Systems are regions of space that are setup for thermodynamic study of the matter within that region. The system is defined by an imaginary or real surface that is called the boundary layer. Depending how you define the boundary layer you can make the problem easier or more difficult to solve. Outside of the system is the systems environment.

Types of Systems

There are different types of systems, and depending how you define the system, your analysis could be affected. The first type of system that I am going to discuss is the closed system. For a system to be considered a close system there has to be a fixed amount of control mass within the system where the mass cannot cross the boundary layer. Energy, however, can cross the boundary layer of a closed system, and since energy can cross the systems boundary either the pressure in the system or the volume will increase or decrease depending if the energy is flowing in or out of the system. For the pressure to increase the volume of the system is fixed which means the boundary is fixed, while if the volume increases the system boundary system will have to have a moving boundary. An example of a closed system can be seen below.


There is a special type of closed system called and isolated system. Like a closed system, and isolated system has a fixed amount of control mass which cannot cross the boundary layer. However, unlike closed systems isolated systems prevent energy from crossing the boundary layer. Now in reality there is no such system that is perfectly isolated. However, well insulated systems can come very close to becoming isolated.

The next and final type of system is called an open system, or sometimes referred to as a control volume. An open system has both real and imaginary surfaces for its boundary layer. This means mass as well as energy can cross the imaginary boundary layer of the system. An example of an open system would be a nozzle; refer to the image below.



Properties of a System

There are many different properties that can define a system. For example there’s pressure, temperature, volume, and mass to name a few of the common properties. However, you need to be able to define if those properties are intensive or extensive properties.

For a property to be an intensive property, the property needs to be independent of the system’s mass. Temperature, pressure, and density are examples of properties that are intensive properties. Extensive properties on the other hand are properties that depend on the size or extent of the system. Total mass and volume are examples of extensive properties. In addition, extensive properties can be broken into a subcategory called specific properties. Specific properties are extensive properties per unit mass such as specific energy (equation 1) and specific volume (equation 2).

(Eq 1) $e=\frac{E}{m}$

(Eq 2) $ν = \frac{V}{m}$


When a system is analyzed, it is common to assume that the system is in equilibrium. Equilibrium means that all of the matter within in the system is acting the same. By doing this the engineer doesn’t have to worry about what each molecule is doing within in the system, but instead the engineer can say the molecules are acting generally acting the same way.

There are several different types of equilibrium states that could be considered. The first one is thermal equilibrium. For a system to be in thermal equilibrium you are stating that the temperature is same throughout the system. At a molecular level you are say that most of if not all of the molecules within the system are moving around at the same energy level.

Another type of equilibrium to consider is mechanical equilibrium. For a system to be in mechanical equilibrium the pressure throughout the system is the same. Looking at pressure at the molecular level this would mean that the molecules are bouncing off of each other or the systems boundary at the same rate and energy if the fluid in the system is a gas. Otherwise a liquid, which is considered incompressible or a solid would have all of the molecules pushing back at the boundary at the same force.

The phase of the matter within in the system can be used to say if the system is within equilibrium or not. Phase means that the matter is changing from a solid to a liquid, or a liquid to a gas, or vise versa. For the phase to be in equilibrium the phase of the matter will changing to one form while the same amount of matter will changing back to the other phase. This means the percentage of one phase to another phase will remain the same instead of favoring one of the phases.

Finally, the last type of equilibrium I’m going to mention is chemical equilibrium. For a system to have chemical equilibrium it means that there aren’t any chemical reactions occurring which would change the molecular makeup of the system.

If you consider these states of equilibrium and they are holding true you can normally assume that the system is in equilibrium. However, if your control volume becomes smaller or system becomes smaller, the random actions of the molecules within the system will have more of an influence until you can’t assume the system is in equilibrium. This however only needs to be considered for very small systems or control volumes.

Processes and Cycles

When a system moves from one state of equilibrium to another state of equilibrium this in known as a process. As a system goes from one state of equilibrium it follows what is called the path. Typically during thermodynamics analysis an engineer would ideally like to have the path be in a state of quasi equilibrium. This means that system will stay close to an equilibrium state as it follows the path. Now this is an ideal process, and in a real process this would not happen. The only way for this to occur in real life is the process has to occur at a very slow rate.

For a process to occur the system is gaining something or loosing something in relation to its properties. There are few ideal processes that you should know about. The first ideal process is the isothermal process. During the isothermal process the temperature will remain constant within the system as the process occurs. The next ideal process is called the isobaric process. For an isobaric process to occur, the pressure in the system will remain constant during the process. Finally, the next ideal process is the adiabatic process. For an adiabatic process occur there cannot be any heat gain or loss as the cycle occurs. The reason why these are ideal is because one of the properties are allowed to remain constant making it easier to analyze the process.

Finally, as you start to combine processes that will bring the system back to its original state you have created a thermodynamic cycle. There are many different types of thermodynamic cycles, such as the Carnot Cycle. The Carnot cycle is known as the ideal cycle, meaning it is impossible to create a cycle that is more efficient then the Carnot Cycle. The Carnot Cycle is made of two isothermal processes and two adiabatic processes. Refer to figure to below to view a P-V diagram of the Carnot Cycle.



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