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Thermodynamics: Equilibrium


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.

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