Nothing is lost, nothing is not created, all changes.

The first principle of the Thermodynamique stipulates that at the time of any transformation, there is Conservation of energy.

In the case of the thermodynamic systems closed, it is stated in the following way: During a unspecified transformation of a system closed, the variation of sound energy is equal to the quantity of energy exchanged with the external medium, in the form of heat and work.

Expression

This principle can be expressed by the formula:

$\backslash \; Delta\; E\; =\; \backslash \; Delta\; U\; +\; \backslash \; Delta\; EC.\; +\; \backslash \; Delta\; Ep=\; W\; +\; Q\; \backslash ,$

• $\backslash \; Delta\; E\; \backslash ,$ is the total variation of energy of the thermodynamic Système.

• $\backslash \; Delta\; U\; \backslash ,$ is the variation of the internal energy of the system; i.e. its clean energy corresponding to the kinetic and potential energies microscopic, of the particles which constitute it.
• $\backslash \; Delta\; EC.\; \backslash ,$ is the variation of the kinetic energy on the scale Macroscopique (movement of the system in a given reference frame).
• $\backslash \; Delta\; Ep\; \backslash ,$ is the variation of the potential energy on a macroscopic scale, of the interacting system with fields gravitational or electromagnetic.

• $W\; \backslash ,$ is the part of the energy which corresponds to the work exchanged with the external medium. The work is not a function of state but a mode of ordered transfer of energy between the external medium and the system.

• $Q\; \backslash ,$ is the quantity of energy concerned in the form of heat. It is transmitted primarily by three processes of heat exchange: thermal Conduction, Convection, Radiation. Heat is not either a function of state but a mode of transfer of microscopic energy disordered. It is to some extent a thermal transfer of Agitation between the system and the external medium, which is by nature disordered.

Remarks

• When the systems are at rest on a macroscopic scale what in general, is the case for the thermodynamic transformations, the energies kinetic $Ec~$ and potential $Ep~$ remain constant and only energy interns $U~$ system varies (for example: a chemical engine, a calorimeter…).

The first principle is written then:

$\backslash \; Delta\; U\; =\; W\; +\; Q\; ~$

• There exist several kinds of ordered transfer of energy: work of the compressive forces, electric work provided during an electrochemical reaction of pile, work of to electromagnetic interactions…

In the case of the work induced by the compressive forces which correspond to the most current form of work met in traditional thermodynamics, it is expressed as follows: $\backslash \; delta\; W\_\; \{FP\}\; =\; -\; p\_\; \{ext.\}\; FD\; \backslash ,$

The sign - takes account owing to the fact that at the time of a relaxation there is increase in volume (FD > 0) and the system provides in this case, of work with the external medium; work counted negatively according to the rule of the signs.

$p\_\; \{ext.\}\; \backslash ,$ being pressure of the external medium,

$dV\; \backslash ,$ being an infinitesimal variation of volume which corresponds on a mathematical level to the Différentielle of volume.

See too

Related articles

• Thermodynamic

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