Question

A thermodynamic process is said to be reversible if:
A. The process can be turned back such that the system and surroundings return to their original states.
B. The process can be turned back with no change in the internal energy of the system.
C. The process can be turned back with a finite heat transfer with the surroundings over a period of time.
D. None of the above.

Answer 1

In our day-to-day life, we see so many things happening around us. For example, if we put some water in the freezer to freeze the water into ice cubes. After it became ice cubes we took that out. It again becomes water. We can say that the above case is an example of a reversible process. But in reality, this cannot be stated as a reversible process. For any reversible process, there should be no change in entropy. But in the above-mentioned example, there is surely a change in entropy. Let us see the condition for the reversible process.

Complete step by step solution:
A process is said to be reversible if both the system and the surroundings return to their original state.
A process must satisfy two conditions to be a reversible process.
There should be no dissipative forces like friction or velocity
The process must be quasi-static which means that the process must be carried out very slowly.
Now we can analyze each option.
Option A is true. We can see from the definition of a reversible process that when both the system and the surroundings return to their original state we can say that the process is reversible.
Option B is not true. This is not necessarily always true. A process can be reversible in two ways. One is externally reversible and the other is internally reversible. So only when that process is internally reversible we can say that change in internal energy is zero. But it cannot be true for all cases.
Option C is also not true. The net heat transfer in a reversible process occurs between two entities—the system and its surroundings—that are arbitrarily close to thermal equilibrium. Such a process is an idealization.
Option D is not true because option A is true.
The correct option is Option A.

Note:
So we can say that a thermodynamic process is reversible only if it changes back to its original state with no other change elsewhere in the universe. But as in reality, there is no such reversibility. Everything in nature is an irreversible process. Therefore the reversible processes can be defined as idealizations or models of real processes.