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ME200 - Lecture 220

# ME200 - Lecture 220 - Law ME200 ThermodynamicsI Lecture22...

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10/18/2010 1 Thermodynamic Cycles and Second Law ME 200 Thermodynamics I L 22 Purdue University , Dr. Tim Pourpoint – [email protected] Lecture 22 October 18 th , 2010 Last Lecture • Second Law of Thermodynamics – Direction of Thermodynamic Processes – Clausius Statement of the Second Law – Thermal Energy Reservoir Hot coffee does not get hotter in cooler room Thermal Energy Reservoir – Kelvin – Planck Statement – Entropy Statement – Reversible and Irreversible Processes All known processes must satisfy both the 1 st Heat engine must reject heat Serve as idealized models to which actual processes ME 200 2 and 2 nd laws to occur A body that can absorb or reject a finite amount of thermal energy without any change in temperature can be compared

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10/18/2010 2 • More on the entropy statement of 2 nd law • Heat Engines • Refrigerators and heat pumps P l i hi This Lecture • Perpetual motion machines • Carnot cycle ME 200 3 From last lecture: • It is impossible for any system to operate in a way that t i d t d Entropy Statement entropy is destroyed. – Entropy can be generated or transferred but not destroyed. This statement of the second law will make more sense after we define entropy of course. At this point we can think of entropy as a measure of: ME 200 4 the disorder in a thermodynamic system, OR: the amount of energy which does no work during energy conversions.
10/18/2010 3 Entropy Statement • Let’s consider a small "universe“… i.e. a thermodynamic system consisting of: – A warm room (i.e., the "surroundings”) – A "system" consisting of glass, ice, cold water • In this universe, some thermal energy δ Q from the warmer room surroundings (at 298 K or 25 C) will spread out to the cooler system of ice and water at its constant temperature T of 273 K (0 C), the melting temperature of ice. • The entropy of the system will change by: ME 200 5 dS syst = δ Q/T δ Q /273 K Note: In this process, δ Q is the energy required to change water from the solid state to the liquid state, the enthalpy of fusion. • The entropy of the surroundings will change by: dS sur. = δ Q /298 K Entropy of the system increases ( dS syst > 0) E f h di d ( dS 0) Entropy Statement Entropy of the surroundings decreases ( sur.

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