Two physical systems are said to be in thermal equilibrium if there is no exchange of heat between them while being connected to each other by a path which is permeable to heat. Thermal equilibrium follows by the zeroth law of thermodynamics. If the temperature within the system is temporarily and spatially uniform, then the system is said to be in a state of thermal equilibrium.
An object with a higher temperature, when comes into contact with an object of lower temperature, there is a transfer of heat from the higher temperature object to the lower temperature object. With exchange of heat, there comes a stage where the two objects approach the same temperature. With absence of other objects, the two objects continue to maintain a constant temperature. They are then known to be in the stage of thermal equilibrium.
The Zeroth law of thermodynamics state that of the two objects are in thermal equilibrium with a third system at the same time; then they all are in the thermal equilibrium with each other. This practically means that all the three objects are at the same temperature, and thus it forms the base for the comparison of the temperatures.
The systems in thermodynamic equilibrium are always in the state of thermal equilibrium but it isn’t always true for the converse. If the connection of the systems allows the flow of heat (energy) but doesn’t permit the transfer of matter or energy as work, then the two systems might reach their thermal equilibrium without being at thermodynamic equilibrium.
VARIETIES OF THERMAL EQUILIBRIUM
- Relation of thermal equilibrium between the two thermally connected bodies.
For the relation of thermal equilibrium, there is a diathermal connection (the contact path only permits transfer of heat but does not permit the passage of work or matter). The equivalence class of systems established are known as isotherms. The relation of thermal equilibrium is reflexive and symmetric,
- The internal thermal equilibrium of an isolated body.
The isolation of body itself refers to the thermal equilibrium of the body. No heat enters or leaves the body and it settles down with its own intrinsic characteristics. After the settlement, (when not even a macroscopic change is detected), the body is said to be in its own thermal equilibrium. With glass as an example, it might be possible for the body to reach internal thermal equilibrium but not be in a state of internal chemical equilibrium.