Thermodynamics

Chapter 4 | 187P, kPa2 AN 2P = const.V 1 = 0.5 m 3 0.52800 J40012120 VP 1 = 400 kPaT 1 = 27°CV, m 3FIGURE 4–31Schematic and P-V diagram for Example 4–9.EXAMPLE 4–10Heating of a Gas at Constant PressureA piston–cylinder device initially contains air at 150 kPa and 27°C. At thisstate, the piston is resting on a pair of stops, as shown in Fig. 4–32, and theenclosed volume is 400 L. The mass of the piston is such that a 350-kPapressure is required to move it. The air is now heated until its volume hasdoubled. Determine (a) the final temperature, (b) the work done by the air,and (c) the total heat transferred to the air.Solution Air in a piston–cylinder device with a set of stops is heated untilits volume is doubled. The final temperature, work done, and the total heattransfer are to be determined.Assumptions 1 Air is an ideal gas since it is at a high temperature and lowpressure relative to its critical-point values. 2 The system is stationary andthus the kinetic and potential energy changes are zero, KE PE 0 andE U. 3 The volume remains constant until the piston starts moving,and the pressure remains constant afterwards. 4 There are no electrical,shaft, or other forms of work involved.Analysis We take the contents of the cylinder as the system (Fig. 4–32).This is a closed system since no mass crosses the system boundary duringthe process. We observe that a piston-cylinder device typically involves amoving boundary and thus boundary work, W b . Also, the boundary work isdone by the system, and heat is transferred to the system.(a) The final temperature can be determined easily by using the ideal-gasrelation between states 1 and 3 in the following form:P 1 V 1T 1 P 3V 3T 3¡ 1150 kPa2 1V 12300 K 1350 kPa212 V 12T 3T 3 1400 K