Chapter 5. The Second Law-student

Chapter 5. The Second Law-student - The Second Law of...

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The Second Law of Thermodynamics
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Differences between heat and work First law: the transformation of energy, heat and work. All the efforts to make perpetual motion machine have failed The second law deals with the reaction direction and efficiency All efforts to devise a process for the continuous conversion of heat completely into work or into mechanical or electrical energy have failed. Conversion efficiencies do not exceed about 40%. All efforts to convert heat to work from a single heat reservoir have failed The flow of heat between two bodies always takes place from the hotter to the cooler body, and never in the reverse direction. The second law No apparatus can operate in such a way that its only effect is to convert heat absorbed by a system completely into work done by the system. No process is possible which consists solely into the transfer of heat from one temperature level to a higher one.
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Heat engine: devices or machines that produce work from heat in a cyclic process. Essential to all heat-engine cycles are absorption of heat into the system at a high temperature, rejection of heat to the surroundings at a lower temperature, and production of work. Q H Q c Engine returns back to original position so ΔU=0
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A heat engine operating in a completely reversible manner is called a Carnot engine (N.L.S. Carnot, 1824) More commonly, Carnot engine has the following cycles: T H T C b c a d P V |Q H | |Q C | a b c a a to b: Adiabatic compression b c b to c: Isothermal expansion, adsorb Q H from hot reservoir d c to d: adiabatic expansion d d to a: Isothermal compression, discard Q c to cold reservoir Q H Q C
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T H T C b c a d P V |Q H | |Q C | For the isothermal steps b → c and d → a For adiabatic processes a → b and c → d 5 . 0 600 300 1 = - = η Actual heat engines are irreversible and η rarely exceed 0.35. For ideal gases If T C =300k and T H = 600 K,
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For two given heat reservoirs no engine can have a thermal efficiency higher than that of a Carnot engine The thermal efficiency of a Carnot engine depends only on the temperature levels and not upon the working substances of the engine. Example
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This note was uploaded on 05/01/2011 for the course CHBE 2110 taught by Professor Gallivan during the Spring '08 term at Georgia Institute of Technology.

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Chapter 5. The Second Law-student - The Second Law of...

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