<P> Two physical systems are in thermal equilibrium if there are no net flow of thermal energy between them when they are connected by a path permeable to heat . Thermal equilibrium obeys the zeroth law of thermodynamics . A system is said to be in thermal equilibrium with itself if the temperature within the system is spatially and temporally uniform . </P> <P> Systems in thermodynamic equilibrium are always in thermal equilibrium, but the converse is not always true . If the connection between the systems allows transfer of energy as heat but does not allow transfer of matter or transfer of energy as work, the two systems may reach thermal equilibrium without reaching thermodynamic equilibrium . </P>

When are two bodies said to be in thermal equilibrium