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Zero law of thermodynamics

Zero law of thermodynamics

The zero law of thermodynamics speaks of what we experience every day: two systems that are in thermal equilibrium with a third are in equilibrium with each other. It is said that two bodies are in thermal equilibrium when, on contacting each other, their state variables do not change. Around this simple idea the zero law is established.

Every law of physics has its relevance, as well as the zero law of thermodynamics, which curiously was the last law to be introduced in literature. After the realization that heat is a form of energy that could be transformed into another, thermology was called thermodynamics. To obtain a logical structure in the presentation of thermodynamics, it was necessary to place another law before those that had already been enunciated (first law of thermodynamics and second law of thermodynamics). Thus, that other law was called the zero law of thermodynamics.

If A is in equilibrium with B and A is also in thermal equilibrium with C, we can conclude that B is in thermal equilibrium with C. With the foundations of this By law, we can guarantee the possibility of using a Z thermometer to find out if two bodies X and Y are in equilibrium. To do this, just check if the two bodies have the same temperature.

History of the zero law of thermodynamics

The ideas of "hot" and "cold" have always been part of man's sensory experiences. In fact, two of the first scientists who expressed these ideas were Leonardo Da Vinci and Galileo. These scientists knew that upon contact with a third body, usually the air, two or more bodies in contact with it "mixed in an appropriate manner until reaching the same condition".

This condition was achieved due to the tendency of hot bodies to diffuse their energy to colder bodies. This flow of energy is called heat or heat energy. Thus, we can perceive the tendency of heat to spread from any hot body to colder ones in its surroundings. This heat transfer takes place until none of the bodies of the system is able to absorb more heat.

Ralph H. Fowler The zero law of thermodynamics has this name thanks to Ralph H. Fowler (1889-1944), a great English physicist, who in the twentieth century after a long time of development of experimental law, considered it as a basic law. If this law the concept of temperature could not be defined and postulated that: "If two bodies are in thermal equilibrium with a third, they will be in thermal equilibrium with each other". However, it became necessary to structure the presentation of thermodynamics in a more logical, as the first and second law had already been formulated, then the term zero law of thermodynamics arose. Since then this denomination has been used in physics.

Concepts to understand the zero law of thermodynamics

To understand the zero law of thermodynamics, let us consider the behavior of two or more systems constituted in the following way.

Systems A and B are separated from each other by an adiabatic wall (a wall that does not allow heat exchange). Each of the two points is in thermal contact with the third system C through diathermic walls (walls that allow the exchange of thermal energy), the whole being surrounded by an adiabatic wall. Our experience says that both systems will reach thermal equilibrium with the third and that no further change will take place if the adiabatic wall separating A and B is replaced by a diathermic wall.

An example that will help to better understand the concept of the zero law of thermodynamics: it considers a B body that consists of a tube with a capillary containing mercury and whose levels of height above the capillary represent different temperatures.

Now consider a body A, for example the human body, if you bring the thermometer to the human body and leave it enough time, the thermometer will reach the value corresponding to its temperature, that is, the thermometer and the human body will be in thermal equilibrium between yes and therefore they will have the same numerical value for the temperature property. The way to choose a thermometric scale is absolutely arbitrary and we will not discuss it here.

Statement of the zero law of thermodynamics

These experiences can be summarized in a law called the zero law of thermodynamics: Two systems in thermal equilibrium with a third are in equilibrium with each other "

The zero law of thermodynamics allows us to differentiate bodies from each other with respect to their "degree of warming". This attribute, which is a property of the system, we will identify with its temperature, which turns out to be a macroscopic (measurable) concept.

Through these concepts we can understand the functioning of the devices called thermometers, which are the devices that precisely measure the temperature property of bodies.

It should be noted that the formulation of the zero law of thermodynamics contains three firm ideas:

  • The existence of a state variable, called temperature.
  • The equality of temperatures as a condition for thermal equilibrium between two systems, or between parts of the same system.
  • The existence of a relationship between the independent variables of the system and the temperature, called the equation of state.
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Last review: December 21, 2017