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BasicThermo3

# BasicThermo3 - Review of classical thermodynamics...

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MSE 3050, Phase Diagrams and Kinetics, Leonid Zhigilei Review of classical thermodynamics Fundamental Laws, Properties and Processes (3) Fundamental equations The Helmholtz Free Energy The Gibbs Free energy Changes in composition Chemical potential Thermodynamic relations Reading: Chapter 5.1 5.9 of Gaskell or the same material in any other textbook on thermodynamics

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MSE 3050, Phase Diagrams and Kinetics, Leonid Zhigilei Fundamental equations Combined statement of 1 st and 2 nd Laws of Thermodynamics: dU = TdS – PdV This equation gives us ¾ Relationship between the dependent variable U and independent variables V and S: U = U(S,V) or dU = ( w U/ w S) v dS + ( w U/ w V) s dV ¾ The criteria for equilibrium: in a system of constant V and S, the internal energy has its minimum value, or, in a system of constant U and V, the entropy has its maximum value. The problem is that the pair of independent variables (V,S) is rather inconvenient entropy is hard to measure or control. We want to have fundamental equations with independent variables that is easier to control. The two convenient choices are: P and T pair the best choice from the practical point of view, easy to control/measure. For systems with constant pressure the best suited state function is the Gibbs free energy (also called free enthalpy) G = H - TS V and T pair easy to examine in statistical mechanics. For systems with constant volume (and variable pressure), the best suited state function is the Helmholtz free energy A = U – TS Any state function can be used to describe any system (at equilibrium, of course), but for a given system some are more convenient than others.
MSE 3050, Phase Diagrams and Kinetics, Leonid Zhigilei The Helmholtz Free Energy A = U – TS dA = dU – TdS – SdT Combining this equation with dU = TdS – PdV we get dA = – PdV – SdT - fundamental equation A = A(T,V) dA = ( w A/ w T) V dT + ( w A/ w V) T dV Comparing the equations we see that S = – ( w A/ w T) V P = – (

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