TERMINOLOGY
The system, surrounding, and boundary
As explained in chap. 2,a thermodynamic system is any three-dimensional region of physical spare on which we wish to focus our attention. Usually we consider only one system at a time and call it simply “the system.” The rest of the physical universe constitutes the surroundings of the system. The boundary is the closed surface that encloses the encloses the system and separates it from the surroundings. The boundary may (and usually does) coincide with real physical surface. the interface between two phases. the inner or outer surface of the wall of a flask or other vessel, and so on. Alternatively, part or all of the boundary may be an imagined surface in space. unrelated to any physical structure. The size and shape of t system, as defined by its boundary, may change in time. In short, our choice of the region that constitutes the system is arbitrary-but s essential that we know exactly what this choice is. We usually think of the system as a part of the physical universe that we are able influence only indirectly through its interaction with the surroundings, and the surroundings as the part of the universe that we are able to directly manipulate with various physical devices under our control. That is, we(the experimenters are part of the surroundings, not the system
For some purposes we may wish to treat the system as being divided into subsystems, or two or more systems as being combined into a super system.
If matter is transferred in either direction across the boundary, the system is open, otherwise it is closed. In an open system, matter may pass through a fixed boundary, or the boundary may move through fixed matter.