Thermodynamics Chapter 7 Review Notes

Thermodynamics Chapter 7 Review Notes - Thermodynamics...

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Thermodynamics Chapter 7 Review Notes: Entropy 7.1 Entropy - the second law of thermodynamics leads to inequalities, namely, an irreversible prosess is less efficient than a reversible one. Clausius Inequality: 0 T Q δ - this indicates that the cyclic integral is always less than or equal to zero. - The change in heat per unit time is always negative (this is entropy) - This equation is proved on page 338 - This inequality is valid for ALL thermodynamic cycles! - This is only equal to zero during a reversible cycle - The cyclic integral of volume is also zero! o This is because for a cycle it has to return to original state! - This clausius integral is based only on state and not process! Therefore it is a property. Entropy – S – disorder in a system: = - = = 2 1 1 2 T Q S S S T Q dS - this is actually the chane in entropy. - Oonly applies over a reversible path Special Case: Internally Reversible Isothermal Heat Transfer Processes - Equation for special process: 0 T Q S = - T = the constant temperature of the system and Q = the heat transferred 7.2 The Increase of Entropy Principle - in every process the following equation applies: T Q dS - This means that in every process, entropy is generated! - Entropy generation is always positive or zero and depends on the process – therefore, ti is not a property of a substance. Entropy Generation – the amount of entropy generated during a process + = - = 2 1 1 2 gen sys S T Q S S S - for a simple adiabatic closed system, ΔS isolated 0 o the entropy of an isolated system during a process always increases, or in the case of a reversible process, remains constant! – this is known as the Increase in entropy principle - gs surroundin sys total gen S S S S + = = Some Remarks about entropy 1. process can occur in a certain direction only, not in any direction!
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2. Entropy is nonconservative property and there is no such thing as the conservation of entropy principle! 3. the performance of engineering systems is degraded by the presence of irreversibilities and the entropy generation is a measure of the magnitudes of irreversibilities associate with a process. 7.3 Entropy Change of Pure Substances - entropy is assigned a value based on a reference point – (0 degrees for water) - for a saturated mixture: fg f xS S S + = - for a saturated or compressed liquid: T f p T s s @ , @ 2245 - Entropy change is: ) ( 1 2 s s m s m S - = = - When analyzing entropy, you should always draw a TS diagram of what is
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Thermodynamics Chapter 7 Review Notes - Thermodynamics...

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