A thermodynamic system is the portion of the universe selected for study in thermodynamics. It is separated from everything else by a boundary that can be real or imaginary; everything outside that boundary is the surroundings or environment. The chosen system may be as small as a sample in a test tube or as large as a planet, and the way the boundary is treated determines which exchanges (mass, heat, work) are permitted. The term "working substance" is a historical name sometimes used for the material within a system that participates in energy transfers. For an introduction to the broader subject, see thermodynamics.

Classification by boundary and exchange

Systems are commonly categorized by what can cross their boundaries. The three standard idealized classes are:

  • Isolated system: neither energy nor matter cross the boundary; the system's total energy and mass remain constant in principle.
  • Closed system: energy (heat and work) may cross the boundary, but mass does not. A rigid sealed container holding a gas is a typical example.
  • Open system: both matter and energy can cross the boundary; many engineering applications (pipes, turbines) use this description.

Other useful distinctions include control mass (a fixed quantity of matter) versus control volume (a region in space through which matter flows). Practical treatments also consider idealizations such as reservoirs and thermal baths that approximate large surroundings at fixed temperature or pressure.

State, properties and processes

The state of a thermodynamic system is specified by a set of properties, which may be intensive (independent of system size, such as temperature and pressure) or extensive (scale with system size, such as mass and internal energy). A state function depends only on the state and not on the path taken, while path-dependent quantities (often called process variables) include heat and work. Important concepts include equilibrium (mechanical, thermal, and chemical), reversible and irreversible processes, and thermodynamic potentials that summarize how systems exchange energy.

History and development

The idea of isolating and analyzing a portion of the world for energetic accounting grew with studies of heat engines and steam power in the 18th and 19th centuries. Early theoretical foundations were laid in the nineteenth century by scientists and engineers who formalized conservation of energy for thermal processes and clarified the limits on conversion between heat and work. These developments led to the general formulation of the laws of thermodynamics, which underpin the modern description of systems and their interactions.

Examples and applications

Thermodynamic systems are a unifying concept across many fields. Examples include:

  • a gas inside a piston or cylinder used to model engines;
  • a chemical solution in a test tube studied in laboratory thermochemistry;
  • an organism where biochemical energy and matter flow across membranes (living organism);
  • a planetary atmosphere treated as a large open system exchanging radiation and mass (planet).

Practical applications span heat engines, refrigeration, power generation, chemical reaction engineering, atmospheric science, and biological energetics. Engineers often model devices as control volumes to compute energy flows; scientists use closed-system or isolated-system idealizations when appropriate to simplify analysis.

Notable distinctions and cautions

When using the system concept it is important to choose the boundary and level of description that match the question at hand. Macroscopic thermodynamics treats bulk properties and averages; microscopic statistical mechanics links those properties to the behavior of many particles. Some quantities are conserved globally (total energy in an isolated system), while others may change during processes. Finally, ideal classifications simplify reasoning but real systems may require hybrid or time-dependent descriptions to capture transport, phase change, or chemical reactions.

For practical calculations and deeper study, textbooks and reference articles introduce equations of state, energy balances, entropy production, and thermodynamic potentials. Additional resources and introductory material can be consulted through general thermodynamics overviews at related summaries and subject-specific pages for experimental setups such as the cylinder model or laboratory examples like the test-tube experiment.