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Unformatted text preview: Solution of Homework 4 4-31 The table is to be completed using conservation of energy principle for a closed system. Analysis The energy balance for a closed system can be expressed as ) ( 1 2 1 2 out in energies etc. potential, kinetic, internal, in Change system mass and work, heat, by nsfer energy tra Net out in e e m E E W Q E E E- =- =- =- 43 42 1 43 42 1 Application of this equation gives the following completed table: Q in (kJ) W out (kJ) E 1 (kJ) E 2 (kJ) m (kg) e 2 - e 1 (kJ/kg) 280 440 1020 860 3 -53.3 -350 130 550 70 5 -96 -40 260 300 0 2 -150 300 550 750 500 1 -250 -400 -200 500 300 2 -100 4-35 An insulated rigid tank is initially filled with a saturated liquid-vapor mixture of water. An electric heater in the tank is turned on, and the entire liquid in the tank is vaporized. The length of time the heater was kept on is to be determined, and the process is to be shown on a P- v diagram. Assumptions 1 The tank is stationary and thus the kinetic and potential energy changes are zero. 2 The device is well-insulated and thus heat transfer is negligible. 3 The energy stored in the resistance wires, and the heat transferred to the tank itself is negligible. Analysis We take the contents of the tank as the system. This is a closed system since no mass enters or leaves. Noting that the volume of the system is constant and thus there is no boundary work, the energy balance for this stationary closed system can be expressed as ) ( V 0) = PE = KE (since ) ( 1 2 1 2 in , energies etc. potential, kinetic, internal, in Change system mass and work, heat, by nsfer energy tra Net out in u u m t I Q u u m U W E E E e- = =- = = =- 43 42 1 43 42 1 The properties of water are (Tables A-4 through A-6) H 2 O V = const....
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- Spring '06