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chapter1 - Chapter 1 Review of the First and Second Laws of...

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Chapter 1 Review of the First and Second Laws of Thermodynamics The purpose of this chapter is twofold: ( i ) to summarize the de fi nitions and concepts used in thermodynamics, ( ii ) to explain what the fi rst and second laws of thermodynamics are and show how to simplify the equations expressing these laws for various systems. 1.1 DEFINITIONS 1.1.1 System Any region which occupies a volume and has a boundary is called a system . The volume outside the boundary is called the surroundings of the system. The sum of the system and its surroundings is called the universe . Thermodynamics considers systems only at the macroscopic level. It is convenient to distinguish between three general types of systems. Isolated system : These are the set of systems which exchange neither mass nor energy with the surroundings. For example, the universe is an isolated system. Closed system : These are the set of systems which exchange energy (in the form of heat and work) but not mass with the surroundings. Open system : These are the set of systems which exchange both mass and energy with the surroundings. The equations available to analyze closed and open systems are di ff erent from each other. Therefore, one should properly de fi ne the system before solving the problem. 1.1.2 State In order to describe and analyze a system, some of the quantities that are characteristic of it must be known. These quantities are called properties and include volume, mass, temperature, pressure, etc. A complete list of the properties of a system describes its state . Consider a function w = f ( x, y ) (1.1-1) in which there are three variables: w is dependent, x and y are independent. In thermodynamics we would say that " the state of the system, w , is fi xed when the thermodynamic properties x and y are speci fi ed ." Note that the mathematical term "point" is equivalent to the thermodynamic term "state". A change of state is called a process , which can occur in a number of ways. Work and heat can occur only during processes and only across the boundary of the system. The path followed in going from one state to another is known as the process path . 1
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1.1.3 Intensive and Extensive Properties Thermodynamic properties are considered to be either intensive or extensive . When the prop- erty is proportional to the mass of the system, the property is extensive , i.e., volume, kinetic energy, potential energy. On the other hand, when the property is independent of the mass of the system, the property is intensive , i.e., viscosity, refractive index, density, temperature, pressure, mole fraction. Speci fi c (or molar) properties are extensive properties divided by the total mass (or total moles) of the system, i.e., Speci fi c property = Extensive property Total mass Molar property = Extensive property Total moles (1.1-2) If ϕ represents any extensive property, then Eq. (1.1-2) is expressed as b ϕ = ϕ m and e ϕ = ϕ n (1.1-3) where m and n are the total mass and moles, respectively. Note that all speci fi c (or molar) properties are intensive.
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