# What is thermal equilibrium?

## What Does thermal equilibrium Mean

The term equilibrium , which comes from the Latin word aequilibrium , has several meanings. It may be the state that a body registers when the forces that influence it compensate each other; to a weight that, being identical to another, counteracts it; to the situation of the object that does not fall despite its instability; or to harmony in different elements.

Thermal , on the other hand, is that linked to heat (the energy that goes from one body to another, causing changes in state) or to temperature (the magnitude that allows expressing the level of heat). An object is also said to be thermal when it manages to maintain temperature.

With these ideas clear, we can focus on defining thermal equilibrium . This is the name given to the situation that is reached when two bodies that had different temperatures become the same temperature . In this way, the flow of heat is interrupted : energy no longer passes from one body to another precisely because both are in thermal equilibrium.
When two bodies or systems that have different temperatures are in contact, a heat transfer always occurs. This transfer can be carried out by radiation , convection or conduction , from the body with more heat to the body with less heat. The process is only suspended when the bodies reach thermal equilibrium; until then, either faster or slower, the transfer will continue.
Thermal equilibrium explains how thermometers work . By bringing a thermometer into thermal contact with a body, heat transfer begins to occur. Only when thermal equilibrium is reached and the two bodies register the same heat, the flow slows down and it is possible to observe on the thermometer what its temperature is (which, due to thermal equilibrium, is the same as that of the body in question).
In 1931, the British-born astronomer and physicist RH Fowler enunciated the so-called Zero Law of Thermodynamics , a phenomenological principle for systems that is based precisely on the concept of thermal equilibrium. The statement of this law is as follows:
If we have systems A and B and we place them each in a different place but in thermal equilibrium with a third, which we can call C, the first two are also in thermal equilibrium .
It is enough to make a careful reading of this statement to understand its logical basis: saying that A and C are in thermal equilibrium is the same as saying that their temperatures are equal, so if the temperature of A is equal to that of C, and that of B is equal to that of C, so that of A is equal to that of B. It is interesting to mention that RH Fowler enunciated this law when the first and second already existed, but since it became a fundamental source for these two was that they assigned the name of zero law .

Thermodynamics is the part of physics that is responsible for the description of the equilibrium states of systems and does so at the macroscopic level, that is, without focusing on the phenomena that take place at the microscopic level within the hypotheses it presents. Despite this, disciplines such as statistical physics do carry out a microscopic study of the phenomena belonging to thermodynamics, and thermal equilibrium is one of them.
If we look at this phenomenon from a microscopic perspective, we can understand the temperature in relation to the average kinetic energy of the particles that make up the system, among which are its molecules and atoms.

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