A thermodynamic system is a part of the physical universe with a specific boundary for observation. This boundary may be defined by real or imaginary walls.
A system contains what is called an object of study. An object of study is a substance with a large number of molecules or atoms. This object is made up of a geometric volume of macroscopic dimensions subjected to controlled experimental conditions.
A thermodynamic system can undergo internal transformations and exchange energy and/or matter with the external environment.
Definition: What is a thermodynamic system?
A thermodynamic system is defined as a quantity of matter or a region in space on which attention is focused in the analysis of a problem.
Everything that is part of the outside of the system is called the surroundings or environment. The system is separated from the environment by the system boundary.
The boundary can be fixed or mobile. A system and its surroundings together.
Classification of thermodynamic systems
In thermodynamics, systems can be classified into three main categories according to their ability to exchange matter and energy with the environment:
Open system
A system is considered open when it allows the exchange of both matter and energy with its surroundings across its boundary. The transfer of energy can occur in the form of heat, work or other energy manifestations.
Example: A swimming pool filled with water is an open system, as water can flow in and out, and the water temperature can vary due to factors such as heating or wind action, which can cause evaporative cooling.
Closed system
A closed system is one that allows the exchange of energy with its surroundings across its boundary (whether in the form of heat, work, or other forms of energy), but does not allow the transfer of mass.
Example: A cylinder with gas sealed by a valve is a closed system as long as the valve remains closed, since it can be heated or cooled without losing mass. However, if the valve is opened, the gas can escape and the system would become open.
Isolated system
An isolated system meets two fundamental conditions:
- It does not exchange matter with the environment.
- It does not exchange energy with the environment.
Example: The universe is generally considered an isolated system, since there is no evidence of exchanging matter or energy with anything outside of it.
Additional subdivisions of thermodynamic systems
Since systems can present great internal complexity, it is possible to subdivide them into smaller subsystems according to different criteria.
According to its internal composition:
- Simple thermodynamic system: It is delimited by a single boundary and does not contain internal subdivisions.
- Composite thermodynamic system: Contains within its boundary one or more subsystems separated by internal boundaries.
According to its homogeneity:
- Homogeneous systems: They present uniform macroscopic properties throughout their entire extension.
- Heterogeneous systems: They exhibit variations in their macroscopic properties in different regions. An example of a heterogeneous system is a liquid in equilibrium with its vapor.
Characteristics of thermodynamic systems
Thermodynamic systems have various characteristics that determine their behavior and the laws that govern them:
- System boundaries : These can be real or imaginary and determine the limits of the system. These boundaries can be rigid or mobile, depending on the type of system.
- Exchange of matter and energy : Depending on the type of system, there may or may not be a transfer of mass and energy with the environment. Open systems allow both exchanges, closed systems only energy, and isolated systems none.
- States and Equilibrium : A system can be described by thermodynamic variables such as temperature, pressure, and volume. A system in equilibrium experiences no net change in these variables.
- Thermodynamic processes : These are the transformations that a system undergoes due to interaction with the environment, such as expansion, compression and heat transfer.
- Laws of thermodynamics : Systems obey fundamental principles, such as the conservation of energy and the tendency toward equilibrium.