CONCEPTS

Thermodynamics

We explain what thermodynamics is and what a thermodynamic system consists of. Also, what are the laws of thermodynamics.

  1. What is thermodynamics?

The branch of physics that studies the mechanical actions of heat and other similar forms of energy is called thermodynamics (from Greek thermós , “heat” and dynamos , “power, force”) . His study addresses objects as real macroscopic systems, through the scientific methodand deductive reasoning, paying attention to extensive variables such as entropy , internal energy or volume ; as well as non-extensive variables such as temperature , pressure or chemical potential, among other types of magnitudes.

However, thermodynamics does not offer an interpretation of the magnitudes it studies , and its objects of study are always systems in a state of equilibrium, that is, those whose characteristics are determinable by internal elements and not so much by external forces acting on them. For that reason, he believes that energy can only be exchanged from one system to another by way of heat or work.

The formal study of thermodynamics began thanks to Otto von Guericke in 1650 , a German physicist and jurist who designed and built the first vacuum pump, refuting Aristotle with his applications and his maxim that “nature abhors a vacuum.” After this invention, scientists Robert Boyle and Robert Hooke perfected their systems and observed the correlation between pressure, temperature and volume. Thus the principles of thermodynamics were born.

  1. Thermodynamic system

Thermodynamics
Open systems exchange energy and matter with their surroundings.

Thermodynamic system is understood as a part of the universe that, for the purpose of study, is conceptually isolated from the rest and attempts to understand autonomously , taking note of the ways in which energy changes or is preserved, and at the same time, if there are , their exchanges of matter and / or energy with the environment or with other similar systems. It is, therefore, a method of studying thermodynamics.

The main classification criteria for these systems is based on their degree of isolation from the environment, thus distinguishing between:

  • Open systems. Those who freely exchange energy and matter with their surroundings, as do most of the systems known in everyday life: a glass with cold water will slowly heat up due to the action of the heat of the surrounding air .
  • Closed systems. Those who exchange energy with their environment, but not matter. It is what happens with a closed container, like a can, whose content is invariable, but loses heat over time , dissipating it in the surrounding air.
  • Isolated systems. Those who, to some extent, do not exchange energy or matter with the environment. There are no perfectly isolated systems, of course, but to a certain degree: a thermos containing hot water will preserve its temperature for a while, enough to keep it isolated for a while.
  1. Laws of thermodynamics

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

Thermodynamics is governed by the provisions of its four fundamental principles or laws, formulated by various scientists throughout the history of this discipline . These principles or laws are:

  • First principle, or Law of Conservation of energy. It states that the total amount of energy in any physical system isolated from its environment will always be the same even if it can be transformed from one form of energy to many different ones. In fewer words: “Energy cannot be created or destroyed, only transformed.”
  • Second principle, or Law of Entropy. This law dictates that “the amount of entropy in the universe tends to increase over time,” which means that the degree of disorder of the systems (entropy) increases once they reach a point of equilibrium. Thus, given enough time, all systems will tend to imbalance. This law explains the irreversibility of physical phenomena: once a paper is burned, it cannot cause it to return to its initial form.
  • Third principle, or Law of absolute zero. It dictates that the entropy of a system that is brought to absolute zero will always be a definite constant, which in other words means that when reaching absolute zero (-273.15 ° C or 0 K), the processes of physical systems they stop, and entropy will have a constant minimum value.
  • Zero principle or Law of thermal equilibrium. It is called “zero law” because, although it was the last to run, the basic and fundamental precepts it establishes take precedence over the other three laws. It dictates that “If two systems are in thermal equilibrium independently with a third system, they must also be in thermal equilibrium with each other.”
  1. Chemical thermodynamics

Chemical thermodynamics is a separate field of study, focused on the correlation between heat and work, and chemical reactions , all framed by the principles of thermodynamics. That is, it is the application of the laws of thermodynamics, especially the first two, to the world of reactions between substances and compounds , to obtain the so-called “fundamental equations of Gibbs”, which govern the way in which The chemical energy contained in the different compounds changes and is transmitted, or how the degree of entropy of the universe increases each time a spontaneous reaction occurs.

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