Thermodynamic System: Properties, Types, Examples

A thermodynamic system or working substance is a part of the universe that is chosen to be studied by thermodynamics. This is a very broad definition, because a thermodynamic system can be vast like a galaxy or it can be a small amount of gas contained in a cylinder.

Everything that surrounds the thermodynamic system, including mass and space, is its environment or environment. It is not about the rest of the universe, but only about the environment capable of affecting the system.

In a thermodynamic system, the exchange of mass or energy between the system and the surroundings is possible, depending on the kind of system, being the environment where the observers who study it are located.

In the analysis of a thermodynamic system it is important to establish the  boundaries (limits or walls), which can be fixed or mobile. A glass or metal jar has fixed walls, but a plunger or piston has movable walls.

As such, the boundary can be a real or imaginary surface, but ideally it has zero thickness, some stiffness, and other characteristics that are described mathematically.

Imaginary boundaries are necessary when studying systems that are themselves part of much larger systems, such as a star in a nebula.

Heat exchanges between the system and its surroundings, if any, are the primary goals of thermodynamics. That is why the walls that allow exchange are given a special name: adiabatic, while those that do not allow it and isolate the system are diathermic.

Types of thermodynamic systems

There are different types of thermodynamic systems, which are classified according to their ability to exchange matter and energy with the environment, in this way we have:

Open systems

The exchange of matter and energy with the outside is allowed, for this reason they are also called control volume .

Many systems in engineering are modeled this way, for example home water heaters and car radiators.

Closed systems

They are also known as control mass and are characterized because there is no exchange of matter with the environment. Therefore its mass is fixed, however energy can cross its limit, either as heat or as work. In this way the volume of the system can vary.

Isolated systems

These are closed systems in which the exchange of heat, work or any form of energy with the environment is prevented.

Homogeneous systems and heterogeneous systems

The systems described were classified according to their ability to exchange with the environment, but this is not the only criterion. Thermodynamic systems can also be homogeneous and heterogeneous.

Pure substances are good examples of homogeneous systems, such as table salt stored in the salt shaker. On the contrary, a combination of liquid water and water vapor is a heterogeneous system, since it is two different states, whose properties differ.

Examples of thermodynamic systems

As we have seen, thermodynamic systems range from the simplest to the most complex. However, when studying them, it is convenient to define them carefully and try to find all the ways to simplify the analysis.

Let’s see some examples of real systems that accompany us in everyday life:

Container immersed in water

This is a very simple and illustrative thermodynamic system, and also a good way to prepare food. The content of the container, be it eggs, a mixture for a flan or whatever preparation, is the thermodynamic system, while the water bath constitutes the environment or environment.

As the water is heated, the heat flows into the system through the boundary of the container.

How quickly the food cooks depends on several factors, one of which is the material of the pan: whether it is ceramic or metal. We know that metal is a good conductor of heat, so we expect the contents to heat up quickly if a steel or aluminum pot is used.

Instead of heating the system, you may want to cool, for example warm a bottle of baby milk that has been overheated. In this case the system transfers heat to the water bath.

Cooking in an uncovered pan

The kitchen is an excellent place to find thermodynamic processes. A sauce, chicken or other food stewed in an open pan is an example of an open system, since all kinds of exchange with the environment are allowed: adding seasonings, other ingredients and modifying the heat that is added.

Internal combustion engine

In the internal combustion engine of cars, motorcycles, airplanes and boats there is a mixture of gas (air) and fuel that is prepared in the carburettor and is taken to the cylinder, where by the effect of combustion it becomes a mixture of gases various.

As the composition of the mixture varies throughout the work cycle, it is a complex and heterogeneous thermodynamic system.

Coffee or tea in a thermos

Coffee or tea stays hot longer in a thermos. It is a container fitted with insulating walls, precisely to prevent our system – the hot drink – from giving up its heat to the environment and cooling.

Actually a thermos are two containers, between which a partial vacuum is made to avoid the presence of a medium such as air, which helps to conduct heat from the inside to the outside of the container.

Of course, the thermos also works in reverse, it serves to keep drinks cold for longer.


The eggs that we consume as food are excellent examples of closed thermodynamic systems, but they allow the exchange of energy with the environment. The eggshell allows the heat of the hen to hatch the embryo, as well as the passage of gases.


The cell is the fundamental unit of living things and is a wonderfully efficient thermodynamic system. By extension, any living being can also be considered a complex thermodynamic system.

The cell membrane that lines internal structures such as the nucleus and mitochondria is the border between the system and the environment. This allows energy exchange, the arrival of nutrients from the outside and the exit of waste.


Canned foods are typical examples of closed thermodynamic systems.

Water heater

The water inside the heater is an open thermodynamic system, since it is necessary to allow heat to reach the water, usually from an electrical resistance that is heated, if the heater is electric, or by solar energy or the flame that comes from a lighter to gas.


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