In Thermodynamic traditional, one calls thermodynamic system , a portion of the universe which one isolates by the thought from the remainder from the universe that one baptizes then external medium.

The thermodynamic system is not inevitably defined by a material border, it is not either necessarily related. The drops of liquid in a fog, for example, define a thermodynamic system.

The term of universe used here should not lend to confusion, it actually acts of the portion of universe in interaction with the studied system. To be more precise, the movement of the planet Jupiter or the hour of the tide does not need to be considered to interpret the gas reaction of contained in a soap bubble.

Separation, even fictitious, between the system and the external medium is called wall or enclosure. According to the nature and the properties of this wall, a thermodynamic system will be described as:

• open system if there can be exchange of Matière and of energy between the system and the external medium. (example: living beings)

• system closed if this system exchanges only energy with the external medium, in the form of heat or of work. (examples: Batteries, refrigerator under operation)

• system isolated if there is no exchange with the external medium. In this case the wall is described as Adiabatique and must be indeformable . (examples: a Thermos bottle, a Calorimeter)