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lecture5 - 8/30/2010 MovingBoundaryWork:...

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8/30/2010 1 Moving Boundary Work: Expansion and Compression Work ME 200 Thermodynamics I Purdue University , Dr. Tim Pourpoint – timothee@purdue.edu Lecture 5 September 1 st , 2010 Last Lecture •Some concepts from mechanics: – Kinetic and potential energy –Wo rk – Power •Two fundamental energy interactions between thermodynamic system and its surroundings –Hea t transfer ME 200 2 – Work
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8/30/2010 2 Today’s agenda Moving boundary work Concepts of: Non quasi static work This Lecture Non quasi static work And: Cyclic work ME 200 3 Acknowledgement: Prof. Eckhard A. Groll (ME) for slides •Re ca l l from Lecture 4: mechanical work involves a force acting through a distance Introduction – Constant force: Fds W s •Mo v ing boundary work: – Form of mechanical work –Va r iab le force: 2 1 Fds W ME 200 4 –In te ra c t ion between system and surroundings: •Mu s t have a force acting on boundary • Boundary must move (e.g. force must act through a distance) •I f either is absent, then no boundary work
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8/30/2010 3 Area under process path on P V diagram represents QE boundary work •Ga s can follow several different paths as it expands from Moving Boundary Work State 1 to State 2 • Different areas so different work WORK IS A PATH FUNCTION ME 200 5 Boundary Work •Con s ide r Piston Cylinder Device: Forces on the Piston p atm (Force Balance): p atm A F rod ME 200 6 Assumes no acceleration of the piston. p gas AF friction rod atm friction gas F pAF pA 
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8/30/2010 4 Boundary Work •E x p a n s i o n or compression of a gas in a piston cylinder device •C a n only evaluate work analytically if we have a QE process (slow) BECAUSE: work is a path function and we need to know the path p atm note WF d s Fp A Wp A d s Ads dV For the Control Mass (Gas inside the Piston Cylinder): ME 200 7 2 1 W pdV W pdV  CM Boundary Work xpan s ion or compression work: Moving boundary work –I f the process is too fast, equilibrium is nor reached at every point 2 1 W pdV ME 200 8 f the process is slow, near equilibrium conditions can be achieved: Quasi Equilibrium Process or Quasi Static Process
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8/30/2010 5 Boundary Work •W h a
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lecture5 - 8/30/2010 MovingBoundaryWork:...

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