Thermodynamic system

A defined quantity of matter or region of space selected for thermodynamic study, separated from its surroundings by a boundary. Its exchanges of matter and energy depend on the system’s classification.

Thermodynamic boundary: A real or imagined surface separating a thermodynamic system from its surroundings. It defines the system’s extent and determines which transfers cross its edge.
Open system (thermodynamics): A thermodynamic system that exchanges both matter and energy with its surroundings. Its boundary permits mass flow as well as energy transfer.
Piston-cylinder device: A device in which a movable piston changes the volume of gas inside a cylinder. The gas can be modeled as a system doing boundary work as the piston moves.
Thermodynamic system classification: The grouping of thermodynamic systems by the matter and energy they exchange with their surroundings. This framework distinguishes open, closed, and isolated systems by permitted transfers.
Surroundings (thermodynamics): Everything outside a chosen thermodynamic system that can potentially interact with it. The system is defined relative to what lies beyond its boundary.
Closed system (thermodynamics): A thermodynamic system that exchanges energy but not matter with its surroundings. Its boundary prevents matter transfer while allowing energy exchange.
Heat engine: A device that converts heat transfer into net work while operating in a cycle. Its working substance is analyzed as a system receiving and rejecting energy.
Thermodynamic control mass: A fixed quantity of matter followed during a thermodynamic process. Unlike a control volume, it follows the same matter rather than a fixed region in space.
Thermodynamic state: The condition of a system specified by its macroscopic properties at a given time. A chosen system is characterized by the state variables describing its condition.
Isolated system: A thermodynamic system that exchanges neither matter nor energy with its surroundings. It is the limiting case in which no transfer crosses the boundary.

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