Isolated system

An isolated system exchanges neither matter nor energy with its surroundings. It is an idealization used to analyze systems without external exchange.

Thermodynamic system: A defined quantity of matter or region of space chosen for thermodynamic analysis. Isolation is a condition imposed on a thermodynamic system.
Conservation of energy: The principle that energy is transferred or transformed but not created or destroyed. With no energy crossing the boundary, the system's total energy remains constant.
Thermal equilibrium: A state in which bodies in thermal contact have no net heat flow between them. An isolated system can approach equilibrium through internal energy redistribution.
Adiabatic wall: A boundary that prevents heat transfer between a system and its surroundings. It blocks one energy-transfer route, unlike complete isolation.
Surroundings: Everything outside a defined system that may interact with it. An isolated system exchanges nothing with these surroundings.
First law of thermodynamics: The thermodynamic energy balance relating changes in internal energy to heat and work. In an isolated system, its energy balance has no heat or work transfer terms.
Microcanonical ensemble: A statistical-mechanical ensemble describing fixed energy, volume, and particle number. It models an isolated system with specified conserved quantities.
Diathermal wall: A boundary that permits heat transfer between a system and its surroundings. It allows energy exchange that an isolated boundary forbids.
System boundary: A real or imagined surface separating a system from its surroundings. The boundary is where prohibited matter and energy transfers would occur.
Internal energy: The energy contained within a system, excluding its bulk motion and external position. It can change form internally even when total energy cannot cross the boundary.

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