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Monday, 29 February 2016

WHAT IS THERMODYNAMIC SYSTEM, SURROUNDINGS AND BOUNDARY?

We have seen in our previous post about the first, second and third law of thermodynamics. We have also seen the Zeroth law of thermodynamics in our previous post.

Today we will see here the basic and fundamental concepts in thermodynamics, those are quite important for any engineering student to understand. Thermodynamics starts with the concept of thermodynamic system. 
We will discuss here the concept of thermodynamic system and simultaneously we will see here the various classes of thermodynamic system with the help of this post.

Thermodynamic system

Thermodynamic system is basically defined as the finite quantity of matter or prescribed region in space where thinking will be concentrated during analyzing a problem.
Whatever thing those are outside from the system will be termed as surrounding and system will be separated by the surrounding with the help of a boundary which is termed as system boundary.  System boundary might be fixed or movable.
System and surrounding together will be considered as universe.

Classes of thermodynamic system

There are basically three classes of a thermodynamic system as mentioned here

Closed system

In case of closed system in thermodynamics, there will not be any transfer of mass across the system boundary. However energy may transfer across the system boundary. Therefore close system in thermodynamics could be considered as fixed mass system also.

Open system

In case of open system in thermodynamics, mass transfer across the system boundary will take place and energy may also transfer across the system boundary. You will see that maximum engineering accessories or devices will follow the concept of open system.

Isolated system

In case of isolated system, there will not be any interaction in between system and it’s surrounding or we can say that there will not be any transfer of mass or energy across the system boundary in case of isolated system and therefore isolated system in thermodynamics could be considered as fixed energy and fixed mass system.

Let us see one example of cooking rice in pressure cooker.

When we will go for cooking the rice in pressure cooker, what we will do? We will use rice and water as contents for pressure cooker and will place over the gas stove. Now, pressure cooker placed over the gas strove here will be considered as closed system if pressure cooker lid is perfectly closed, safety valve is also perfectly closed and whistle is in position i.e. not blowing.

Because there will not be any transfer of mass across the pressure cooker here, while heat energy is being provided to the contents of pressure cooker i.e. rice and water. Therefore it could be considered as closed system as only heat energy transfer take place across the pressure cooker not mass.

When whistle starts blowing, at that time we will consider the contents of pressure cooker as open system because generated steam will leave the pressure cooker when whistle blows. Now let us think the condition here during the blowing of cooker whistle, mass and energy both are transferring across the pressure cooker so we may call this as open system.

If we are having one vessel which is perfectly insulated, closed and rigid, then we may refer this as isolated system because there is no transfer of energy or mass across the vessel.
There are few more types of systems those are quite important to be considered here i.e. adiabatic system, homogeneous system and heterogeneous system.

Adiabatic system

Adiabatic system will not permit the exchange of heat energy with its surrounding, however it will permit to exchange the work energy with its surrounding.

Homogeneous system and heterogeneous system

Homogeneous system will consist single phase such as mixture of water vapor and air.
Heterogeneous system will consist two or more than two phases such as steam and water

We will start our discussion with thermodynamic equilibrium, intensive and extensive properties in our next post.
Do you have any suggestion? Please write in comment box.

Image courtesy: Google

Reference:

Engineering thermodynamics by P.K Nag

Engineering thermodynamics by R.K Rajput

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