FIRST LAW OF THERMODYNAMICSThermodynamicsHeat and/or work can be directed into or out of the control volume. But, for convenience andas a standard convention, the net energy exchange is presented here with the net heat exchangeassumed to be into the system and the net work assumed to be out of the system. If no masscrosses the boundary, but work and/or heat do, then the system is referred to as a "closed"system. If mass, work and heat do not cross the boundary (that is, the only energy exchangestaking place are within the system), then the system is referred to as an isolated system. Isolatedand closed systems are nothing more than specialized cases of the open system. In this text, theopen system approach to the First Law of Thermodynamics will be emphasized because it ismore general. Also, almost all practical applications of the first law require an open systemanalysis.An understanding of the control volume concept is essential in analyzing a thermodynamicproblem or constructing an energy balance. Two basic approaches exist in studyingThermodynamics: the control mass approach and the control volume approach. The former isreferred to as the LeGrange approach and the latter as the Eulerian approach. In the control massconcept, a "clump" of fluid is studied with its associated energies. The analyzer "rides" with theclump wherever it goes, keeping a balance of all energies affecting the clump.Figure 15 Control Volume ConceptsHT-01 Page 56 Rev. 0
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