[Encyclios]

Constant volume (isochoric) transformation

A constant-volume transformation (also called isochor) is a type of thermodynamic transformation that can occur in a system bounded by rigid, fixed walls. Clearly, since it results null the work related to volume changes, the first principle assumes the formulation \(\Delta U_{1,2} = Q_{1,2}\), according to which the thermal energy given (or subtracted) to the system is found entirely as an increase (or decrease) of internal energy of the system.

An interesting thing, in a transformation of this type, is the following: in general, the calculation of the heat exchanged in a transformation needs the knowledge of the transformation itself, state by state, and this is possible only if the transformation is quasi-static; on the contrary, when the volume remains constant, the heat exchanged is equal to the variation \(\Delta U_{1,2}\) of the internal energy and therefore its calculation needs only the knowledge of the initial state and the final state.