lecture_10_04_06_2011 - In this lecture you will learn more...

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In this lecture you will learn more about entropy and how to calculate entropy changes for an ideal gas. Lecture 10
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A thermodynamic process is reversible if at any stage, a differential, infinitesimal decrease in the variables causing a change will result in a reversal of the process itself, both in direction and all its quantitative effects. After a reversible change, it is possible to restore both the system and surroundings to their original conditions. Recall that we discussed an idealized process called reversible A transfer of heat could be reversible if the body accepting the heat were always infinitesimally cooler than the body losing the heat. To transfer a finite amount of energy or to do a finite amount of work by a reversible process would require an infinite time. A reversible process is a limiting case which may be closely approximated but never actually realized. Reversible processes are most useful for logical analysis from H.C. Weber, Thermodynamics for Chemical Engineers, John Wiley and Sons, 1939 (pg. 11).
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0 out in c h Q Q T T - = defines entropy difference ΔS Recall that when analyzing the Carnot engine, we found for a reversible process
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The thermodynamic function called S “entropy” is a state function, just like P, V, T and E int . P V S V Because entropy is a state function, the change in entropy depends ONLY on the initial and final states of the system, not on the details of the process by which the change occurs. KEY IDEAS Entropy measures the amount of microscopic disorder in a thermal system.
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dQ rev is defined as the heat in a reversible process required to drive the system from an initial to a final state. In principle, the above equation only applies when transfers of 0 = = rev dQ dS T Defining Entropy Change energy occur as heat flow and when any volume change occurs quasistatically. Reversible processes do not occur in nature. For irreversible processes, the above equation becomes 0 = actual dQ dS T
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“New” thermodynamic variable Thermodynamic state of a system ? ? S (Entropy) Allowed, irreversible llowed, ENTROPY (from the Greek, “to turn”, as in “to turn useful energy into useless energy”) time New idea : The entropy of a closed system can never decrease. Once the entropy of a system increases , it cannot return to its original value Which processes are allowed? Which processes are forbidden? ? ? ? time Allowed, reversible Forbidden
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Calculating the entropy change of the “universe” in an isothermal process system dQ reservoir dS reservoir =-dQ/T res T res Heat lost but temperature of reservoir does not change dS system =dQ/T system T system u=“Universe” In a reversible process, the entropy change of the universe (ΔS u ) is zero u system reservoir S dS dS = + Absorbs heat but temperature of system does not change
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u system res system res S dS
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This note was uploaded on 04/23/2011 for the course PHYS 242 taught by Professor Staff during the Spring '08 term at Purdue University-West Lafayette.

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lecture_10_04_06_2011 - In this lecture you will learn more...

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