5_60_lecture11

# 5_60_lecture11 - MIT OpenCourseWare http/ocw.mit.edu 5.60...

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MIT OpenCourseWare http://ocw.mit.edu 5.60 Thermodynamics & Kinetics Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .

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1 5.60 Spring 2008 Lecture #11 page Fundamental Equations, Absolute Entropy, and The Third Law Fundamental Equations relate functions of state to each other using 1 st and 2 nd Laws 1 st law with expansion work: dU = đ q - p ext dV need to express đ q in terms of state variables because đ q is path dependent Use 2 nd law: đ q rev = TdS For a reversible process p ext = p and đ q = đ q rev =TdS So…… ** dU = TdS – pdV ** This fundamental equation only contains state variables Even though this equation was demonstrated for a reversible process, the equation is always correct and valid for a closed (no mass transfer) system, even in the presence of an irreversible process. This is because U, T, S, p, and V are all functions of state and independent of path. AND The “best” or “natural” variables for U are S and V, ** U(S,V) **
_______________ 2 5.60 Spring 2008 Lecture #11 page ** U(S,V) ** U U From dU = TdS – pdV ** = T ; = − p ** S V V S We can write similar equations for enthalpy H = U + pV dH = dU + d(pV) = dU + pdV + Vdp inserting dU = TdS – pdV ** dH = TdS + Vdp ** The natural variables for H are then S and p ** H(S,p) ** H H From dH = TdS + Vdp ** = T ; = V ** S p p S We can use these equations to find how S depends on T. S

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5_60_lecture11 - MIT OpenCourseWare http/ocw.mit.edu 5.60...

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