Thermodynamics is mainly based on a set of four laws that are universally valid when applied to systems that fall within the constraints implicit in each.
The first principle that was established was the second law of thermodynamics, as formulated by Sadi Carnot in 1824. The 1860 already established two "principles" of thermodynamics with the works of Rudolf Clausius and William Thomson, Lord Kelvin. Over time, these principles have become "laws." In 1873, for example, Willard Gibbs claimed that there were two absolute laws of thermodynamics in his graphical methods in fluid thermodynamics. Currently a total of four laws are enunciated. In the last 80 years, some authors have suggested other laws, but none of them was accepted unanimously.
In the various theoretical descriptions of thermodynamics, these laws can be expressed in apparently different forms, but the most outstanding formulations are the following:
- The zero principle of thermodynamics has different contents, in different authors and in different contexts; that is, it can refer to one or other of the following two aspects of the equilibrium states of a thermodynamic system: establishing thermodynamic equilibrium, or transitivity of thermal equilibrium.
- The first principle of thermodynamics establishes the equivalence between mechanical work and the amount of heat as forms of energy exchange between a system and the surrounding world. One of its consequences is the existence of a state function called internal energy.
- The second law of thermodynamics is compatible with a primary form, the incapability of thermal machines that received a little heat from a source to produce equivalent mechanical work. One of its consequences is the existence of a state function called entropy.
- The third principle of thermodynamics states that when the temperature tends to absolute zero, the entropy of any system tends to zero. It is not the result of the direct abstraction of the experimental facts, but the extension of the consequences of the preceding principles.
Zero law of thermodynamics
The thermodynamic equilibrium of a system is defined as the condition of the system in which the empirical variables used to define a state of the system (pressure, volume, electric field, polarization, magnetization, linear tension, surface tension, among others) have arrived at a point of equilibrium and therefore do not vary over time, ie they are not dependent on time. These empirical (experimental) variables of a system are known as thermodynamic coordinates of the system.
This principle is called thermodynamic equilibrium. If two systems A and B are in thermodynamic equilibrium, and B is in thermodynamic equilibrium with a third C system, then A and C are in thermodynamic equilibrium. This principle is fundamental, despite being widely accepted; it was not formulated formally until after the other three laws had been enunciated. Hence, it receives position 0.
First law of thermodynamics
The first law of thermodynamics is also known as the principle of conservation of energy for thermodynamics. This thermodynamic law states that, if work is done on a system or it exchanges heat with another, the internal energy of the system will change.
Seen in another way, this law allows to define heat as the necessary energy that the system must exchange to compensate for the differences between work and internal energy. It was proposed by Antoine Lavoisier.
Second law of thermodynamics
The second law of thermodynamics regulates the direction in which thermodynamic processes have to be carried out and, therefore, the impossibility of occurring in the opposite direction. It also establishes, in some cases, the impossibility of completely converting all energy from one type to another without losses. In this way, the second law imposes restrictions for energy transfers that hypothetically could be carried out taking into account only the first principle of thermodynamics.
This law allows us to define a physical quantity called entropy such that, for an isolated system, that is, it does not exchange matter or energy with its environment, the change in entropy must always be greater than or equal to zero and only equal to zero if the process is reversible.
Third law of thermodynamics
The third of the laws of thermodynamics, proposed by Walther Nernst, states that it is impossible to reach a temperature equal to absolute zero by a finite number of physical processes. The third principle of thermodynamics can also be formulated as that as a given system approaches absolute zero, its entropy tends to a specific constant value.
The entropy of pure crystalline solids can be considered with the value of zero at temperatures equal to absolute zero. It is not a notion required by classical thermodynamics, so it is probably inappropriate to speak of "law".