Overview
A closed system in thermodynamics is a region of matter chosen for study whose mass remains fixed: no mass crosses the system boundary. Energy, however, may cross that boundary in the form of heat or work. The closed-system concept simplifies analysis when composition and total mass are conserved while energy interactions are important.
Key characteristics
- Mass: constant within the system boundary (no mass flow across the boundary).
- Energy exchange: allowed as heat transfer and mechanical work; internal energy can change.
- Boundary: may be rigid or movable, impermeable to mass but not necessarily adiabatic.
Mathematical and practical notes
Energy balances for a closed system usually take the form deltaU = Q - W, where deltaU is the change in internal energy, Q is heat added, and W is work done by the system. Because mass is constant, terms for mass flow that appear in open-system (control volume) balances are absent. A fixed-mass analysis is often called a control-mass approach.
Examples and applications
Common examples include a sealed piston–cylinder assembly in which the piston can move (mechanical work) while no gas leaks, a calorimeter used to measure heat changes, and many laboratory vessels. Engineers use closed-system models to study compression and expansion, heating processes, and to formulate thermodynamic cycles at the component level.
Distinctions and common misconceptions
Closed systems differ from open systems (which exchange mass and energy) and isolated systems (which exchange neither). Saying a system is "closed" does not mean it cannot exchange energy—only that mass transfer is prevented. In other disciplines, such as systems theory, "closed" may carry different meanings (for example, a closed-loop control system), so context matters.
Historical and conceptual context
The closed-system idea arose as thermodynamics developed methods to track energy and matter separately. It remains a foundational modeling assumption in teaching and in practical engineering when mass conservation simplifies analysis while allowing heat and work interactions to capture important behavior.