Thermodynamic system

A thermodynamic system is a specific body of matter or radiation. It is separated from its surroundings by a boundary or a wall. Scientists study these systems to understand how energy and matter behave. The space outside the system is called the surroundings or the environment.

The properties of the walls determine what can pass between the system and its surroundings. These walls can be real, like a metal container. They can also be imaginary surfaces used for mathematical study. A wall might be fixed, like a constant volume reactor. It might also be moveable, such as a piston in an engine.

Thermodynamic systems are classified by how they interact with their environment. There are three main types based on the permeability of their walls. An open system allows both matter and energy to pass through its boundaries. A closed system allows energy to pass as heat or work, but it does not exchange matter.

An isolated system is the most restrictive type. It does not exchange any matter or energy with its surroundings. In practice, an isolated system is an idealization. Some transfer, such as through gravitational forces, is almost always possible in nature.

Internal processes allow for another way to categorize these systems. In 2010, Boris Dobroborsky proposed a classification based on how energy behaves inside. He identified passive systems and active systems. A passive system only redistributes its available energy. For example, if one end of a metal rod is warmer than the other, heat will transfer until the temperature is equal. An active system actually converts one type of energy into another. This occurs during chemical reactions or when an electric motor operates. Active processes often cause a system to move away from a state of balance.

Thermodynamic equilibrium is a central concept in this field of science. A system is in equilibrium when there are no macroscopic flows of matter or energy. This means there is no visible tendency toward change on a large scale. In such a state, the physical properties of the system do not change over time. For instance, pressures and temperatures will eventually equalize. Matter may also arrange itself into a few stable, homogeneous phases.

Equilibrium states are easier to describe using deterministic math than non-equilibrium states.

Scientists study two main branches of this subject. Equilibrium thermodynamics looks at bodies that are in a state of internal balance. It uses the concept of thermodynamic processes to describe how bodies move from one equilibrium state to another. This field relies on a well-defined quantity called entropy. Non-equilibrium thermodynamics is a more complex and growing field. It studies systems that are not in balance and involve the flow of matter and energy. These systems often have spatial gradients, such as a change in temperature over a distance. Because these systems are changing, it is difficult to find an exactly defined entropy for them.

History shows how our understanding of these systems has evolved. The study of thermodynamic processes began with early theories of heat engines. Sadi Carnot from France published work on this in 1824. Later, Ilya Prigozhin from Belgium studied dissipative structures in 1971. These studies focused on how systems interact with their surroundings. The classification of systems into open, closed, and isolated grew alongside the science itself. This progression has allowed us to model everything from tiny atoms to massive steam engines.

Understanding these systems is vital for many scientific and engineering tasks. In engineering, many processes are described as flow processes. These approximate equilibrium concepts to make practical calculations possible. Even though nothing in nature is in strict thermodynamic equilibrium, the idea is a very useful tool. It provides a way to create models and perform experiments. By using these idealizations, we can predict how energy will move and how matter will react in the real world.