Ch18 - Chapter 18: Heat,Work,the First Law of...

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Chapter 18: Heat,Work,the First Law of Thermodynamics Thermodynamic systems ± Thermodynamic system Thermodynamic system is any collection of objects that is convenient to regard as a unit, and that may have the potential to exchange energy with its surroundings. A process in which there are changes in the state of a thermo- dynamic system is called a thermodynamic process. ± Signs for heat and work in thermodynamics Q W Q is + if heat is added to system Q is if heat is lost by system W is + if work is done by the system. W is if work is done on the system.
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Work done during volume changes ± Work done Piston in a cylinder moves by dl due to expansion of gas at pressure p Force on piston: F = p·A Incremental work done: dW = F·dl = p·A dl = p·dV ∫∫ = = f i V V pdV dW W : done work Total p
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Work done during volume changes (cont’d) ± Work and pV diagram p p A p A p B Work done depends on the path taken. • Process A B : If V B >V A , W > 0 • Process B A : If V B >V A , W < 0 • Process A B : If V A =V B , W = 0 = = B A V V pdV W curve V - p under area
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Work done during volume changes (cont’d) ± Work and pV diagram (cont’d) Work done depends on the path taken. C A C : W = 0 C B : W = p B (V B -V A ) B C : W = p B (V A -V B ) D A C B : W = p B (V B -V A ) A D : W = p A (V B -V A ) D B : W = 0 p p A p B A D B : W = p A (V B -V A )
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Internal energy and the first law of thermodynamics ± Internal energy Tentative definition: The internal energy U of a system is the sum of the kinetic energies of all of its constituent particles, plus the sum of all the potential energies of interactions among these particles. ± The first law of thermodynamics W Q U U U = = 1 2 Now define the change in internal energy using the first law of thermodynamics. It is known from experiments that while Q and W depend on the path, U does not depend on the path.
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Internal energy and the first law of thermodynamics ± A cyclic process A B : is a process in which the initial and final states are the same. p A p B upper A B lower B A upper A B lower B A W W W W < < > , 0 , 0 V A V B 0 < + = upper A B lower B A A B A W W W If the directions of the arrows are reversed, then 0 > + = lower A B upper B A A B A W W W
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Internal energy and the first law of thermodynamics ± Infinitesimal changes in state The first law of thermodynamics: dW dQ dU = For the system we will discuss, the work dW is given by pdV pdV dQ dU =
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Ch18 - Chapter 18: Heat,Work,the First Law of...

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