Open systems
Formalism and principles of thermodynamics

An open system is delimited by a possibly deformable envelope with at least one opening that allows material transfer.

The envelope and the portion of space it delimits are called the control surface and control volume respectively. Describing such a system requires, in particular, precise marking of openings and their identification in relation to material transfers (inputs or outputs) (figure 11 ).

A priori, material transfer modifies the total mass of the material contained in the control volume for the duration of the transfer.