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Unformatted text preview: Chapter 4 Energy Transfer by Heat, Work, and Mass 4-1 Chapter 4 ENERGY TRANSFER BY HEAT, WORK, AND MASS Heat Transfer and Work 4-1C Energy can cross the boundaries of a closed system in two forms: heat and work. 4-2C The form of energy that crosses the boundary of a closed system because of a temperature difference is heat; all other forms are work. 4-3C 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. 4-4C It is a work interaction. 4-5C It is a work interaction since the electrons are crossing the system boundary, thus doing electrical work. 4-6C It is a heat interaction since it is due to the temperature difference between the sun and the room. 4-7C 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). 4-8C 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. 4-9C The caloric theory is based on the assumption that heat is a fluid-like 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 4-10C It represents the boundary work for quasi-equilibrium processes. 4-11C Yes. 4-12C The area under the process curve, and thus the boundary work done, is greater in the constant pressure case. 4-13C 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 4-2 4-14 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 quasi-equilibrium. Properties Noting that the pressure remains constant during this process, the specific volumes at the initial and the final states are (Table A-4 through A-6) /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). 4-15 Refrigerant-134a 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
- Heat Transfer