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Thermodynamic system

Thermodynamic system

A thermodynamic system is a portion of the material space, separated from the rest of the thermodynamic universe (that is, from the external environment) by means of a real or imaginary control surface (or edge), rigid or deformable.

A thermodynamic system can be the seat of internal transformations and exchanges of matter and / or energy with the external environment (that is, everything external to the system that interacts with it).

Classification of thermodynamic systems

Within thermodynamics there are three main types of thermodynamic systems: open, closed and isolated. In particular:

  • Open thermodynamic system. It is said that a system is open if it allows a flow with the external environment, both mass and energy (through heat and / or work and / or another form of energy), through its limit; an example of an open system is a pond filled with water, in which water can enter or leave the pool and can be heated by a system of heating and cooling by wind.
  • Thermodynamic closed system. In thermodynamics it is said to be closed if it allows a flow of energy with the external environment, through its frontier, (by means of heat and / or work and / or another form of energy), but not by mass; an example is a cylinder held closed by a valve, which can heat or cool, but does not lose mass (while the same cylinder behaves like an open system if we open the valve).
  • Adiabatic system. A system is adiabatic when it does not exchange heat with the environment.
  • Isolated thermodynamic system. It is said that a system is isolated if it does not allow a flow of energy or mass with the external environment.

Each of these systems can still be schematized due to its internal complexity. The possibility to subdivide into smaller subsystems. In this way we will obtain that an open, adiabatic, closed, adiabatic and isolated system can be:

  • Simple thermodynamic system: a system is simple if it is limited by a limit, within which there are no other walls.
  • Composite thermodynamic system: a system is composed if it is delimited by a boundary, within which other walls exist.

Microscopic and macroscopic description of the thermodynamic system

A thermodynamic system can be seen from a macroscopic and microscopic point of view.

Characteristics of a macroscopic description of a simple system:

  • No assumptions are made about the structure of the system.
  • The quantities needed to describe it are small quantities: pressure, volume, temperature, amount of gas.
  • They are perceptible by our senses.
  • There is the equation of state of the ideal gas, which is particularly simple and versatile; in addition, other transformation equations facilitate the calculation of energies and mass exchanged.

Characteristics of an ecosystem description

It is a more complicated description, treated at a macroscopic level; but transformations are usually not ideal and the approach requires a broader basic preparation.

  • We have to make numerous assumptions about the structure of the system, which is composed of different substances in different phases.
  • The quantities are in large quantities.
  • Sometimes the causes and effects of friction escape perception.
  • They are composed of many elements that interact, sometimes in a complex way.
  • Sometimes the mathematical competence of dealing with very large numbers is required.

Characteristics of the molecular description of a system

  • It is a more complicated description, the approach requires a broader basic preparation, generally requires statistical thermodynamic bases.
  • It is necessary to make numerous assumptions about the structure of the system, which consists of different substances in different phases.
  • The quantities are in large quantities.
  • Sometimes the causes and effects of friction escape perception.
  • They are composed of many elements that interact independently.
  • Sometimes, mathematical competence is necessary to handle very large numbers or fairly abstract concepts.

Sometimes this level is called microscopic, but the atoms the molecules are not visible under the microscope; in addition, the Heisenberg uncertainty principle is almost always important at the molecular level.

Simple system

To describe an ideal gas in the cylinder macroscopically, it is sufficient to take into account the pressure, the temperature, the amount of gas and the volume.

Molecular system

To describe a molecular system, it is necessary to consider molecules and atoms and mathematically describe all the positions they assume as pressure, volume and temperature change, taking into account the principle of uncertainty, which causes the behavior of the system and its elementary components.

Control surface

The control surface (more commonly called the boundary, or the wall), is that entity, material or purely geometric, that separates the system from the external environment; An example of a material wall is the surface of a cylinder (usually cast iron), while an example of a geometric wall is the contact surface between air and water in a vessel (or even between water and glass).

Classification of a control area

The wall of a thermodynamic system can be classified by three essential parameters: permeability, rigidity and thermality.

According to its impermeability the wall can be impermeable that does not allow a flow of matter) or porous allowing a flow of matter, even selectively (as the name suggests).

Depending on the stiffness the wall can be rigid (it does not allow changes in volume, therefore it works) or mobile (it allows volume variations, so work)

Depending on the thermality the wall can be adiabatic wall (does not allow heat exchange) or diathermic wall (allows heat exchange).

Classification according to their thermodynamic systems. The aforementioned thermodynamic systems are nothing more than a combination of these properties:

  • Open system: porous, mobile and diathermic wall
  • Open adiabatic system: porous, mobile and adiabatic wall
  • Closed system: waterproof, mobile and diathermic wall
  • Closed adiabatic system: waterproof, mobile and adiabatic wall
  • Isolated system: waterproof, rigid and adiabatic wall.
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Last review: December 19, 2017