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# chapter20 - Reversibility Processes in nature are always...

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1 Reversibility Reversible process: idealized process infinitely close to thermodynamic equilibrium (quasi-equilibrium) Processes in nature are always irreversible : far from equilibrium Necessary conditions for Reversibility 1. No work must be done by friction, viscous forces, or other dissipative forces that produce heat (no dissipation). 2. Heat conduction can only occur isothermally. 3. The process must be quasi-static so that the system is always in an equilibrium state (or infinitesimally close to one).

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2 CPS question A. a b B. b c C. c a D. two or more of A., B., and C. E. none of A., B., or C. An ideal gas is taken around the cycle shown in this pV –diagram, from a to b to c and back to a . Process b c is isothermal. Which of the processes in this cycle could be reversible ? Natural processes Consider a free expansion: Why is this process irreversible? Why the gas doesn’t compress itself to its original volume? (it wouldn’t violate the 1 st law) We have missed something… We need another principle…
3 Heat engines •Heat flows from the hot reservoir to the cold reservoir, producing work •The process is cyclic Work during a cycle: | | | | C H C H Q Q Q Q Q W - = + = = We define thermal “efficiency” as H C H C H Q Q Q Q Q W e - = + = = 1 1 Refrigerators •Mechanical work induces a heat flow from the cold reservoir to the hot reservoir •The process is cyclic Work during a cycle: | | | | | | H C C H Q Q Q Q Q W - = + = = - We define the”coefficient of performance” as | | | | | | | | | | C H C C Q Q Q W Q K - = =

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4 2 nd Law of Thermodynamics
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