# Laws of thermodynamics

The laws of thermodynamics are a set of laws on which the physics of thermodynamics is based. Specifically, these are four laws that are universally valid when applied to systems that fall within the restrictions implicit in each one.

Over time, these principles have become "laws." Currently, a total of four laws are stated. In the last 80 years, some authors have suggested other laws, but none of them were unanimously accepted.

Curiously, the zeroth law was formulated after having stated the other three laws and is a consequence of all of them. For this reason, it has position 0.

## What are the laws of thermodynamics?

In the various theoretical descriptions of thermodynamics, these laws can be expressed in apparently different ways, but the most prominent formulations are the following:

• The zeroth law states that “if two thermodynamic systems that are in thermal equilibrium with a third, they are also in equilibrium with each other.”
• The first law states that “The total energy of an isolated system is neither created nor destroyed, it remains constant.”
• The second law states that the amount of entropy in the universe tends to increase. From this law, it is extracted that there is no such thing as 100% efficiency of a heat engine. It is also noted that not all thermodynamic processes are reversible.
• The third law states that it is impossible to reach a temperature equal to absolute zero (0 kelvin ).

## Zeroth law: the law of thermodynamic equilibrium

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 system C, then A and C are in turn in thermodynamic equilibrium.

This principle is fundamental. Principle 0 was not formally formulated until after the other three laws had been stated. Hence it receives position 0.

The thermodynamic equilibrium of a system is defined as the condition in which the empirical variables used to define a state of the system have reached an equilibrium point. Being in balance, they do not vary over time.

These empirical (experimental) system variables are known as the system's thermodynamic coordinates. Among other empirical variables, we have pressure, volume, electric field, polarization, magnetization, linear tension, surface tension, etc.

## First law: conservation of energy

The first law of thermodynamics postulates that the total energy of an isolated system remains constant; It is neither created nor destroyed, it simply transforms from one form to another. For example, in a heat engine, the thermal energy resulting from combustion is converted into mechanical energy.

This law, also known as the law of conservation of energy, states that by doing work on a system or exchanging heat with another, the internal energy of the system will change. In simpler terms, the law allows heat to be conceptualized as the amount of energy that a system must exchange to balance the discrepancies between the work done and the internal energy. Antoine Lavoisier was the pioneer in proposing this fundamental thermodynamic law.

## Second law: principle of entropy

The second law of thermodynamics regulates the direction in which thermodynamic processes must be carried out and, therefore, the impossibility of them occurring in the opposite direction. For example, heat transfer can occur from a hot body to a cold one, but not the other way around.

It also establishes, in some cases, the impossibility of completely converting all energy from one type to another without losses. For example, in an ideal engine, the amount of heat supplied is converted into mechanical work. However, in a real engine, some of the heat supplied is lost.

This law allows us to define entropy. The change in the amount of entropy of an isolated system must always be greater than or equal to zero and is only equal to zero if the process is reversible.

The first and second laws emerged simultaneously in the 1850s. They were mainly the result of the works of William Rankine, Rudolf Clausius and William Thomson (Lord Kelvin).

## Third law: the principle of absolute zero

The third law of thermodynamics postulates that it is impossible to reach a temperature equal to absolute zero through a finite number of physical processes. Absolute zero is located at 0 kelvin, equivalent to -273 degrees Celsius, and this law, fundamentally proposed by Walther Nernst, establishes fundamental limitations in approaching these extreme conditions.

As the temperature approaches absolute zero, the entropy of any system tends to zero, implying maximum ordering and minimum molecular agitation. In other words, the disorder of the system is reduced to its minimum as the lowest possible thermal limit is reached.

This principle can also be formulated as follows: as a specific system approaches absolute zero, its entropy converges toward a specific constant value. This aspect of the third law of thermodynamics highlights the intrinsic regularity that manifests itself in systems at extremely low temperatures.

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Data de publicació: August 28, 2018
Última revisió: February 21, 2024