# Thermodynamics - reversibility and heat addition

• Haftred
In summary, the speaker discusses the concept of entropy as a state function and the Clausius inequality, which states that a change in entropy is greater for a reversible change compared to an irreversible change. However, the speaker questions why more heat is needed for a reversible change and where the additional heat goes if the final states are the same. The other person clarifies that during an irreversible change, entropy is generated within the system, so to achieve the same final states with a reversible process, additional heat needs to be transferred to compensate for the lack of generated entropy.
Haftred
I understand entropy is a state function, insofar as we deny the existence of irreversible cycles. However, for a said change of state, the heat transferred as a result of a reversible change is greater than that for an irreversible change. This is simply a reiteration of the Clausius inequality, as because entropy is a state function, a change dS is greater for a dq/T if dq is irreversible. However, it seems to me that it does not logically make sense for more heat having to be added for a reversible change. What is it about reversibility that requires more heat to be added to change states? I understand that entropy works out if the former statement is true; however, I guess I just don't understand how logically more heat is needed to effect a reversible change of state rather than an irreversible one. Where does the extra heat go if the final states are the same?

- Thanks

Last edited:
You pretty much have it backwards. During an irreversible change, there is entropy generated within the system so, to get between the same two ends states with a reversible process, you need to compensate for the entropy which is not generated within the system during the reversible process by transferring additional heat into the system.

## 1. What is the concept of reversibility in thermodynamics?

Reversibility in thermodynamics refers to a process that can be reversed without causing any change in the system or its surroundings. In other words, the system and its surroundings can be returned to their original state after the process is reversed. This is an idealized concept and often not achievable in real-world processes.

## 2. How is reversibility related to the second law of thermodynamics?

The second law of thermodynamics states that in any spontaneous process, the total entropy of the universe always increases. Reversible processes, on the other hand, have no change in entropy and therefore do not violate the second law. However, most real-world processes are irreversible and result in an overall increase in entropy.

## 3. What is heat addition in thermodynamics?

Heat addition refers to the transfer of thermal energy from one system to another. In thermodynamics, heat is considered as a form of energy that is transferred due to a temperature difference. Heat addition can occur in various forms, such as conduction, convection, or radiation.

## 4. How does heat addition affect the reversibility of a process?

Heat addition can affect the reversibility of a process by increasing the entropy of the system and its surroundings. This increase in entropy makes it more difficult to reverse the process and return the system to its original state. In reversible processes, heat addition is often avoided or minimized to maintain the reversibility of the process.

## 5. Can all processes in thermodynamics be considered as reversible or irreversible?

No, not all processes in thermodynamics can be classified as reversible or irreversible. Some processes, such as phase changes or ideal gas expansions, can be reversible under certain conditions. However, most real-world processes involve some degree of irreversibility due to factors such as friction, heat transfer, and energy losses.

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