A reversible process is defined as a process which, having taken place, can be reversed and in so doing leave no change in either the system or the surroundings.
Features of reversible process:
Reversible process refers to both the system and the surroundings.
The process obviously has to be a quasiequilibrium process; additional requirements are:
1. No friction is involved in the process.
2. Heat transfer occurs due to an infinitesimal temperature difference only.
3. Unrestrained expansion does not occur.
The mixing of different substances and combustion also lead to irreversibilities.
To illustrate that friction makes a process irreversible consider the system of block plus inclined plane shown in Fig.
- Weights are added until the block is raised to the position shown in part ( b ) .
- Now, to return the system to its original state some weight must be removed so that the block will slide back down the plane, as shown in part (c).
Note that the surroundings have experienced a significant change; the weights must be raised, which requires a work input. Also, the block and plane
are at a higher temperature due to the friction, and heat must be transferred to the surroundings to return the system to its original state.
- This will also change the surroundings. Because there has been a change in the surroundings as a result of the process and the reversed process, we conclude that the process was irreversible.
- To demonstrate the fact that heat transfer across a finite temperature difference makes a process irreversible, consider a system composed of two blocks, one at a higher temperature than the other.
- Bringing the blocks together results in a heat transfer process; the surroundings are not involved in this process. To return the system to its original state, we must refrigerate the block that had its temperature raised.
- This will require a work input, demanded by the second law, resulting in a change in the surroundings. Hence, the heat transfer across a finite temperature difference is an irreversible process.
For an example of unrestrained expansion, consider the high-pressure gas contained in the cylinder of Fig. a.
- Pull the pin and let the piston suddenly move to the stops shown. Note that the only work done by the gas on the surroundings is to move the piston against atmospheric pressure.
- Now, to reverse this process it is necessary to exert a force on the piston. If the force is sufficiently large, we can move the piston to its original position, shown in part ( d ) .
- This will demand a considerable amount of work, to be supplied by the surroundings.
- In addition, the temperature will increase substantially, and this heat must be transferred to the surroundings to return the temperature to its original value.
- The net result is a significant change in the surroundings, a consequence of irreversibility.