Earth system/boundary/surroundings

These ideas may seem obvious, but the choice of system and surroundings is not always clear. For example, if the system to be studied is the atmosphere of the earth, the definition of the boundary is somewhat challenging. The atmosphere simply gets gradually less dense, and we must make some arbitrary decision as to where it ends. In most cases, the main requirement is that we be consistent. The same choice of system and surroundings must be used throughout a particular problem, even if the choice is somewhat arbitrary. 

A system is the region in space that is the subject of the thermodynamic study. It can be as large or small, or as simple or complex, as we want it to be, but it must be carefully and consistently defined. Sometimes the system has definite and precise physical boundaries, such as a gas enclosed in a cylinder so that it can be compressed or expanded by a piston. However, it may be also something as diffuse as the gaseous atmosphere surrounding the earth. 

In physics and chemistry we call an ensemble of substances a thermodynamic system consisting of atomic and molecular particles. The system is separated from the surroundings by a boundary interface. The system is called isolated when no transfer is allowed to occur of substances, heat, and work across the boundary interface of the system as shown in Fig. 1.1. The system is called closed when it allows both heat and work to transfer across the interface but is impermeable to substances. The system is called open if it is completely permeable to substances, heat, and work. The open system is the most general and it can be regarded as a part of a closed or isolated system. For instance, the universe is an isolated system, the earth is regarded as a closed system, and a creature such as a human being corresponds to an open system. 

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