When the system undergoes a change from its initial state to the final state, the system is said to have undergone a process. During the thermodynamic process, one or more of the properties of the system like temperature, pressure, volume, enthalpy or heat, entropy, etc. changes. The second law of thermodynamics enables us to classify all the processes under two main categories: reversible or ideal processes and irreversible or natural processes.
The reversible process is the ideal process which never occurs, while the irreversible process is the natural process that is commonly found in nature. When we break the wood stick, we cannot change this and ‘un-break’. This is an irreversible process. Whereas when water evaporates, it can also be condensed in the form of rains. This is a reversible process. Let us study more about them below.
Reversible Process :
A process is said to be reversible in which system return to it’s original state without leaving any effect on the surrounding.
- For the reversible process to occur both forward and reverse path has to be same.
- Reversible process are shown by continuous lines or curve where as irreversible process are represented by dotted line or curve.
- Reversible process is impossible to achieve practically.
Irreversible Process :
An irreversible process is what we encounter in reality almost all the time. The system and it’s environment cannot be restored to their original states at the same time. Because this is what happens in nature, it also called a natural process. The sign of an irreversible process comes from the finite gradient between the states occurring in the actual process.
For example, when heat flows from one body to another, there is a finite temperature difference (gradient) between the two bodies. More importantly, at any given moment of the process, the system most likely is not at equilibrium or in a well defined state. This phenomenon is called as irreversible process.
Method which make the process irreversible :
2.) Free Expansion
3.) Mixing of fluid
4.) Heat transfer through a finite temperature difference
- In the irreversible process the initial state of the system and surroundings cannot be restored from the final state.
- During the irreversible process the various states of the system on the path of change from initial state to final state are not in equilibrium with each other.
- During the irreversible process the entropy of the system increases decisively and it cannot be reduced back to its initial value.
- The phenomenon of a system undergoing irreversible process is called as irreversibility.
Cyclic Process :
A process is said to be cyclic in which initial state and the final state are same.
Minimum number of process required to make a cycle is two.