Closed exchange matter , while open .

Please note: the open or closed nature of a system depends on the you choose to define it, which can lead to small difficulties.

Most of the components involved in thermal machines operate in , all being crossed .

A closed system exchanges with the exterior work, denoted W, and heat, denoted Q. The state function representing its own energy is the .

An open system exchanges useful work with the exterior, denoted τ, and heat, denoted Q. The state function representing its own energy is the