<P> Thermodynamic equilibrium is an axiomatic concept of thermodynamics . It is an internal state of a single thermodynamic system, or a relation between several thermodynamic systems connected by more or less permeable or impermeable walls . In thermodynamic equilibrium there are no net macroscopic flows of matter or of energy, either within a system or between systems . In a system in its own state of internal thermodynamic equilibrium, no macroscopic change occurs . Systems in mutual thermodynamic equilibrium are simultaneously in mutual thermal, mechanical, chemical, and radiative equilibria . Systems can be in one kind of mutual equilibrium, though not in others . In thermodynamic equilibrium, all kinds of equilibrium hold at once and indefinitely, until disturbed by a thermodynamic operation . In a macroscopic equilibrium, almost or perfectly exactly balanced microscopic exchanges occur; this is the physical explanation of the notion of macroscopic equilibrium . </P> <P> A thermodynamic system in its own state of internal thermodynamic equilibrium has a spatially uniform temperature . Its intensive properties, other than temperature, may be driven to spatial inhomogeneity by an unchanging long range force field imposed on it by its surroundings . </P>

When is a system said to be in a state of equilibrium
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