= Isolated system
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.
Back to article page
