Partt of the physics in which the relationships between heat and the remaining forms of energy are studied.”

Thermodynamics (from the Greek thermo, which means “heat” and dynamis, which means “force”) is the branch of physics that describes the states of equilibrium at the macroscopic level. It constitutes a phenomenological theory, based on deductive reasoning, that studies real systems, without modeling and follows an experimental method.The equilibrium states are studied and defined by means of extensive magnitudes such as internal energy, entropy, volume or molar composition of the system, or by means of non-extensive magnitudes derived from the previous ones such as temperature, pressure and chemical potential; Other quantities such as magnetization, electromotive force and those associated with the mechanics of continuous media in general can also be treated by means of thermodynamics.

It is important to emphasize that Thermodynamics offers a formal apparatus applicable only to equilibrium states, defined as that state towards “the one that every system tends to evolve and characterized in that in it all the properties of the system are determined by intrinsic factors and not by influences previously applied external” Such terminal states of equilibrium are, by definition, independent of time, and the entire formal apparatus of Thermodynamics – all laws and thermodynamic variables – are defined in such a way that it could be said that a system is in equilibrium if its properties they can be consistently described using thermodynamic theory. The equilibrium states are necessarily consistent with the contours of the system and the restrictions to which it is subject. By means of the changes produced in these restrictions (that is, by removing limitations such as preventing the expansion of the volume of the system, preventing the flow of heat, etc.), the system will tend to evolve from one state of equilibrium to another; comparing both equilibrium states, Thermodynamics allows to study the processes of mass and thermal energy exchange between different thermal systems. To have greater handling, it is specified that heat means “energy in transit” and dynamic refers to “movement”, so, in essence, thermodynamics studies the circulation of energy and how energy infuses movement. Historically, thermodynamics developed from the need to increase the efficiency of the first steam engines.

Typical thermodynamic system showing the input from a heat source (boiler) on the left and the output to a heat sink (condenser) on the right. The work is extracted in this case by a series of pistons.

As a phenomenological science, thermodynamics is not concerned with offering a physical interpretation of its magnitudes. The first of these, internal energy, is accepted as a macroscopic manifestation of the laws of conservation of energy at the microscopic level, which allows to characterize the energy state of the macroscopic system.The starting point for most thermodynamic considerations are the principles of thermodynamics, which postulate that energy can be exchanged between systems in the form of heat or work, and that it can only be done in a certain way. A magnitude called entropy is also introduced, which is defined as that extensive function of internal energy, volume and molar composition that takes maximum equilibrium values: The principle of maximization of entropy defines the sense in which the system evolves from one state of equilibrium to another. It is Statistical Mechanics, intimately related to Thermodynamics, which offers a physical interpretation of both magnitudes: internal energy is identified with the sum of the individual energies of the atoms and molecules of the system, and entropy measures the degree of order and the dynamic state of the systems, and has a very strong connection with information theory. In thermodynamics, the interactions between different systems are studied and classified, which leads to defining concepts such as thermodynamic system and its contour. A thermodynamic system is characterized by its properties, related to each other through state equations.

With these tools, thermodynamics describes how systems respond to changes in their environment. This can be applied to a wide variety of science and engineering topics, such as motors, phase transitions, chemical reactions, transport phenomena, and even black holes. The results of thermodynamics are essential for chemistry, physics, chemical engineering, etc., to name a few.

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