A closed system is a concept from thermodynamics and systems theory that refers to a physical system that does not exchange matter with its surroundings but can exchange energy. In a closed system, energy can enter or exit the system, but the total mass remains constant because there is no flow of matter across the system's boundaries. ### Key Characteristics of a Closed System: 1. **No Mass Transfer**: There is no exchange of matter with the environment.
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A closed system exchanges no matter with its surroundings, but can exchange energy through heat or work. A sealed cylinder with a moving piston is a typical idealized example. “Closed” fixes the material inventory, rather than the volume or the internal energy. An insulated cylinder can receive compression work; a rigid sealed vessel can receive heat. Neither is necessarily an isolated system.