# Irreversible process

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Pushoam

Does this mean that any process in which energy goes away\comes into from the system to the surrounding as heat is irreversible and in any irreversible process, energy must go away
\come into from the system to the surrounding through heat?

Staff Emeritus
Conservation of energy is not the only law relevant. Think of entropy as well as energy.

Pushoam
Conservation of energy is not the only law relevant. Think of entropy as well as energy.
This reminds me the following:
Irreversible process is that process in which entropy of the system increases.
So, if I get a system whose energy remains conserved ( that is it doesn't get lost through heat) ,but entropy increases, then the system will be in irreversible process. For example, isolated ideal gas as ideal gas has only elastic collision, their energy remains conserved but the entropy increases, hence the ideal gas are in irreversible process. But, what is the name of this irreversible process?

So, in irreversible process, loss of energy through heat is not necessary. Right?

Mentor
This reminds me the following:
Irreversible process is that process in which entropy of the system increases.
So, if I get a system whose energy remains conserved ( that is it doesn't get lost through heat) ,but entropy increases, then the system will be in irreversible process. For example, isolated ideal gas as ideal gas has only elastic collision, their energy remains conserved but the entropy increases, hence the ideal gas are in irreversible process. But, what is the name of this irreversible process?

So, in irreversible process, loss of energy through heat is not necessary. Right?
Your definition of an irreversible process in terms of an entropy increase is incorrect. The entropy of a system in an irreversible process can also decrease.

Pushoam
The entropy of a system in an irreversible process can also decrease.
Entropy of an irreversible process always increases.
Entropy of a reversible process remains constant.
I have never heard that the entropy of a system in an irreversible process can also decrease.
Will you please give me an example?

Mentor
Entropy of an irreversible process always increases.
Entropy of a reversible process remains constant.
I have never heard that the entropy of a system in an irreversible process can also decrease.
Will you please give me an example?
I remove heat from a hot body (my system) by contacting it with a colder reservoir.

Pushoam
I remove heat from a hot body (my system) by contacting it with a colder reservoir.
But then the hotter body is not an isolated system.
The hotter body with the cold reservoir is the isolated system. And the entropy of this isolated system increases.

Mentor
But then the hotter body is not an isolated system.
The hotter body with the cold reservoir is the isolated system. And the entropy of this isolated system increases.
This is correct. But, in you previous posts, you did not identify the system under consideration as being isolated. You just identified it as a "system."

Pushoam
This is correct. But, in you previous posts, you did not identify the system under consideration as being isolated. You just identified it as a "system."
I am sorry for not being clear in the earlier post.
I assumed that by system it is meant an isolated system.

Thanks for pointing it out.
From next time I would be clear.