Piston-cylinder assembly example

For example, say we wish to study the piston-cylinder assembly in Figure 1.1. The usual choice of system, surroundings, and boundary are labeled. The boundary is depicted by the dashed line just inside the walls of the cylinder and below the piston. The system contains the gas within the piston-cylinder assembly but not the physical housing. The surroundings are on the other side of the boundary and comprise the rest of the universe. Likewise the system, surroundings, and boundary of an open system are labeled in Figure 1.2. In this case, the inlet and outlet flow streams, labeled in and out, respectively, allow mass to flow into and out of the system, across the system boundary. 

Figure 1.8 provides a schematic representation of each of these quantities. The two piston-cylinder assemblies depicted on the left represent cases for which the saturation pressure is defined. In these systems, pure species a is in vapor-liquid equihbiium at temperatures Ty and Tg, respectively, where T2 is greater than Ti. In each case, there is a unique pressure at which the two phases can be in equilibrium—defined as the saturation pressure, Pf. For example, pure water at 293 K (20°C) has a saturation pressure of 2.34 kPa. Said another way, for pure water to boil at 293 K, the pressure of the system must be 2.34 kPa. If the pressure is higher, water will exist only as a liquid. Conversely,... 

Equation (2.7) is often encountered in thermodynamics the work described by this equation will be referred to as Pv work. On a molecular scale, the energy transfer by Pv work can be understood in terms of momentum transfer of the molecules in the system when they bounce off the moving boundary, as discussed in Section 1.3. A piston-cylinder assembly is a common system that is used to obtain work (e.g., in your automobile). Example 2.3 illustrates how work is calculated for such a system. 

Figure E23 Example of a process in which energy is transferred from the system to the surroundings by Pv work expansion of a gas in a piston—cylinder assembly. The surroundings are maintained at 1 bar.

To help solidify these abstract ideas, a concrete example is illustrative. We will compare the value of work for six processes. We will label these cases process A through process F. Three processes (A, C, and E) entail isothermal expansion of a piston-cylinder assembly between the same states state 1 and state 2. The other three (B, D, and F) consist of the opposite process, isothermal compression between state 2 and state 1. An isothermal process results in the limit of fast heat transfer with the surroundings. We could perform a similar analysis on adiabatic processes where there is no energy transfer via heat between the system and the surroundings. 

Figure E2.10A Piston-cylinder assembly with a spring attached to the piston. The initial state of the system for Example 2.10 is shown.

Figure 2.16 An ideal gas in a piston-cylinder assembly undergoing a reversible, adiabatic expansion. In this example, is constant. See if you can predict the signs of At/, (), and W for this process in the table.

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