Thermodynamics: Concept of System and Surrounding

The system and its surroundings are effectively "everything." This is due to the fact that the total energy in the system and its surroundings are constant. Although energy can flow between these two, the total energy remains constant. As a result, we refer to the system and its surroundings as the "universe." Because energy is constant in the universe (from a chemistry thermodynamics standpoint), we can discuss changes in the system and its surroundings as changes in the universe.

Universe = System + Surroundings

?It is critical in thermodynamics to define a system and its surroundings because this concept serves as the foundation for many different types of descriptions and calculations. A thermodynamic system is a body of matter and/or radiation that is enclosed in space by walls with defined permeabilities that isolate it from its surroundings. Other thermodynamic systems or physical systems that are not thermodynamic systems may be present in the surroundings. When a wall of a thermodynamic system is described as being 'permeable' to all matter, all radiation, and all forces, it may be purely fictitious. A thermodynamic system's state can be fully described in a variety of ways using various sets of thermodynamic state variables.

In a nutshell, a thermodynamic system is a quantity of matter or a region in a space of interest. The surrounding mass or region is referred to as the surroundings, and the surface that separates the system and the surroundings are referred to as the boundary. A thermodynamic system can be (1) isolated, (2) adiabatic, (3) closed, or (4) open, depending on the type of exchange between the system and its surroundings.

Closed system and isolated system

A closed thermodynamic system is bounded by impermeable-to-matter walls while it allows heat to be exchanged with the surroundings. In the closed system, the exchange of mass is not allowed and consequently, the mass in the system is always conserved. If the exchange of energy is also not allowed, the system is called the isolated system. In the piston-cylinder device shown below, the boundary is movable but no mass crosses it if gas leakage is assumed negligible. In this case, the device shown above is classified as a closed system.

Isolated system

An isolated thermodynamic system has heat-insensitive walls that are impermeable to all forms of matter and forces.

In another word, an isolated system is impermeable to both energy and matter

Open system

An open system can exchange both energy and matter with its surroundings. An example of the open system is shown below. This device is similar to that shown above but it is classified as an open system since the mass flow crossing the boundary is present. If the boundary is stationary and the inlet and outlet mass flow rates are equal and do not change with time, the system is called a steady flow system. For example, the main components used in the thermal power plant such as the boiler, turbine, condenser, and pump are often treated as steady flow systems.

Thermodynamic system and state functions

The state of a thermodynamic system is defined by its internal energy, U, and entropy, S, as well as a set of state variables, including temperature T, hydrostatic pressure p, volume V, and a number of moles of components n. The former two thermodynamic quantities are functions of the state variables. Among these, temperature and pressure constitute intensive variables, which define properties independent of the size of the system, whereas volume, internal energy, and entropy constitute extensive variables, values of which vary with the size of the system. Intensive molar properties are obtained by dividing extensive properties by the number of moles n. The molar volume Vm is defined, for instance, by Vm=V/n. It is important to point out that the aforementioned state functions depend on the conditions, or in other words, the state of the system, but not on the path, process, or history that was followed to reach it.

A thermodynamic system is subject to external interventions called thermodynamic operations; these alter the system's walls or its surroundings; as a result, the system undergoes transient thermodynamic processes according to the principles of thermodynamics. Such operations and processes affect changes in the thermodynamic state of the system.

Credit: Google