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.
Types of thermodynamic system
Based on the type and combination of properties of the control surfaces, the following types of thermodynamic system can be distinguished:
- open system: its boundaries are permeable, albeit partially, to the passage of matter;
- closed system: it is delimited by boundaries impermeable to the passage of matter; in other words, a closed system cannot exchange matter with the environment, but only energy;
- isolated system: a system is said to be isolated when, in addition to being closed, it does not exchange energy with the external environment.
Intensive and extensive properties of a thermodynamic system
Each characteristic of a thermodynamic system is called property; the properties of a system are divided into:
- intensive properties: these are those that do not depend on the size of the system, for example: temperature, pressure, density (if a system is large, or small, it does not influence the temperature, pressure or density of the system);
- extensive properties: these are those that depend on the size, or extent, of the system, for example: mass, volume, total energy (if a system is large, or small, it influences the mass, the volume and the total energy of the same, as the mass will be larger, or smaller, the volume will be larger, or smaller, the total energy contained will be larger or smaller).
Thermodynamic transformation of a system
It takes the name of thermodynamic transformation (or thermodynamic process) of a system, any modification which involves the variation of at least one of its internal properties. Depending on whether this variation is infinitesimal or finite we will have an infinitesimal or finite transformation.
It is necessary to distinguish a transformation from a simple physical phenomenon: for example, a simple system consisting of a fluid which changes its position in space without other consequences, represents a physical phenomenon, but certainly not a transformation: in fact, if the fluid changes its position, probably changes its potential energy (related to quota), but this is an external properties of the system. The same applies if the system under examination changes shape but remains unchanged in volume. A particular transformation is the so-called cycle: it is a finite transformation that brings the system back to the same state from which it started.
Energy transfers: heat and work
The energy that, during any transformation, crosses the surfaces of the system is given the name of heat or work: we speak of heat when the energy is transferred as a consequence of a temperature difference existing between the system and the environment; otherwise, if the energy flow does not derive from a temperature difference, this is called work. According to these last definitions, it is possible to speak of heat and work only in the presence of an energy flow between the system and the environment. In other words, heat and work are not state system properties.
Symbols and sign conventions for work and heat
Heat and work are indicated, respectively, with the symbols Q and W. Furthermore, although heat and work are not state properties, it still makes sense to consider the amount of heat exchanged per unit of mass (or weight) of the system and the work done per unit of mass (or weight) of the system. For the energy balances of a system it is always necessary to give a sign to the numerical value of heat and work, depending on the direction of their flow; then there are two different conventions for work and heat:
- the heat is positive if the energy is supplied to the system, while it is negative otherwise;
- the work is positive if the energy is supplied to the environment, while it is negative otherwise.