# How can a process be isentropic but not reversible or adiobatic?

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• bardia sepehrnia
In summary, the book discusses the concept of "internally reversible" processes, which refers to the system passing through a continuous sequence of thermodynamic equilibrium states. This does not take into account any irreversibilities in the surroundings. An example of this is a gas expanding isothermally and reversibly while the surroundings experience an irreversible change. An isentropic process can be not reversible nor adiabatic, such as an irreversible engine cycle, while a reversible process can be not adiabatic and not isentropic, like a reversible isothermal expansion of a gas. It is important to differentiate between the entropy change of a system and the entropy change of the combination of system and surroundings.
bardia sepehrnia
TL;DR Summary
Thermodynamics, entropy and reversibility
In the book for our thermodynamics, it states that a process that is internally reversible and adiabatic, has to be isentropic, but an isentropic process doesn't have to be reversible and adiabatic. I don't really understand this. I always thought isentropic and reversible mean the same thing, process that leads to no entropy generation should be by definition, reversible?
The book also keeps mentioning the word "internally" reversible which I don't understand what it means despite reading about it from 100 different sources.

So can anyone give me an example of:
1- An isentropic process that is not reversible not adiabatic? (no entropy generated, yet heat is transferred?)
2- A reversible process that is "not" adiabatic and "not" isentropic? (A graph in the book demonstrate that a process can be reversible, despite heat transfer)

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Let's first talk about an "internally reversible" process. An internally reversible process is one in which the system passes through a continuous sequence of thermodynamic equilibrium states. This does not say anything about what is happening in the surroundings (which may contain irreversibilities). An example of such a situation is one in which the system is gas in a cylinder with a piston that is allowed to expand isothermally and reversibly. The surroundings may consist of a constant temperature reservoir at a higher temperature than the system, with an intervening low thermal conductivity "insulation" through which the heat flows. The gas experiences a reversible expansion, but the surroundings experience an irreversible change because of the entropy generated within the intervening insulating layer. But, for the system, the process is internally reversible. However, overall, the combination of system plus surroundings experiences an increase in entropy.

An example of an isentropic process that is not reversible nor adiabatic is one in which an engine is working in a cycle, and the working fluid is undergoing an irreversible cycle. In this case, since the working fluid returns to its original state after each cycle, the process is isentropic with respect to the system (the working fluid). This doesn't say anything about the entropy change of the surroundings, which may consist of constant temperature reservoirs.

An example of a reversible process that is not adiabatic and not isentropic is the reversible isothermal expansion of a gas. This process is internally reversible with respect to the gas, even though its entropy increases.

So, in summary, some of the problem you are experiencing may be the result of confusing the entropy change of a system with the entropy change for the combination of system plus surrounding. The former can be positive, negative, or zero, depending on the nature of the process, but the latter can only be positive (irreversible process) or zero (reversible process).

Frabjous, bardia sepehrnia, etotheipi and 1 other person
Thank you

## 1. What does it mean for a process to be isentropic?

Isentropic refers to a process in which there is no change in entropy. This means that the energy of the system remains constant and there is no loss or gain of heat.

## 2. How is an isentropic process different from a reversible process?

An isentropic process is a specific type of reversible process where there is no change in entropy. In a reversible process, the system can be returned to its original state without any loss of energy or increase in entropy.

## 3. Can a process be isentropic but not reversible?

Yes, it is possible for a process to be isentropic but not reversible. This means that there is no change in entropy, but the process cannot be reversed without some loss of energy or increase in entropy.

## 4. What is an example of an isentropic but not reversible process?

An example of an isentropic but not reversible process is the expansion of a gas through a porous plug. The gas expands without any change in entropy, but the process cannot be reversed without some loss of energy due to friction.

## 5. How is an isentropic process different from an adiabatic process?

An isentropic process refers to a process with no change in entropy, while an adiabatic process refers to a process with no exchange of heat. An isentropic process can be adiabatic, but an adiabatic process may not necessarily be isentropic.

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