An isolated system exchanges neither matter nor energy with its surroundings. Its total energy is fixed, while the Second law of thermodynamics requires its total entropy to be nondecreasing. Its internal energy alone is fixed when bulk kinetic energy and external potential energy also remain unchanged. For example, an initially spinning stirrer inside an insulated rigid vessel can lose kinetic energy while the contents gain internal energy, with no external input.
An isolated system need not yet be in thermodynamic equilibrium. Two enclosed bodies at different temperatures can exchange heat internally until they reach thermal equilibrium; their combined energy stays constant. Their internal transfer is absent from the outer boundary ledger. A vacuum-insulated container is only an approximation to isolation: residual thermal conduction and thermal radiation can still carry energy across its boundary.
An isolated system is a physical system that does not exchange any matter or energy with its surroundings. In other words, an isolated system is completely self-contained; it is defined by its boundaries and has no interactions with the environment outside of those boundaries. ### Key Characteristics of an Isolated System: 1. **No Energy Exchange**: There is no transfer of energy (heat, work, etc.) into or out of the system. This means that the total energy of the isolated system remains constant.
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