Thermodynamics Notes 15

Thermodynamics - ME 311 THERMODYNAMICS S Masutani FALL 2007 CHAPTER 6 The preceding lectures attempted to provide insight on how entropy can be

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Unformatted text preview: 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: ) , ( M V U S S r = ; M r = magnetization vector b. Intensive entropy (entropy/unit mass) is defined as M S s ≡ ; 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 g f s s s χ χ + − = ) 1 ( Thermodynamic Definition of Temperature Review : 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 ME 311 THERMODYNAMICS S. Masutani FALL 2007 Consider δ Q A B C Rigid wall • A and B are control masses; C is the combined system; • A and B initially are in a state of thermodynamic equilibrium with themselves (but not with each other); • for simplicity, assume A and B are SCS;...
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This note was uploaded on 05/19/2008 for the course ME 311 taught by Professor Masutani during the Fall '07 term at Hawaii.

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Thermodynamics - ME 311 THERMODYNAMICS S Masutani FALL 2007 CHAPTER 6 The preceding lectures attempted to provide insight on how entropy can be

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