Thermodynamics Notes6

Thermodynamics Notes6 - ME 311 FALL 2007 CHAPTER 3...

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ME 311 THERMODYNAMICS S. Masutani FALL 2007 CHAPTER 3 The State Postulate & Properties of Simple Substances It is clear that, to perform an energy (thermodynamic) analysis of a system, we need information on the properties of the system and relationships between properties. Recall that: 1. the state (at equilibrium) is described by the values of properties; 2. thermodynamic properties are not independently variable, e.g., from chemistry, we know that for an ideal gas, PV = NkT where P = pressure; V = volume; N = number of gas particles (atoms or molecules); k is the Boltzmann constant; and T = absolute temperature. Note that P and T are intensive properties, while V and N are extensive properties. In terms of all intensive properties, this can also be expressed as Pv = RT where v is the specific volume and R = M kN M R A = (the universal gas constant R = 8.314 J/mole; k = 1.381 x 10 -23 J/K; M is the molecular weight of the gas; and N A is the Avogadro number = 6.022 x 10 23 mol -1 ). Observe that there are three properties (unknowns; e.g., P, v, and T) in this one equation only two of these can be varied independently in situations where the above Equation of State holds. 3. Thermodynamics deals with energy and thermodynamic properties are those which are, in some way, related to energy. It follows that: The number of ways in which the energy of a given substance (or system) can be varied independently is an indicator of the number of independent thermodynamic properties. 4. Consider ways to transfer energy as work to a closed system: i. if compressible medium: energy can be increased through PdV work; ii. if magnetic: energy can be increased through magnetization work ( ) ( 0 V M d H r r μ ). it can be shown that there will be at least one independently variable property for each relevant work mode. Moreover, we can hold these properties fixed and vary energy through transfer of energy as heat. This gives us one more free variable. 1
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ME 311 THERMODYNAMICS S. Masutani FALL 2007 Look closer at this statement : = 2 1 12 W W δ where FdX W = Recall that: F generalized force dX generalized displacement If F is independent of the direction and rate of change of the process, then the amount of energy transferred to the system going from X 1 to (X 1 + dX) is exactly equal to amount transferred from the system as we move from (X 1 + dX) to X 1 the work mode is reversible: i.e., ∫∫ = 2 1 1 2 FdX FdX (note: nothing has been specified regarding the process in general case, the process path must be same) Thus, if F is a property of the thermodynamic state of the system, then the associated work mode is reversible; however, if F depends on the rate or direction of the process and not just on the
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Thermodynamics Notes6 - ME 311 FALL 2007 CHAPTER 3...

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