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Sunday, 7 August 2016

CAUSES OF IRREVERSIBILITY IN A THERMODYNAMIC PROCESS

We were discussing the concept of reversible and irreversible thermodynamic process in our previous post, where we have discussed the basics of a reversible and irreversible thermodynamics process with the help of few examples.

Today we will try to find out here the main causes of irreversibility of a process with the help of this post.

There are basically two reasons, as mentioned below, of irreversibility of a thermodynamic process. We will see here both causes of irreversibility of process.
  1. Lack of thermodynamic equilibrium during the process
  2. Involvement of dissipative effect during the process

So, first we will see here the causes of irreversibility of a process due to lack of thermodynamic equilibrium during the process

As we know that a thermodynamic process will have thermodynamic equilibrium if process has following characteristic.
  1. Mechanical equilibrium
  2. Thermal equilibrium
  3. Chemical equilibrium
In order to secure mechanical equilibrium, there must not be any change in pressure i.e. change in pressure or dP must be zero. Similarly In order to secure thermal equilibrium, there must not be any change in temperature or we can say that change in temperature i.e. dT must be zero. In order to have chemical equilibrium, there should not be any difference between concentrations or dc must be zero.

Now let us analyze the heat transfer process, in order to have heat transfer process there must be temperature difference and if there is no temperature difference then heat transfer will never take place because heat transfer will always take place from higher temperature side to lower temperature side in nature.
Hence, in order to have heat transfer process there must be temperature difference or dT will not be equivalent to zero. Similarly, mass transfer will only takes place when there will be difference in the mass concentration or dc will not be zero. In similar way, work transfer will only take place when there will be difference between pressure or dP will not be zero.

Therefore, each and every natural process will have lack of thermodynamic equilibrium or in other way we can also say that lack of thermodynamic equilibrium will be the reason for initialing of a natural process.

Therefore, lack of thermodynamic equilibrium will be the main reason behind the irreversibility of a thermodynamic process.

Let us see the second cause of irreversibility of a process i.e. involvement of dissipative effect during the process

Involvement of dissipative effect during the process will also be a cause of irreversibility of a process. There are following various types of dissipative effect during a process and these are as mentioned below
  1. Mechanical friction
  2. Magnetic hysteresis
  3. Electrical resistance
  4. Viscosity or fluid viscosity
  5. Inelasticity
Therefore if there is no involvement of dissipative effect during a process and there is no lack of thermodynamic equilibrium then process will be considered as reversible process but we must note it here that each and every natural process will have involvement of dissipative effect and lack of thermodynamic equilibrium.

Let’s go to the next post i.e.thermodynamic reversible cycle,reversible heat engine and reversible heat pump in the category of thermal engineering.
Do you have suggestions? Please write in comment box.

Reference:

Engineering thermodynamics by P. K. Nag
Basic thermodynamics by Prof. S.K. Som
Image courtesy: Google

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