5_60_lecture2

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MIT OpenCourseWare http://ocw.mit.edu 5.60 Thermodynamics & Kinetics Spring 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .

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5.60 Spring 2008 Lecture #2 page 1 Work, Heat, and the First Law Work: w = F A applied force distance A Expansion work F = p ext A p ext ext p w = − ( p ext A ) A = − p ext V convention: Having a “-“ sign here implies w > 0 if V < 0 , that is, positive work means that the surroundings do work to the system. If the system does work on the surroundings ( V > 0 ) then w < 0 . If p ext is not constant, then we have to look at infinitesimal changes d - w = − p ext dV d - means this is not an exact differential Integral w = − 1 2 p ext dV depends on the path!!! Path dependence of w Example: assume a reversible process so that p ext = p Ar (g, p 1 , V 1 ) = Ar (g, p 2 , V 2 ) Compression V 1 > V 2 and p 1 < p 2
5.60 Spring 2008 Lecture #2 page 2 p ext = p 1 p ext = p 2 compression ,V p 1 1 initial Two paths: (1) First V 1 V 2 at p = p 1 then p 1 p 2 at V = V 2 Ar(g, p 1 , V 1

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