A THERMODYNAMIC SYSTEM AND THE CONTROL VOLUME
A thermodynamic system is a device or combination of devices containing a quantity of matter that is being studied. To deﬁne this more precisely, a control volume is chosen so that it contains the matter and devices inside a control surface. Everything external to the
Schematic diagram of a steam power plant
control volume is the surroundings, with the separation provided by the control surface. The surface may be open or closed to mass ﬂows, and it may have ﬂows of energy in terms of heat transfer and work across it. The boundaries may be movable or stationary. In the case of a control surface that is closed to mass ﬂow, so that no mass can escape or enter the control volume, it is called a control mass containing the same amount of matter at all times.
Schematic diagram of a refrigerator
Selecting the gas in the cylinder of Fig. 1.4 as a control volume by placing a control surface around it, we recognize this as a control mass. If a Bunsen burner is placed under the cylinder, the temperature of the gas will increase and the piston will move out. As the piston moves, the boundary of the control mass also changes. As we will see later, heat and work cross the boundary of the control mass during this process, but the matter that composes the control mass can always be identiﬁed and remains the same.
An isolated system
An isolated system is one that is not inﬂuenced in any way by the surroundings so that no mass, heat, or work is transferred across the boundary of the system. In a more typical case, a thermodynamic analysis must be made of a device like an air compressor which has a ﬂow of mass into and out of it, as shown schematically in Fig. 1.5. The real system includes possibly a storage tank, as shown later in Fig. 1.20. In such an analysis, we specify a control volume that surrounds the compressor with a surface that is called the control surface, across which there may be a transfer of mass, and momentum, as well as heat and work.
Example of a control mass
Thus, the more general control surface deﬁnes a control volume, where mass may ﬂow in or out, with a control mass as the special case of no mass ﬂow in or out. Hence, the control mass contains a ﬁxed mass at all times, which explains its name. The general formulation of the analysis is considered in detail in Chapter 4. The terms closed system (ﬁxed mass) and open system (involving a ﬂow of mass) are sometimes used to make this distinction. Here, we use the term system as a more general and loose description for a mass, device, or combination of devices that then is more precisely deﬁned when a control volume is selected. The procedure that will be followed in presenting the ﬁrst and second
Example of a control volume
laws of thermodynamics is ﬁrst to present these laws for a control mass and then to extend the analysis to the more general control volume.
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