Systems, States, and Processes

Thermodynamics uses abstract models to represent real-world systems and processes. These processes may appear in a rich variety of situations, including controlled laboratory conditions, industrial production facilities, living systems, the environment on Earth, and space. A key step in applying the methods of thermodynamics to such diverse processes is to formulate the thermodynamic model for each process. This step requires precise definitions of thermodynamic terms. Students (and professors ) of thermodynamics encounter—and sometimes create—apparent contradictions that arise from careless or inaccurate use of language. Part of the difficulty is that many thermodynamic terms also have everyday meanings different from their thermodynamic usage. This section provides a brief introduction to the language of thermodynamics. 

A system is that part of the universe of immediate interest in a particular experiment or study. The system always contains a certain amount of matter and is described by specific parameters that are controlled in the experiment. For example, the gas confined in a closed box may constitute the system, characterized by the number of moles of the gas and the fixed volume of the box. But in other experiments, it would be more appropriate to consider the gas molecules in a particular cubic centimeter of space in the middle of a room to be the system. In the first case, the boundaries are physical walls, but in the second case, the boundaries are conceptual. We explain later that the two kinds of boundaries are treated the same way mathematically. In the second example, the system is characterized by its volume, which is definite, and by the number of moles of gas within it, which may fluctuate as the system exchanges molecules with the surrounding regions. 

Thermodynamics is concerned with macroscopic properties of systems and changes in these properties during processes. Such properties are of two kinds extensive and intensive. To distinguish between them, consider the following 

FIGURE 12.1 TheP- /-Tsurfaceof 1 mol of ideal gas. Each point on the surface represents a combination of pressure (P), volume (V), and temperature (7) allowed by the equation of state of the gas. Along an isotherm (Tconstant), the pressure varies inversely with volume along an isochore ( /constant), it varies linearly with temperature. Two processes are shown connecting states A and B along paths that satisfy the equation of state at every point. 

A thermodynamic state is a macroscopic condition of a system in which the properties of the system are held at selected fixed values independent of time. The properties of the system are held constant by its boundaries and the surroundings. For example, a system comprising 2 mol helium (He) gas can be held in a piston-cylinder apparatus that maintains the system pressure at 1.5 atm, and the apparatus may be immersed in a heat bath that maintains the system temperature at 298 K. The properties of pressure (P) and temperature (T) are then said to be constrained to the values 1 atm and 298 K, respectively. The piston-cylinder and the heat bath are the constraints that maintain the selected values of the properties P and T. 

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