A thermodynamic system, or simply system, is defined as a quantity of matter or a region in space chosen for study.  The region outside the system is called the surroundings.  The real or imaginary surface that separates the system from its surroundings is called the boundary.  The boundary of a system may be fixed or movable.

Surroundings are physical space outside the system boundary.


Systems may be considered to be closed or open, depending on whether a fixed mass or a fixed volume in space is chosen for study.

A closed system consists of a fixed amount of mass and no mass may cross the system boundary.  The closed system boundary may move.

Examples of closed systems are sealed tanks and piston cylinder devices (note the volume does not have to be fixed).  However, energy in the form of heat and work may cross the boundaries of a closed system.


An open system, or control volume, has mass as well as energy crossing the boundary, called a control surface.  Examples of open systems are pumps, compressors, turbines, valves, and heat exchanger.


An isolated system is a general system of fixed mass where no heat or work may cross the boundaries.  An isolated system is a closed system with no energy crossing the boundaries and is normally a collection of a main system and its surroundings that are exchanging mass and energy among themselves and no other system.


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