Thermodynamics

Chapter 4 | 175P, kPaH 2 Om = 25 gP 1 = P 2 = 300 kPaSat. vapor0.2 A120 V30012EXPERIMENTQ out = 3.7 kJ5 minvFIGURE 4–13Schematic and P-v diagram for Example 4–5.Analysis We take the contents of the cylinder, including the resistance wires,as the system (Fig. 4–13). This is a closed system since no mass crosses thesystem boundary during the process. We observe that a piston–cylinder devicetypically involves a moving boundary and thus boundary work W b . The pressureremains constant during the process and thus P 2 P 1 . Also, heat is lostfrom the system and electrical work W e is done on the system.(a) This part of the solution involves a general analysis for a closed systemundergoing a quasi-equilibrium constant-pressure process, and thus we considera general closed system. We take the direction of heat transfer Q to beto the system and the work W to be done by the system. We also express thework as the sum of boundary and other forms of work (such as electrical andshaft). Then the energy balance can be expressed asE in E out ¢E systemUse actual data from the experimentshown here to verify the first law ofthermodynamics. See end-of-chapterproblem 4–176.© Ronald Mullisen⎫ ⎪⎬⎪⎭⎫⎪⎬⎪⎭Net energy transferChange in internal, kinetic,by heat, work, and masspotential, etc., energiesQ W ¢U ¢KE ¢PE¡0¡0Q W other W b U 2 U 1For a constant-pressure process, the boundary work is given as W bP 0 (V 2 V 1 ). Substituting this into the preceding relation givesEXPERIMENTQ W other P 0 1V 2 V 1 2 U 2 U 1However,P 0 P 2 P 1 S Q W other 1U 2 P 2 V 2 2 1U 1 P 1 V 1 2Also H U PV, and thusQ W other H 2 H 1 1kJ2(4–18)which is the desired relation (Fig. 4–14). This equation is very convenient touse in the analysis of closed systems undergoing a constant-pressure quasiequilibriumprocess since the boundary work is automatically taken care ofby the enthalpy terms, and one no longer needs to determine it separately.Use actual data from the experimentshown here to verify the first law ofthermodynamics. See end-of-chapterproblem 4–177.© Ronald Mullisen