Definition of a reversible process

[spaghetti3451]

I have seen a reversible process defined as one in which the system and surroundings are restored to their initial state without change elsewhere. As far as I am aware, the system and the surroundings completely occupy the universe. So, I am failing to understand what elsewhere means in this context.

Please enlighten me.

 

[spaghetti3451]

I am also wondering why reversible processes do not occur in nature. Anyone wants to take on the challenge?

 

[Andrew Mason]

I have seen a reversible process defined as one in which the system and surroundings are restored to their initial state without change elsewhere. As far as I am aware, the system and the surroundings completely occupy the universe. So, I am failing to understand what elsewhere means in this context.

Please enlighten me.

The system and surroundings = universe. But you are only interested in the part of the universe that is affected by the process. The universe is constantly changing so you can never actually restore the same state of the universe. So we say that the system and surroundings can be restored to their initial state IF you were to assume that there were no other changes occurring other than those relating to the particular thermodynamic process you are concerned with.

AM

 

[Andrew Mason]

I am also wondering why reversible processes do not occur in nature. Anyone wants to take on the challenge?

Reversible processes require heat to flow due to infinitessimal temperature differences and changes in volume to occur due to infinitesimal pressure differences, so they take an infinite time. The reversible process represents a limit that can be approached but never achieved.

AM

 

[Studiot]

I am also wondering why reversible processes do not occur in nature. Anyone wants to take on the challenge?

Change of state eg melting is an example of a natural reversible process.

 

[Pythagorean]

A (The?) Grandfather:

http://en.wikipedia.org/wiki/Ilya_Prigogine

where a lot of his contributions went:

http://onlinelibrary.wiley.com/bookseries/10.1002/SERIES2007

 

[jtbell]

The reversible process represents a limit that can be approached but never achieved.

To put it another way, a reversible process is to thermodynamics what a frictionless surface is to classical mechanics.

 

[spaghetti3451]

But you are only interested in the part of the universe that is affected by the process.

How can we be sure that the entire universe is not affected by the process?

The universe is constantly changing so you can never actually restore the same state of the universe.

But are we not considering an ideal universe where the parameters of only the system can be altered? In other words, are we not considering a universe where the surroundings can modify only if there is a change within the system?

 

[Andrew Mason]

How can we be sure that the entire universe is not affected by the process?

A reversible process is a theoretical limit. The assumption is that the rest of the universe is not affected during the time it takes for the process to occur.

But are we not considering an ideal universe where the parameters of only the system can be altered? In other words, are we not considering a universe where the surroundings can modify only if there is a change within the system?

I am not sure what you mean. Can you give an example?

AM

 

[Andrew Mason]

Change of state eg melting is an example of a natural reversible process.

A change of state due to an infinitessimal temperature difference between the substance and the surroundings would be reversible. But this does not occur in nature. It would take an infinite amount of time. A change of state in nature can be close to reversible. For example, an ice cube in a glass of ice water at 0C. For it to melt reversibly, the room would have to be kept at 0C. But if it is in a room at 0C the ice will never melt. If the room is 1C, the ice will eventually melt but slowly. That would be close to a reversible process. If the room is kept at 20C it will melt much faster, but at the expense of a greater increase in entropy.

AM