Thermodynamics Zeroth Law
Law1
If system $A,B,C$ is in thermodynamic equilibrium with $A$ and $B$, and $B$ is in thermodynamic equilibrium with $C$, then $A$ and $C$ are also in thermodynamic equilibrium.
Description
Mathematically, the Zeroth Law of Thermodynamics can be expressed as the ’transitivity of thermodynamic equilibrium’. It is a fundamental law similar to the [First Postulate of Euclidean Geometry $A = B \land B=C \implies A = C$ and underpins many aspects of physics, being naturally employed across much of the discipline.
Thermal contact refers to the process in which two different systems exchange heat. If the left side is the hot system and the right side is the cold system, heat flows from the hot side to the cold side, as depicted below.
Thermodynamic Equilibrium is described as follows:
Over time, the heat flowing from left to right will equalize with the heat flowing from right to left. This state is referred to as thermodynamic equilibrium. It is critical to understand that there is no (macroscopic) flow of heat in such a state, but that does not imply that there is absolutely no (microscopic) flow of energy. Although there is no observable change in heat (macroscopically), it is not accurate to assert that the (microscopic) flow of energy has ceased entirely. Any changes in heat, however minimal, are considered too slight to be observed as a flow.
See Also
Stephen J. Blundell and Katherine M. Blundell, Concepts in Thermal Physics, translated by Jae-Woo Lee (2nd Edition, 2014), p41-43 ↩︎