# Thermal Equilibrium

Thermal equilibrium is a theoretical physical concept, used especially in theoretical texts, that means that all temperatures of interest are unchanging in time and uniform in space. When the temperatures of interest are just those in the different parts of one body, the concept also requires that any flow of heat by thermal conduction or by thermal radiation into or out of one part of the body be balanced by a flow of heat in the opposite sense into or out of another part of the body. When the temperatures of interest belong to several bodies, the concept also requires that flows of heat between each pair of bodies balance to a zero net flow, but it allows the several bodies to gain or lose heat to several external reservoirs provided that their total rate of inflow from all reservoirs is equal to their total rate of outflow to all reservoirs and that each flow is unchanging in time. For some situations, the definition of transfer of heat can be problematic.

Some writers use the term thermal equilibrium in a different sense. They mean by it that the spatial temperature distribution of the body is not necessarily uniform, and indeed is likely to be non-uniform, but is maintained unvarying in time, by flows of energy; for example they mean that there is spatially distributed radiative cooling of the body and equal and opposite spatially distributed energy addition by condensation of water vapour, just so as on average to keep the spatial distribution of temperature time-invariant.

Thermal equilibrium does not mean the same as thermodynamic equilibrium, because the latter requires that there be equilibrium of all kinds, not only thermal, and that there be no flow of any kind, in the system of interest.

### Other articles related to "thermal equilibrium, thermal, equilibrium":

... Thermal equilibrium - When two objects A and B are in thermal contact and there is no net transfer of thermal energy from A to B or from B to A, they are said to be in ... It should be noted that the majority of objects experiencing thermal equilibrium still do exchange thermal energy but do so equally so that the net heat transfer is zero ... Perfect thermal contact ...
Laws Of Thermodynamics
... are Zeroth law of thermodynamics If two systems are in thermal equilibrium with a third system, they must be in thermal equilibrium with each other ... The entropy of any isolated system not in thermal equilibrium almost always increases ... Isolated systems spontaneously evolve towards thermal equilibrium—the state of maximum entropy of the system—in a process known as "thermalization" ...
Thermal Equilibrium - Theoretical Foundations
... The concept of thermal equilibrium is thus coherent with the concepts of temperature and of heat transfer ... or mathematical elegance, prefer to define thermal equilibrium by relying on a presupposed notion of thermodynamic equilibrium, in which all mechanically measurable properties of a body ... not 'thermally connected' to be put into 'thermal connection' ...
Thermodynamic Equations - Laws of Thermodynamics
... B, C are thermodynamic systems such that A is in thermal equilibrium with B and B is in thermal equilibrium with C, then A is in thermal equilibrium with C ... that systems that are in thermodynamic equilibrium with each other have the same temperature ...
Temperature - Theoretical Foundation - Zeroth Law of Thermodynamics
... It has long been recognized that if two bodies of different temperatures are brought into thermal connection, conductive or radiative, they exchange heat ... the two connected bodies eventually reach a state of thermal equilibrium in which no further changes occur ... of the zeroth law of thermodynamics is that if two systems are each in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other ...

### Famous quotes containing the word equilibrium:

When a person hasn’t in him that which is higher and stronger than all external influences, it is enough for him to catch a good cold in order to lose his equilibrium and begin to see an owl in every bird, to hear a dog’s bark in every sound.
Anton Pavlovich Chekhov (1860–1904)