MasteringPhysics20 - MasteringPhysics Assignment Print View...

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View Full Document Right Arrow Icon .. 1 of 18 12/19/2007 1:09 PM [ Print View ] PHCC 141: Physics for Scientists and Engineers I - Fall 2007 20. Second Law of Thermodynamics Due at 11:59pm on Thursday, December 6, 2007 Hide Grading Details Number of answer attempts per question is: 8 You gain credit for: correctly answering a question in a Part, or correctly answering a question in a Hint. You lose credit for: exhausting all attempts or requesting the answer to a question in a Part or Hint, or incorrectly answering a question in a Part. Late submissions: reduce your score by 100% over each day late. Hints are helpful clues or simpler questions that guide you to the answer. Hints are not available for all questions. There is no penalty for leaving questions in Hints unanswered. Grading of Incorrect Answers For Multiple-Choice or True/False questions, you lose 100% / ( # of options - 1 ) credit per incorrect answer. For any other question, you lose 3% credit per incorrect answer. Heat Engines and Carnot Cycle Heat Engines Introduced Learning Goal: To understand what a heat engine is and its theoretical limitations. Ever since Hero demonstrated a crude steam turbine in ancient Greece, humans have dreamed of converting heat into work. If a fire can boil a pot and make the lid jump up and down, why can't heat be made to do useful work? A heat engine is a device designed to convert heat into work. The heat engines we will study will be cyclic: The working substance eventually returns to its original state sometime after having absorbed a quantity of heat and done some work. A cyclic heat engine cannot convert heat into work without generating some waste heat in the process. Although by no means intuitively obvious, this is an important fact of nature, since it dramatically affects the technology of energy generation. If it were possible to convert heat into work without any waste heat, then one would be able to build refrigerators that are more than 100% efficient! Consequently, the "impossible heat engine" pictured schematically here cannot exist, even in theory. Engineers tried hard for many years to make such a device, but Sadi Carnot proved in 1824 that it was impossible. The next figure shows an "ideal" heat engine, one that obeys the laws of thermodynamics. It takes in heat at a temperature and does work . In the process of doing this it generates waste heat at a cooler temperature . Take and to be the magnitudes of the heat absorbed and emitted, respectively; therefore both quantities are positive. Part A A heat engine is designed to do work. This is possible only if certain relationships between the heats and temperatures at the input and output hold true. Which of the following sets of statements must apply for the heat engine to do work? ANSWER:
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MasteringPhysics20 - MasteringPhysics Assignment Print View...

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