By contacting two bodies that are at different temperatures, the one that is warmer yields part of its energy to the lower temperature, to the point where both temperatures are equalized.
This situation is known as thermal equilibrium, and it is precisely the state in which the temperatures of two bodies that initially had different temperatures are equalized. It happens that when the temperatures are equalized, the flow of heat is suspended, and then the equilibrium situation is reached.
Theoretically, thermal equilibrium is fundamental in what is known as the Zero Law or the Zero Principle of Thermodynamics , which explains that if two separate systems are at the same time in thermal equilibrium with a third system, those are in thermal equilibrium. with another. This Law is fundamental for the whole discipline of thermodynamics, which is the branch of physics that deals with describing the states of equilibrium at a macroscopic level.
Amount of balance heat
The equation that gives rise to the quantification of the amount of heat that is exchanged in the transfers between the bodies, has the form:
Q = M * C * ?T
Q being the amount of heat expressed in calories, M the mass of the body under study, C the specific heat of the body, and ?T the difference in temperature.
In an equilibrium situation, the mass and the specific heat retain their original value, but the temperature difference becomes 0 because precisely the equilibrium situation was defined where there are no changes in temperature.
Another important equation for the idea of thermal equilibrium is that which seeks to express the temperature that the unified system will have. It is accepted that when a system of N1 particles, which is at temperature T1, is brought into contact with another system of N2 particles that is at temperature T2, the equilibrium temperature is obtained by the formula:
(N1 * T1 + N2 * T2) / (N1 + N2) .
In this way, it can be seen that when both subsystems have the same amount of particles, the equilibrium temperature is reduced to an average between the two initial temperatures. This can be generalized for relations between more than two subsystems.
Examples of thermal equilibrium
Here are some examples of situations where thermal equilibrium occurs:
- The measurement of body temperature through a thermometer works that way. The prolonged duration that the thermometer must have in contact with the body in order to be able to truly quantify the temperature degrees is due precisely to the time it takes to reach thermal equilibrium.
- The products sold ‘al natural’ could have gone through a refrigerator. However, after some time outside the refrigerator, in contact with the natural environment, they reached thermal equilibrium with it.
- The permanence of glaciers in the seas and at the poles is a particular case of thermal equilibrium. Precisely, warnings about global warming have much to do with an increase in the temperature of the seas, and then a thermal equilibrium where much of that ice melts.
- When a person comes out to bathe, he has a relative cold because the body had come into balance with the hot water, and now must come into balance with the environment.
- When you are looking to cool a cup of coffee, adding cold milk.
- Substances such as lard are very sensitive to changes in temperature, and with very little time in contact with the environment at natural temperature come into balance and melt.
- When you put your hand on a cold railing, for a time, the hand goes to have a colder temperature.
- A bottle with a kilo of ice cream will melt slower than another one with a quarter of a kilogram of the same ice cream. This is produced by the equation in which the mass is conditioning the characteristics of thermal equilibrium.
- When an ice cube is placed in a glass of water, thermal equilibrium also occurs. The only difference is that the balance implies a change of state, because it crosses the 100 ° C where the water passes from solid to liquid.
- Add cold water at a rate of hot water, where very quickly equilibrium is reached at a cooler temperature than the original.