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Understanding the Second Law of Thermodynamics Through Hypothetical Devices

This document explores various hypothetical devices that illustrate the principles of the second law of thermodynamics. We analyze a device extracting 1000 J of heat from a hot reservoir at 300 K, ejecting 200 J to a cold reservoir at 100 K, and producing 800 J of work. Additionally, we look at a refrigerator that takes 1000 J from a cold reservoir, ejects 1200 J to a hot reservoir, and assess the implications on entropy. Results demonstrate a violation of thermodynamic principles, specifically regarding heat flow direction and efficiency limits.

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Understanding the Second Law of Thermodynamics Through Hypothetical Devices

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  1. correct Chapter 15,Problem 3 Consider a hypothetical device that takes 1000 J of heat from a hot reservoir at 300K, ejects 200 J of heat to a cold reservoir at 100K, and produces 800 J of work. Does this device violate the second law of thermodynamics ? 1. Yes 2. No • W (800) = Qhot (1000) - Qcold (200) • Efficiency = W/Qhot = 800/1000 = 80% • Max eff = 1 - 100/300 = 67% .

  2. TH QH W QC TC Chapter 15,Problem 4 Consider a hypothetical refrigerator that takes 1000 J of heat from a cold reservoir at 100K and ejects 1200 J of heat to a hot reservoir at 300K. 1. How much work does the refrigerator do? 2. What happens to the entropy of the universe? 3. Does this violate the 2nd law of thermodynamics? Answers: 200 J Decreases yes QC = 1000 J QH = 1200 J Since QC + W = QH, W = 200 J DSH = QH/TH = (1200 J) / (300 K) = 4 J/K DSC = -QC/TC = (-1000 J) / (100 K) = -10 J/K DSTOTAL = DSH + DSC = -6 J/K  decreases (violates 2nd law)

  3. Chapter 15,Problem 5 Which of the following is forbidden by the second law of thermodynamics? 1. Heat flows into a gas and the temperature falls 2. The temperature of a gas rises without any heat flowing into it 3. Heat flows spontaneously from a cold to a hot reservoir 4. All of the above Answer: 3

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