Basic thermodynamics, the study of energy

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All physical or chemical process is in the inherent need to know and studies the various factors that compose them and for this thermodynamics intervenes, which is basically who is responsible for the study of energy and its transformations in all microscopic systems, ie in points or concrete physical states while in balance. This means that their properties do not vary with time when they are isolated and have no hysteresis

Thermodynamics is a theory of a great generality, applicable to systems of a very elaborate structure with all forms of complex mechanical, electrical and thermal properties.

When entering into thermodynamics it is important to be clear that a system is a place you want to study, limited by a border. The system and its environment form the universe

However, these systems can be:

Open: Allow entry and exit of the mass
Closed: Where does not enter or leave the dough
Isolated: Where there is no transfer of mass and energy

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In the same order of ideas, we must emphasize that every thermodynamic system is defined by variables or system coordinates, which are the set of properties that characterize the system. There are two types:

a) Physical variables: The fundamentals are Pressure (P), Volume (V) and Temperature (T); P and T are intensive variables (independent of the size of the system) and V is extensive (depends on the size of the system).
b) Chemical Variables: Usually the numbers of moles of each component are used. Strictly speaking, thermodynamics is more interested in chemical potentials.

After defining and knowing the basic principles of thermodynamics it is important that we know that thermodynamics, in turn, is governed by 4 fundamental laws.

Zero law of thermodynamics.

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This law is known as "thermal equilibrium". The thermal equilibrium should be understood as the point where balanced systems have the same temperature. This law states that if two systems A and B are at the same temperature and B is at the same temperature as a third C system, then A and C have the same temperature.

First Law of Thermodynamics.

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This was the first law that was conceived to explain and govern thermodynamic principles and is perhaps the most important. This law states that energy can not be created or destroyed but transformed from one type of energy to another.
This law also relates the work and heat transferred exchanged in a system through a new thermodynamic variable, the internal energy.
For this, we must be clear that the internal energy.
The general equation of energy conservation is as follows: .

That applied to thermodynamics taking into account the thermodynamic sign criterion, it remains of the form: .

Where U is the internal energy of the system (isolated), Q is the amount of heat contributed to the system and W is the work done by the system. This last expression is just as often found in the form ΔU = Q + W.

The second law of thermodynamics.

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This law basically states that you can only perform a job by passing the heat of a body with a higher temperature to one that has a lower temperature. This law also gives a fairly accurate definition of entropy, feel this a fraction of energy of a system that can not be converted into work.

That is, entropy can never decrease. Therefore, when an isolated system reaches a maximum configuration of entropy, it can no longer undergo changes (equilibrium has been reached).
In this way, the second law imposes restrictions on energy transfers that hypothetically could be carried out taking into account only the First Principle.
Due to this law, it is also necessary that the spontaneous heat flow is always unidirectional, from the higher temperature bodies to the lower temperature ones, until reaching a thermal equilibrium.

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Third law of thermodynamics.

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This law states that it is impossible to reach a temperature equal to absolute zero by a finite number of physical processes. That is to say that it is basically possible to approach absolute zero indefinitely, but you can never reach it. However, the behavior of entropy when we approach absolute zero is the reason for this.
In conclusion, it can also be stated that as a given system approaches absolute zero, its entropy tends to a specific constant value. The entropy of pure crystalline solids can be considered zero under temperatures equal to absolute zero.