What are the laws of thermodynamics?

We explain to you what the laws of thermodynamics are, what is the origin of these principles and the main characteristics of each one.

  1. What are the laws of thermodynamics?

When we talk about the laws of thermodynamics or the principles of thermodynamics, we refer to the most elementary formulations of this branch of physics , interested as the name implies (from the Greek thermos, “heat”, and dynamos, “power” , “Force”) in the dynamics of heat and other forms of known energy.

These laws or principles of thermodynamics are a set of formulas and equations that describe the behavior of the so-called thermodynamic systems , that is, a portion of the universe theoretically isolated for study and understanding, using its fundamental physical quantities: temperature, energy and entropy .

There are four laws of thermodynamics, listed from zero to three, and serve to understand the physical laws of the universe , as well as the impossibility of certain phenomena such as perpetual motion.

  1. Origin of the laws of thermodynamics

The four principles of thermodynamics have different origins, and some were formulated from the previous ones . The first to settle, in fact, was the second, the work of French physicist and engineer Nicolás Léonard Sadi Carnot in 1824.

However, in 1860 this principle would be formulated again by Rudolf Clausius and William Thompson, adding then what we now call the First Law of Thermodynamics. Later the third, more modern, would appear, thanks to Walther Nernst’s studies between 1906 and 1912, which is why he is known as Nernst’s postulate.

Finally, the so-called “zero law” would appear in 1930 , proposed by Guggenheim and Fowler. It should be said that not in all areas it is recognized as a true law.

  1. First Law of Thermodynamics

First Law of Thermodynamics
Energy cannot be created or destroyed, only transformed.

The title of this law is “Energy Conservation Law”, as it dictates that, in any physical system isolated from its surroundings, the total amount of energy will always be the same , even though it can be transformed into a form of energy to different ones. Or in other words: “Energy cannot be created or destroyed, only transformed.”

Thus, by supplying a certain amount of heat (Q) to a physical system, its total amount of energy can be calculated by finding the difference in the increase of its internal energy (ΔU) plus the work (W) performed by the system on its surroundings. Or expressed in a formula: Q = ΔU + W, or also: ΔU = Q – W , which means that the difference between the energy of the system and the work done will always be detached from the system as heat energy (heat).

To exemplify this law, let’s imagine the engine of an airplane . It is a thermodynamic system to which fuel enters which, reacting with the oxygen in the air and the spark generated by combustion, releases a significant amount of heat and work. The latter is precisely the movement that pushes the plane forward. So: if we could measure the amount of fuel consumed, the amount of work (movement) and the amount of heat released, we could calculate the total energy of the system and conclude that the energy in the engine remained constant during the flight: nor was it created nor was energy destroyed, but it was changed from chemical energy to caloric energy and kinetic energy (movement, that is, work).

  1. Second law of thermodynamics

Second law of thermodynamics
Given enough time, all systems will eventually tend to imbalance.

This second principle, sometimes called the Entropy Law, can be summed up in that “the amount of entropy in the universe tends to increase in time . ” That means that the degree of disorder of the systems increases once they have reached the equilibrium point, so given enough time, all systems will eventually tend to imbalance.

This law explains the irreversibility of physical phenomena, that is, the fact that once a paper is burned, it cannot be returned to its original form . And in addition, it introduces the entropy state function (represented as S), which in the case of physical systems represents the degree of disorder, that is, its inevitable loss of energy. Therefore, entropy is linked to the degree of energy not usable by a system, which is lost to the environment . Especially if it is a change from a state of equilibrium A to a state of equilibrium B: the latter will have a higher degree of entropy than the former.

The formulation of this law establishes that the change in entropy (dS) will always be equal to or greater than the heat transfer (Q) , divided by the temperature (T) of the system. That is, that dS ≥ δQ / T.

And to understand this with an example, just burn a certain amount of matter and then gather the resulting ashes. By weighing them, we will verify that it is less matter than in its initial state. Why? Because part of the matter became irrecoverable gases that tend to dispersion and disorder, that is, that are lost in the process. That is why this reaction cannot be reversed.

  1. Third law of thermodynamics

Third law of thermodynamics
Upon reaching absolute zero the processes of the physical systems stop.

This principle concerns temperature and cooling, stating that the entropy of a system that is brought to absolute zero will be a definite constant . In other words:

  • Upon reaching absolute zero (0 K), the processes of the physical systems stop.
  • Upon reaching absolute zero (0 K), the entropy will have a constant minimum value.

It is difficult to reach the so-called absolute zero daily (-273.15 ° C), as if to give a simple example of this law. But we can equate it to what happens in our freezer: the food we store there will be cooled so cold and at such low temperatures that it will slow down or even stop the biochemical processes inside. This is the reason that its decomposition is delayed and lasts much longer suitable for consumption .

  1. Zero law of thermodynamics

Thermal equilibrium
The “zero law” is logically expressed as follows: if A = C and B = C, then A = B.

The “zero law” is known by that name because although it was the last to run, it establishes basic and fundamental precepts regarding the other three . But in reality its name is the Law of Thermal Equilibrium. This principle dictates that: “If two systems are in thermal equilibrium independently with a third system, they must also be in thermal equilibrium with each other.” It is something that can be expressed logically as follows: if A = C and B = C, then A = B.

To put it more simply, this law allows us to establish the principle of temperature , based on the comparison of the thermal energy of two different bodies: if they are in thermal equilibrium with each other, then they will necessarily have the same temperature. And, therefore, if both are in thermal equilibrium with a third system, then they will also be in each other.

Everyday examples of this law are easy to find. When we get into the hot or cold water, we will notice the temperature difference only for a while , since our body will then enter into thermal equilibrium with the water and we will not notice the difference anymore. It also happens when we enter a hot or cold room: we will notice the temperature initially, but then we will stop perceiving the difference because we will enter into thermal equilibrium with it.

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