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Unformatted text preview: Work w What are the types of work? See table 2.1 Now for mechanical work in particular, work involved in changing the size (volume) of the system. First. . comments about pressure. What are the units? Pascal, bar, atm., standard pressure. Expansion of a gas in a piston-cylinder machine Derive: ext dw P dV = -and 2 1 ( ) V ext V w P V dV = -Sign of w: negative Compression of gas in piston-cylinder machine: 2 1 ( ) V ext V w P V dV = -Sign of w: positive Note: w depends on P ext , NOT P int (see just below equation 2.6) Plot these processes on a diagram: Plot P ext vs. V Sign of the area; sign of w w is a path function (like q) The first law of thermodynamics Law of conservation of energy applied to q and w Introduce a new function: U, the internal energy Here is the first law: 2 1 U U U q w =-= + Also. . U is a state function The sum of two path functions gives a state function!...
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