Key Difference – Cyclic vs Reversible Process
Cyclic process and reversible process relate to initial and final states of a system after a work has been completed. However, the initial and final states of the system affect these processes in two different ways. For example, in a cyclic process, the initial and final states are identical after completing the process but, in a reversible process, the process can be reversed to get its initial state. Accordingly, a cyclic process can be considered as a reversible process. But, a reversible process is not necessarily a cyclic process, it is only a process that is capable of being reversed. This is the key difference between a cyclic and reversible process.
What is Cyclic Process?
The cyclic process is a process where the system returns to the same thermodynamic state as it started. The overall enthalpy change in a cyclic process is equal to zero since, there is no change in the final and the initial thermodynamic state. In other words, the internal energy change in a cyclic process is also zero. Because, when a system undergoes a cyclic process, the initial and final internal energy levels are equal. The work done by the system in a cyclic process is equal to the heat absorbed by the system.
What is Reversible Process?
A reversible process is a process that can be reversed to get its initial state, even after the process has been completed. During this process, the system is in thermodynamic equilibrium with its surroundings. Therefore, it does not increase the entropy of the system or the surroundings. A reversible process can be done if the overall heat and the overall work exchange between the system and the surroundings are zero. This is not practically possible in nature. It can be considered as a hypothetical process. Because, it is really difficult to achieve a reversible process.
What is the difference between Cyclic and Reversible Process?
Definition:
Cyclic Process: A process is said to be cyclic, if the initial state and the final state of a system are identical, after executing a process.
Reversible Process: A process is said to be reversible if the system can be restored to its initial state after the process has been completed. This is done by doing an infinitesimal change in some property of the system.
Examples:
Cyclic Process: The following examples can be considered as cyclic processes.
- Expansion at constant temperature (T).
- Removal of heat at constant volume (V).
- Compression at constant temperature (T).
- The addition of heat at constant volume (V).
Reversible Process: Reversible processes are ideal processes that can never be achieved practically. But there are some real processes that can be considered as good approximations.
Example: Carnot cycle (a theoretical concept proposed by Nicolas Léonard Sadi Carnot in 1824.
Assumptions:
- The piston moving in the cylinder does not create any friction during motion.
- The walls of the piston and cylinder are perfect heat insulators.
- The transfer of the heat does not affect the temperature of source or sink.
- Working fluid is an ideal gas.
- Compression and expansion are reversible.
Properties:
Cyclic Process: The work done on the gas is equal to the work done by the gas. Moreover, the internal energy and the enthalpy change in the system is equal to zero in a cyclic process.
Reversible Process: During a reversible process, the system is in thermodynamic equilibrium with each other. For that, the process should occur in infinitesimally small time, and the heat content of the system remains constant during the process.Therefore, the entropy of the system remains constant.
Image Courtesy:
1. “Stirling Cycle” by Zephyris at the English language Wikipedia. [CC BY-SA 3.0] via Commons
2. “Carnot heat engine 2” by Eric Gaba (Sting – fr:Sting) – Own work [Public Domain] via Commons
