Thermodynamic system

A thermodynamic system is the portion of the physical world selected for study, treated as a well-defined entity that can (in general) exchange energy and/or matter with its surroundings across a system boundary .

The “system” may be a material body (e.g., a gas in a cylinder), a region of space (e.g., a control volume in fluid flow), or a composite object (e.g., two phases plus an interface), provided the boundary and allowed exchanges are specified.

Physical interpretation

Choosing the system is a modeling step: it decides what you call “inside” and “outside,” and therefore what counts as energy transfer into the system as heat or work . Once the system is fixed, its macroscopic condition at equilibrium is summarized by a thermodynamic state described by state variables (e.g., $T$, $p$, $V$, $N$), and its changes are described as a thermodynamic process .

Key classifications (set by exchanges across the boundary)

The system type is determined by what can cross the boundary:

• No matter or energy exchange: isolated system .
• Energy exchange but no matter exchange: closed system .
• Matter (and typically energy) exchange: open system .

These distinctions are not intrinsic “properties of the material” but of the system + boundary choice.

Core relation to energy bookkeeping

For any thermodynamic system, the energy accounting is organized around internal energy $U$ (a state function ) and transfers across the boundary as heat and work (both path functions ). This is the content formalized by the first law of thermodynamics .