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2303_-_Spr_2010_-_Week_4_-_Entropy

# 2303_-_Spr_2010_-_Week_4_-_Entropy - Week 4 1 Agenda HW...

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Week 4 – 1 Agenda HW Review Engines 2nd Law of Thermodynamics Reading ch. 20 11 Steam engine at K. U. Leuven Museum mech.kuleuven.be

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20-1 Beyond Energy Conservation Certain processes do not occur spontaneously even though energy would be conserved, such as a lukewarm drink forming ice cubes and becoming hot. The second law of thermodynamics accounts for this: 1. Thermal energy cannot be entirely converted to work. 2. Spontaneous heat flow always goes from the higher-temperature body to the lower-temperature one. 22
20-1 Engines 1. An engine must work in cycles to be useful – otherwise, when one cycle does work the engine stops. 2. A cyclic engine must include more than one thermal reservoir. A heat engine is a device that cyclically transforms thermal energy into mechanical energy. 33 c h Q Q W - =

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20-1 Engines: Efficiency Every engine has an efficiency, which is a measure of what fraction of the input heat flow becomes mechanical work: Efficiency
20-1 Spontaneous Processes Compressed gas in a bottle will expand when bottle is opened, but won’t spontaneously go back in – why not? Add dye to water – dye diffuses, and won’t spontaneously go back. Answer: the fraction of all possible distributions that have the gas back in the bottle, or dye in one spot is impossibly small 55 wiley.co

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20-2 The Second Law of Thermodynamics There are many ways of expressing the second law of thermodynamics: Heat will not spontaneously flow from a lower temperature object to a higher temperature object. In an isolated system, a process can occur only if it increases the total entropy of the system. (R. Clausius) One cannot convert heat completely into work. 66 www.wiley.com
20-2 The Second Law of Thermodynamics There are also two historical formulations: The Kelvin form: It is impossible to construct a cyclic engine that converts thermal energy from a body into an equivalent amount of mechanical work without a further change in its surroundings. The Clausius form: It is impossible to construct a cyclic engine whose only effect is to transfer thermal energy from a colder body to a hotter body. 77

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Next Time 4 - 3 Agenda   Carnot Cycle  Stirling Engine  Diesel Engine  Refrigerator Reading: 20.3 – 20.4 88
Quiz #1 99

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Week 4 – 3 10 10 Agenda Carnot Cycle Stirling Cycle Other Cycles Refrigerators Heat Pumps
20-3 The Carnot Cycle An idealized version of an engine has two reservoirs at  different temperatures  Th  and  Tc , and follows a  reversible   cycle known as the Carnot cycle. The Carnot cycle  has four steps: 1.  Isothermal expansion 2.  Adiabatic expansion 3.  Isothermal compression 4.  Adiabatic compression 11 11

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20-3 The Carnot Cycle The work done during the Carnot cycle is the area enclosed  by the curve in the  p-V  diagram:  12 12
13 13

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20-3 The Carnot Cycle The efficiency of the Carnot cycle is This can be related to the reservoir temperatures: 14 14
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2303_-_Spr_2010_-_Week_4_-_Entropy - Week 4 1 Agenda HW...

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