Notes_16_Introduction_to_the_Second_Law

Notes_16_Introduction_to_the_Second_Law - Meeting...

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1 Meeting 16--Sections 6-1–6-7 Second Law of Thermodynamics The Watterson statement of the Second Law:
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Example 2 An insulated rigid tank is initially evacuated. A valve is opened, and atmospheric air at 100 kPa and 27ºC enters the tank until the pressure in the tank reaches 100 kPa, at which point the valve is closed. Determine the final temperature of the air in the tank. Assume constant specific heats at room temperature.
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Team Play 3 On most mornings, the first thing I do after kissing my wife good morning, is go to the kitchen, pour a hot cup of coffee (~10 oz, mass) and turn on The Weather Channel. Fresh brewed coffee is at a temperature of about 180 F which is scaldingly hot. So, I put a couple (~3) of ice cubes (~1 oz per cube, mass) in it. This morning, 2 Btu of heat transfer occurs between the ice and coffee causing the ice to get colder and the coffee to get hotter. Assuming ice (Cp=0.54504Btu/lbm/ºR) comes out of the freezer at 0ºF, coffee can be modeled water, and the mug is well insulated; determine the final temperature of the ice and coffee.
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4 2nd Law of Thermodynamics will help us: determine direction of change for a process, establish the final equilibrium state, define ideal performance for energy conversion devices, determine the quality of energy, determine if a process is irreversible, and define an absolute temperature scale.
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5 2nd Law of Thermodynamics It has implications for economics, theology and philosophy, and other fields. These make fun and interesting discussions…however, we’ll focus on engineering applications of 2nd law. It is a matter of experience, of observations of what happens in nature….experimental evidence.
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6 2nd Law of Thermodynamics It informs about quality of energy and direction of flow; thus, it will be stated in terms of inequalities . It’s a non-conservation law It will typically be negatively stated - tell us what we can’t do No one statement covers all facets of the second law Let’s look at some now.
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7 A sample of statements regarding direction of processes All spontaneous processes result in a more probable state. Heat energy cannot of itself pass from a colder to hotter body. Once you open a can of worms, the only way to recan them is with a larger can. Let’s try to be more scientific!
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Clausius Statement of 2nd Law of Thermodynamics: It is impossible to operate a cyclic device in such a manner that the sole effect external to the device is the transfer of heat from one heat reservoir to another at a higher temperature. Okay, so what’s a thermal reservoir?
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This note was uploaded on 10/04/2011 for the course MEEN 315 taught by Professor Ramussen during the Summer '07 term at Texas A&M.

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Notes_16_Introduction_to_the_Second_Law - Meeting...

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