tailieunhanh - Lecture Thermodynamics: An engineering approach (8/e): Chapter 7 (Part 1) - Yunus A. Çengel, Michael A. Boles
Chapter 7 (part 1) - Entropy: A measure of disorder. The objectives of Chapter 7 are to: Apply the second law of thermodynamics to processes; define a new property called entropy to quantify the second-law effects; establish the increase of entropy principle; Calculate the entropy changes that take place during processes for pure substances, incompressible substances, and ideal gases. | Chapter 7 Entropy: A Measure of Disorder Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Çengel and Michael A. Boles Entropy and the Clausius Inequality The second law of thermodynamics leads to the definition of a new property called entropy, a quantitative measure of microscopic disorder for a system. Entropy is a measure of energy that is no longer available to perform useful work within the current environment. To obtain the working definition of entropy and, thus, the second law, let's derive the Clausius inequality. Consider a heat reservoir giving up heat to a reversible heat engine, which in turn gives up heat to a piston-cylinder device as shown below. We apply the first law on an incremental basis to the combined system composed of the heat engine and the system. where Ec is the energy of the combined system. Let Wc be the work done by the combined system. Then the first law becomes If we | Chapter 7 Entropy: A Measure of Disorder Study Guide in PowerPoint to accompany Thermodynamics: An Engineering Approach, 8th edition by Yunus A. Çengel and Michael A. Boles Entropy and the Clausius Inequality The second law of thermodynamics leads to the definition of a new property called entropy, a quantitative measure of microscopic disorder for a system. Entropy is a measure of energy that is no longer available to perform useful work within the current environment. To obtain the working definition of entropy and, thus, the second law, let's derive the Clausius inequality. Consider a heat reservoir giving up heat to a reversible heat engine, which in turn gives up heat to a piston-cylinder device as shown below. We apply the first law on an incremental basis to the combined system composed of the heat engine and the system. where Ec is the energy of the combined system. Let Wc be the work done by the combined system. Then the first law becomes If we assume that the engine is totally reversible, then The total net work done by the combined system becomes Now the total work done is found by taking the cyclic integral of the incremental work. If the system, as well as the heat engine, is required to undergo a cycle, then and the total net work becomes If Wc is positive, we have a cyclic device exchanging energy with a single heat reservoir and producing an equivalent amount of work; thus, the Kelvin-Planck statement of the second law is violated. But Wc can be zero (no work done) or negative (work is done on the combined system) and not violate the Kelvin-Planck statement of the second law. Therefore, since TR > 0 (absolute temperature), we conclude or Here Q is the net heat added to the system, Qnet. This equation is called the Clausius inequality. The equality holds for the reversible process and the inequality holds for the irreversible process. Example 7-1 For a particular power plant, the heat added and rejected
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