A thermodynamic system is a defined quantity of matter, or a defined portion of space (geometrically delimited). This system is delimited by surfaces (or walls or boundaries), also known as control surfaces; everything that is external to the system, and is able to interact with it, is called an environment.
The control surface of a thermodynamic system represents the boundaries of the thermodynamic system and characterizes it according to its intrinsic properties of interaction between the system and the environment; in fact, depending on the type of control surface we will have closed, open or isolated thermodynamic systems. The boundary of a thermodynamic system can be classified with three essential parameters: permeability, rigidity and thermicity.
- Permeability: the boundary of the control surface may not allow a flow of matter (mass) or porous, that is, allow a flow of matter, even selectively.
- Rigidity: the boundary of the control surface can be rigid not allowing changes in volume (and therefore also in work), or mobile allowing changes in volume (and therefore also in work).
- Thermicity: the boundary of the control surface can be adiabatic, that is, it does not allow heat exchange, or diathermic, that is, it allows heat exchange.
The thermodynamic state of a thermodynamic system is defined as the set of values assumed by those measurable thermophysical properties, or by the physical quantities that characterize it, such as pressure, volume, entropy, temperature and so on. The properties thus characterized are defined quantities or state parameters, because of the correspondence between the values assumed and the state identified. It should be noted that this formulation configures a state of macroscopic character, in the sense that it is identified by values of the quantities deriving from the average on a large number of particles, and can, in fact, correspond to infinite distributions of positions and speeds at the microscopic level.
A thermodynamic state is in equilibrium if the parameters that define the thermodynamic state are stationary or independent of time. You can vary the parameters that define the state, that is, the state variables, in many ways. If the variation of the state variables and therefore of the state itself leads from one state of equilibrium to another state of equilibrium it is said that a thermodynamic transformation has occurred. The graphical representation of the set of thermodynamic states that a system can assume when varying some thermodynamic quantities (for example temperature, pressure, volume, and composition) takes place through state diagrams.