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Thermodynamics is the branch of classical physics that studies and describes the thermodynamic transformations induced by heat and work in a thermodynamic system, as a result of processes that involve changes in the state variables of temperature and energy.

Classical thermodynamics is based on the concept of macroscopic system, that is, a portion of physical mass or conceptually separated from the external environment, which is often assumed for convenience that is not disturbed by the exchange of energy with the system. The state of a macroscopic system that is in equilibrium conditions is specified by quantities called thermodynamic variables or state functions such as temperature, pressure, volume, and chemical composition. The main notations in chemical thermodynamics have been established by the international union of pure and applied chemistry.

However, there is a branch of thermodynamics, called thermodynamics of non-equilibrium that studies the thermodynamic processes characterized by the inability to achieve stable equilibrium conditions.

Laws of thermodynamics

The principles of thermodynamics were enunciated during the nineteenth century and regulate the thermodynamic transformations, their progress, their limits. They are real axioms, unproven and unprovable, based on experience, on which the whole theory of thermodynamics is based.

We can distinguish three basic principles, plus a "zero" principle that defines temperature and that is implicit in the other three.

Zero law of thermodynamics

When two interacting systems are in thermal equilibrium, they share some properties, which can be measured, which gives them a precise numerical value. As a result, when two systems are in thermal equilibrium with a third, they are in equilibrium with each other and the shared property is the temperature. The zero principle of thermodynamics simply says that, if a body "A" is in thermal equilibrium with a body "B" and "B" is in thermal equilibrium with a body "C", then "A" and "C" are in thermal equilibrium equilibrium between them.

This principle explains the fact that two bodies at different temperatures, between which heat is exchanged (even if this concept is not present at the zero principle) end up reaching the same temperature.

In the kinetic zero formulation principle of thermodynamics is a tendency to arrive at a common average kinetic energy of the atoms and molecules of the bodies between which leads to heat exchange: on average, as a result of the collisions of the particles from the hotter body, on average, faster, with the colder body particles, on average slower, there will be energy going from the first to the second, tending to equal temperatures. The efficiency of the energy exchange determines specific heats of the elements involved.

First law of thermodynamics

When a body is placed in contact with a relatively colder body a transformation takes place that leads to a state of equilibrium in which the temperatures of the two bodies are equal. To explain this phenomenon, scientists of the eighteenth century assumed that a substance, present in greater quantities in the hottest body, passed to the coldest body.

This hypothetical substance, called caloric, was thought of as a fluid capable of moving through the mass improperly called matter. The first principle of thermodynamics identifies heat as a form of energy that can be converted into mechanical work and stored, but that is not a material substance. It was experimentally demonstrated that heat, originally measured in calories and work or energy, measured in joules, are actually equivalent. Each calorie is equivalent to approximately 4,186 joules.

The first principle is, therefore, a principle of conservation of energy. In each thermal machine or a thermal engine, a certain amount of energy is transformed into work: there can be no machine that produces work without consuming energy. A similar machine, if it existed, would in fact produce the so-called perpetual motion of the first species.

The first principle is traditionally established as:

The variation of the internal energy of a closed thermodynamic system is equal to the difference between the heat supplied to the system and the work done by the system in the environment.

The corresponding mathematical formulation is expressed as:

& Delta; U = Q - L

where U is the internal energy of the system, Q the heat supplied to the system and L the work done by the system.

Internal energy means the sum of the kinetic energies and the interaction of the different particles of a system. Q is the heat exchanged between the environment and the system (positive if it is supplied to the system, negative if it is transferred by the system) and L the work performed (positive if the system does it in the environment, negative if the environment does the system). The sign convention is influenced by the link with the study of thermal engines, in which heat is (partially) transformed into work.

The alternative and equivalent formulations of the first principle are:

  • For an open system, qw =? And where? E is intended for the variation of the total energy, which is not more than the sum of the changes in the internal energy, the kinetic energy and the potential energy that that system has. We see that for a closed system the variations of kinetic and potential energy are zero and, therefore, we refer to the previous relationship.
  • For a thermodynamic cycle, q = w, since the total energy variation is zero, the system that has, at the end of each cycle, again in the same starting conditions.

Second law of thermodynamics

There are several statements of the second principle, all equivalent, and each of the formulations emphasizes a particular aspect. It establishes that "it is impossible to make a cyclic machine whose only result is the transfer of heat from a cold body to a warm body" (Clausius statement) or, equivalently, that "it is impossible to carry out a transformation whose result is only that of converting heat extracted from a single source into mechanical work "(Kelvin's statement).

This last limitation denies the possibility of carrying out the so-called perpetual movement of the second species. L 'entropy the total of an isolated system remains unchanged when a reversible transformation takes place and increases when an irreversible transformation takes place.

Third law of thermodynamics

It is closely related to the latter and, in some cases, it is considered a consequence of the latter. It can be stated by saying that "it is impossible to reach absolute zero with a finite number of transformations" and provides a precise definition of the magnitude called entropy.

It also states that the entropy for a perfectly crystalline solid, at a temperature of 0 kelvin is equal to 0. It is easy to explain this statement through molecular thermodynamics: a perfectly crystalline solid is composed of a single complex (All of them are ways to organize the molecules, if the molecules are all the same, regardless of the way they are arranged, macroscopically the crystal is always the same) and, being at 0 kelvin, the energy of vibration, translation and rotation of the particles that it is composed of nothing, therefore, of Boltzmann's law S = k ln (1) = 0 where 1 are the complexes (in this case only one).

History of thermodynamics

It was Sadi Carnot, in 1824, the first to demonstrate that work can be obtained from the exchange of heat between two sources at different temperatures. Through the theorem of Carnot and the ideal machine of Carnot (based on the Carnot cycle) he quantified this work and introduced the concept of thermodynamic efficiency.

In 1848, Lord Kelvin, using the Carnot machine, introduced the concept of effective thermodynamic temperature and is responsible for a statement of the second principle of thermodynamics.

In 1850 James Prescott Joule demonstrated the equality of the two forms of energy (then it was believed that the caloric liquid still existed).

Having arrived at this, the problem arose that, if it were possible to obtain the total heat of the work, it would not have been possible to obtain the inverse. This result also landed Clausius who in 1855 presented his inequality to recognize reversible processes of the irreversible and state function of entropy.

In 1876 Willard Gibbs published the treatise "On the balance of heterogeneous substances" (On the balance of heterogeneous substances) that showed how a thermodynamic process could be represented graphically and how to study in this way energy, entropy, volume, temperature and the pressure could foresee the eventual spontaneity of the considered process.

The case of thermodynamics is emblematic in history and in 'the epistemology of science: it is one of those cases in which practice has been a pioneer in the theory itself: the first is designed for the steam engine, below , its theoretical functioning was systematized through its basic principles.

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Last review: March 7, 2018