperature x mass) = J/K-kg or BTU/R-lbm or cal/K-g c. The specific entropy of a liquid-vapor mixture is gfsssχχ+−=)1(Thermodynamic Definition of TemperatureReview: What is temperature? •Bring two masses in contact; if energy transfer as heat occurs between them ⇒temperatures are different. •Energy passes from “hotter” body to “colder” body. •Temperature is an indicator of the direction of heat transfer. •Differences in temperature reflect a lack of equilibrium ⇒may be able to form basic definition of temperature through considerations of thermal equilibrium. Thus:Search for conditions under which no energy transfer as heat occurs when two masses, each at equilibrium with themselves, are brought together. Find a property that has same value for both bodies under this condition of thermal equilibrium. This leads us to the definition of temperature. 1
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