Lecture - 3 (state of the system and processes)

Any physical condition of the system at any instant is called as state of the system. The state of the system is specified by the properties of the system. If any one of the property changes, the state of the system changes. This change in state is called as process. The Infinite state through which the system passes while moving from initial state to the final state is called process path.
The processes can be classified as follows -

Reversible and Irreversible process

A process is said to be a reversible process if it can be reversed in direction following the same path without leaving any change in the system and surroundings, otherwise the process will be Irreversible. We will have a detailed explanation of reversible and Irreversible process in my upcoming lectures.

Quasi Static and non Quasi static process 

A process is said to be a Quasi static process if it take place in a very slow manner, otherwise non Quasi static process.

First of all we understand Quasi static process. So let us consider a piston cylinder arrangement. Here our initial position is '1' and final position is '2' and 'a' is the position somewhere between the process, as shown in below figure.

On this arrangement two pressure measuring device is installed, one is on the cylinder head and second is on the piston. Both devices are showing atmospheric pressure, let atmospheric pressure is 100kpa ( kpa stands for kilo Pascal ) as shown in below figure.

Now Piston will undergoes a process of compression very slowly from position '1' to position '2'. Let us consider that after compression the pressure in the Piston Rises upto 150 kilo Pascal. Now as the Piston start moving the gap present between molecules start decreasing in a gradual manner not instantly. So the rise in pressure will also be gradual. The below figure showing the rise of pressure when piston is at position 'a' somewhere between position 1 and 2.

After reaching at position 2, let piston goes under expansion. Now Piston will move from position 2 to position 1 in slow manner. What we observe is that the gap between the molecules will start increasing gradually and similarly pressure will also decrease from 150 kilo Pascal to 100 kilo Pascal gradually and slowly. After the process Piston again regain it original position. The way in which compression process occur exactly in that way expansion process occur. Now if we consider the Piston cylinder to be frictionless then we can say that the process is absolutely reversible because compression and expansion occurs in exactly similar manner, there is gradual increase and decrease of pressure in both stroke. Also during whole process the properties remain uniform. So from here we can conclude that frictionless Quasi static process is a reversible process.
In below figure the graph is shown which represents above explained reversible process or we can say frictionless Quasi static process. Remember always show a reversible process with a continuous line. In this graph y-axis represent pressure and X-axis is represent volume.

Now let us understand about non Quasi static process. So we have similar condition same piston cylinder arrangement connected with two pressure measuring device one on Piston and one on cylinder head as shown below.

Now let us start compression process here, but the only difference in this process is that, it occurs in very fast manner. At a particular instant when this process is occurring we analyse the compression behaviour of molecules. The figure below shows the position of piston somewhere between position 1 and 2.

Because of this fast process, the space between the molecules which are very near to the piston will decrease rapidly whereas the gap between the molecules which are near to the cylinder head will remain same. Therefore because of rapid decrease of molecules spacing there is an instant rise of pressure which will definitely more than 150 kilo Pascal. Let us consider that this instant rise of pressure reaches to 250 kilo Pascal. And when Piston achieve its final position that is position 2, it's final pressure will going to same as 150 kilo Pascal. We observe this instant increase of pressure during the process because of rapid decreasing of molecules spacing.
Now here if we do expansion in fast manner we see that the pressure will decrease from 150 kilo Pascal directly to 100 kilo Pascal. During expansion process the pressure will not rise up to 250 kilo Pascal as there was in compression process. so if we compare both expansion and compression process here, these both process occurs in very different manner. Therefore after considering the Piston cylinder arrangement to be frictionless we cannot say that the process is reversible. So from this we can conclude that whether the non Quasi static process is frictionless or with friction it will always be a Irreversible process.
The above explained process is shown in below PV graph. Remember Irreversible process is always shown with dashed lines.

The main conclusion of all this explanation of quasi static and non quasi static process is as follows-

A Quasi static process is represented by joined line on property diagrams whereas and non Quasi static proces is generally represented by dashed lines. In Quasi static process the properties are uniform where as in non Quasi static process the properties are non uniform. Frictionless Quasi static process becomes reversible process.

For video of this lecture check out my YouTube channel Mechanical education hub