Thermodynamic system

A thermodynamic system is a part of the universe confined in space by real or imaginary walls that separate it from the rest of the universe (called surroundings). This separation makes it possible to study the processes that the system undergoes when it exchanges matter (M) and/or energy (E) with its surroundings.

Examples of thermodynamic systems are for instance a cell, a person, the engine of a car, a galaxy, a planet’s atmosphere, a tree…

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Thermodynamic systems can be isolated, closed or open.

    • Open system: it exchanges matter and energy with its surroundings.
    • Closed system: it exchanges energy but no matter with its surroundings.
    • Isolated system: it does not exchange neither matter nor energy with its surroundings.

The following figure shows the three types of system: open (a), closed (b) and isolated (c).

When the wall that delimits a system allows the exchange of energy with its surroundings, it is called diathermal wall (case (b) shown in the upper figure).

An adiabatic wall is a wall which prevents the exchange of energy between the system and its surroundings (case (c) shown in the figure).

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We will focus on the study of the thermodynamic processes that a closed system can undergo.

The first step to follow when studying the thermodynamic behavior of a system is to determine the variables used to describe it and the processes it can undergo. Follow the links below to find the definition of thermodynamic variables and processes.

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