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Thermodynamic System

Thermodynamic system

A thermodynamic system is a part of the physical universe with a specific limit for observation.

A system contains a substance with a large number of molecules or atoms, and is made up of a geometric volume of macroscopic dimensions subjected to controlled experimental conditions.

A thermodynamic system can undergo internal transformations and exchanges of matter and / or energy with the external environment.

Classification of thermodynamic systems

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

Open thermodynamic system

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 wind heating and cooling system.

Closed thermodynamic system

In thermodynamics it is said closed if it allows an energy flow with the external environment, through its border, (through heat and / or work and / or another form of energy), but not mass; An example is a cylinder kept closed by a valve, which can heat or cool but does not lose mass (while the cylinder itself 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.

Insulated thermodynamic system

A system is said to be isolated if it does not allow a flow of energy or mass to the external environment.

Other subdivisions

Each of these systems can still be outlined due to their internal complexity. The ability to subdivide into smaller subsystems. In this way we will obtain that an open, adiabatic open, 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 limit, within which other walls exist.

Microscopic and macroscopic description of the thermodynamic system

A thermodynamic system can be viewed from both 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 amounts necessary to describe it are small amounts: pressure, volume, temperature, amount of gas.

  • They are noticeable by our senses.

  • There is the ideal gas equation of state, which is particularly simple and versatile. In addition, other transformation equations facilitate the calculation of the energies and mass exchanged.

Characteristics of an ecosystem description

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

  • We have to make numerous assumptions about the structure of the system, which is made up of different substances in different phases.

  • The quantities are in large quantities.

  • Sometimes the causes and effects of friction escape perception.

  • They are made up of many interacting elements, 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, it 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 proficiency is necessary to handle very large numbers or fairly abstract concepts.

Sometimes this level is called microscopic, but the atoms and molecules are not visible under the microscope. Furthermore, 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

In order to describe a molecular system it is necessary to consider the molecules and the atoms. In addition, it is necessary to describe mathematically all the positions that they assume as the pressure, volume and temperature change.

For this, one must take into account the uncertainty principle that causes the behavior of the system and its elemental components.

Control surface

The control surface 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). An example of a geometric wall is the contact surface between air and water in a glass (or even between water and glass).

Classification of a control surface

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

Depending on its impermeability, the wall can be waterproof or porous, allowing a flow of material, even selectively.

Depending on the stiffness, the wall can be rigid or mobile.

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

Classification according to their thermodynamic systems. The thermodynamic systems mentioned above are just a combination of these properties:

  • Open system. Porous, mobile and diathermic wall

  • Open adiabatic system. Porous, mobile and adiabatic wall

  • System closed. Waterproof, mobile and diathermic wall

  • Closed adiabatic system. Waterproof, mobile and adiabatic wall

  • Isolated system. Waterproof, rigid and adiabatic wall.

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    Published: December 19, 2017
    Last review: May 28, 2020