Sunday, 7 August 2016

We were discussing the concept of work transfer and heat transfer in thermodynamics in our previous posts. We have also seen earlier the fundamentals of thermodynamic state, path, process and cycles.
Let us go ahead to discuss the concept of reversible process and irreversible process. We will discuss reversible process and irreversible process in detail in this post, but let me first write here that each and every natural process is irreversible process.

If we talk about reversible process then we may say that reversible process does not exist practically and we only consider the reversible process as imaginary process or ideal process. However there are some processes those can approach to reversibility.

So let us see, what are reversible process and irreversible process in thermodynamics?

Reversible process is a process which could be stopped at any stage and could be reversed to its initial state without affecting the surrounding or any part of the universe. In simple words, we can say that if a thermodynamic process could be stopped at any stage and reversed back in such as way that we can restore the system and surrounding to their initial state then the process will be termed as reversible process.

Reversible process must be carried out quite slowly so that process could be reversed at any stage in such as way that we can restore the system and surrounding to their initial state.

If a thermodynamic process could be reversed back but it’s surrounding is not able to restore its initial position then we could not say that process is reversible but also the process will be termed as irreversible process.
Irreversible process

Let us see one example of expansion of gas in cylinder piston arrangement

Let us consider two cylinders of similar capacity and consisting with similar amount of gas. Let us assume that gas is compressed in the first cylinder by a 100 gm weight and gas is compressed in second cylinder by 100 numbers of 1 gm weights. Ultimately there will be similar pressure force over the gas in both cases. Let us also think that volume of gas in both cylinders is same.

Now, if we remove the weight of 100 gm from piston in first cylinder, gas will be expanded and pressure force over the piston will be reduced and it will be equivalent to atmospheric pressure and this process will be termed as irreversible process because it could not be reversed back without affecting the surrounding. I have tried below in case 2 about the reason of irreversibility of this process.

Now consider the second cylinder, if we remove 1 gm weight from the piston one by one then gas will be expanded infinitesimal slowly in 100 stages and process will be carried out here infinitesimal slowly with infinite small change in volume and pressure and each stage will be considered in thermal equilibrium. Therefore this process will be considered as reversible process in the limit.
Reversible process

Reversible and Irreversible process: Explanation

Case 1: Heating and cooling an object

Let us consider one example of a thermodynamic process, let us think that we are heating an object up to a particular temperature and then we are cooling that object to its initial temperature. Now let us concentrate and analyze the above case. What do you think? Is it reversible process or irreversible process?

This is an example of irreversible process. Let us find the reason for saying this process as irreversible process. We can heat an object by simply transferring the heat from surrounding to system i.e. object and in order to secure the initial temperature of object we can cool the object by removing heat or rejecting heat to surrounding from the system i.e. object.

Now let us see here, system has secured its initial state as it will have initial temperature because we have cooled the object by removing heat to surrounding. But surrounding has not restored its initial state. Heat transfer to the system from surrounding during heating the object and heat rejected by system to the surrounding during cooling the object will not be similar.

Therefore surrounding has been affected or we can say that surrounding has not restored to initial state and hence the thermodynamic process in above example will be considered as irreversible process.

Case 2: Compression and expansion of gas in cylinder piston arrangement

Let us we have one cylinder piston arrangement with gas as content inside the cylinder. Let we are expanding the gas from higher pressure and temperature to lower pressure and temperature.

What do you think about the process? Is this process will be an example of reversible process?

This will be an example of irreversible process, because surrounding will be affected here or we can say that we could not restore the surrounding to its initial state. Let us focus here why surrounding could not achieve its initial state during the above thermodynamic process.

We are expanding the gas or we can say that system is doing the work on the surrounding. While during compression of the gas to secure the initial state of pressure and temperature of system, surrounding will have to do the work on system. But we must note it here that work given by system from surrounding during expansion and work taken by system from surrounding during compression of the gas will not be similar and therefore surrounding will not be able to restore its initial state.

We will see another topic in our next post “Causes of irreversibility in a thermodynamic processin 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