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Chapter 2 - revised

# Chapter 2 - revised - Chapter 2 Some concepts and denitions...

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Chapter 2 Some concepts and definitions Read BS, Chapter 2 2.1 Thermodynamic system and control volume We take the following definitions: Thermodynamic system : a quantity of fixed mass under investigation, Surroundings : everything external to the system, System boundary : interface separating system and surroundings, and Universe : combination of system and surroundings. The system, surroundings, system-boundary for a universe are shown for a potato-shaped system in Fig. 2.1. We allow two important interactions between the system and its sur- roundings: heat can cross into the system (our potato can get hot), and work can cross out of the system (our potato can expand). Now the system boundaries can change, for example the potato might expand on heating, but we can still distinguish the system and the surroundings. We now define an isolated system : a system which is not influenced by its surroundings. Note that a potato with thick and inelastic skin will be isolated. We distinguish the system, which has constant mass, but possible variable volume, from the Control Volume : fixed volume over which mass can pass in and out of its boundary. The control volume is bounded by the 27

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28 CHAPTER 2. SOME CONCEPTS AND DEFINITIONS system surroundings system boundary heat in work out universe Figure 2.1: Sketch of a universe composed of a system, its surroundings, and the system boundary. Control Surface : boundary of the control volume. The mass within a control volume may or may not be constant. If there is fluid flow in and out there may or may not be accumulation of mass within the control volume. We will mainly study cases in which there is no accumulation, but this need not be the case. A sketch contrasting scenarios in which a fluid is compressed in which the system approach would be used against those where the control volume approach would be used is shown in Fig. 2.2. In summary, system fixed mass, closed, and control volume potentially variable mass, open. 2.2 Macroscopic versus microscopic In principle, we could solve for the forces acting on every molecule and use Newton’s laws to determine the behavior of systems. This is difficult for even modestly sized systems. If we had a volume of 1 m 3 of gas at atmospheric pressure and temperature, we would find that it was composed of 2 . 4 × 10 25 molecules. We would need six equations of motion for each molecule, three for x,y,z , position, and three for u,v,w velocity. This would require then a total of 1 . 4 × 10 26 differential equations to solve simultaneously. CC BY-NC-ND. 2011, J. M. Powers.
2.2. MACROSCOPIC VERSUS MICROSCOPIC 29 piston-cylinder compressor system approach control volume approach system flow in flow out control volume Figure 2.2: Comparison of system (fixed mass) and control volume (fixed volume) approaches in thermodynamics for two common scenarios: piston-cylinder compression (left) and com- pression in a flow device whose details are not shown (right).

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Chapter 2 - revised - Chapter 2 Some concepts and denitions...

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