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Unformatted text preview: Chapter 4 Energy Transfer by Heat, Work, and Mass 41 Chapter 4 ENERGY TRANSFER BY HEAT, WORK, AND MASS Heat Transfer and Work 41C Energy can cross the boundaries of a closed system in two forms: heat and work. 42C The form of energy that crosses the boundary of a closed system because of a temperature difference is heat; all other forms are work. 43C An adiabatic process is a process during which there is no heat transfer. A system that does not exchange any heat with its surroundings is an adiabatic system. 44C It is a work interaction. 45C It is a work interaction since the electrons are crossing the system boundary, thus doing electrical work. 46C It is a heat interaction since it is due to the temperature difference between the sun and the room. 47C This is neither a heat nor a work interaction since no energy is crossing the system boundary. This is simply the conversion of one form of internal energy (chemical energy) to another form (sensible energy). 48C Point functions depend on the state only whereas the path functions depend on the path followed during a process. Properties of substances are point functions, heat and work are path functions. 49C The caloric theory is based on the assumption that heat is a fluidlike substance called the "caloric" which is a massless, colorless, odorless substance. It was abandoned in the middle of the nineteenth century after it was shown that there is no such thing as the caloric. Boundary Work 410C It represents the boundary work for quasiequilibrium processes. 411C Yes. 412C The area under the process curve, and thus the boundary work done, is greater in the constant pressure case. 413C 1 kPa m 1 k(N /m ) m 1 kN m 1 kJ 3 2 3 = = = Chapter 4 Energy Transfer by Heat, Work, and Mass 42 414 Saturated water vapor in a cylinder is heated at constant pressure until its temperature rises to a specified value. The boundary work done during this process is to be determined. Assumptions The process is quasiequilibrium. Properties Noting that the pressure remains constant during this process, the specific volumes at the initial and the final states are (Table A4 through A6) /kg m 1.3162 C 300 kPa 200 /kg m 0.8857 vapor Sat. kPa 200 3 2 2 2 3 kPa 200 @ 1 1 = = = = = = v T P v v P g o Analysis The boundary work is determined from its definition to be kJ 430.5 m kPa 1 kJ 1 /kg m 0.8857) 2 kPa)(1.316 kg)(200 (5 ) ( ) ( 3 3 1 2 2 1 1 2 out , = === = v v mP V V P dV P W b Discussion The positive sign indicates that work is done by the system (work output). 415 Refrigerant134a in a cylinder is heated at constant pressure until its temperature rises to a specified value. The boundary work done during this process is to be determined....
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This note was uploaded on 06/25/2010 for the course MEEN 315 taught by Professor Ramussen during the Spring '07 term at Texas A&M.
 Spring '07
 RAMUSSEN
 Heat Transfer

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