Solving Irreversible Processes: Q-W=U2-U1

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Homework Help Overview

The discussion revolves around the concepts of irreversible processes in thermodynamics, specifically focusing on isentropic processes, the implications of irreversibility on efficiency, and the application of the first law of thermodynamics in different contexts.

Discussion Character

  • Conceptual clarification, Assumption checking, Mixed

Approaches and Questions Raised

  • Participants explore the nature of isentropic processes and their irreversibility, questioning how these concepts coexist. They discuss the desirability of minimizing irreversibility in processes, particularly in relation to efficiency. Questions arise regarding the applicability of the first law of thermodynamics to irreversible processes and the implications of energy loss due to entropy.

Discussion Status

Several participants have contributed insights and examples, with some suggesting that while the first law applies universally, the efficiency and energy losses associated with irreversible processes are significant points of discussion. There is an ongoing exploration of examples and equations relevant to the topic, indicating a productive exchange of ideas without a clear consensus.

Contextual Notes

Participants are navigating the complexities of thermodynamic laws and their implications for real-world processes, with a focus on the definitions and characteristics of reversible versus irreversible processes. The conversation reflects an interest in understanding the underlying principles rather than arriving at definitive solutions.

Dili
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I am having trouble with these questions. Can anyone help please

1. How is it possible to have an isentropic process which is irreversible?

2.Why is it desirable to have processes which are less irreversible?

3. Is the expression for the 1st law of thermodynamics for a closed system process is same for both reversible and irreversible processes?
i.e. Q -W = U2-U1
 
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A reversible process does not imply that system entropy is not changing, but universe entropy not increasing. So, the process can proceed in such a way that system entropy does not change and entropy of the system environment (the univers) will increase. So such a process would be irreversible.

Desirable? Something is desirable if it helps in pursuing a target. If you are looking for a big blast, for sure it won't be reversible. If you are lloking for a rechargeable battery, you would like to design an electrochemical process that proceeds closely reversibly.

For the third point, I would say that for irreversible processes this equation does not apply. There is always some loss of energy when the process is not reversible: this is entropy.
 
Thanks vivesdn

In the 1st question can you give an example for an isentropic irreversible process?

In the second question i was referring in line with efficiency

Can you give an equation of 1st law of thermo that applies to the irreversible process?

Any replies/opinions from others are welcome as well
 
All irreversible processes have an efficiency problem: universe entropy increases, so there is some energy loss (not disappeared, just invested in increasing universe disorder). But this is the toll you have to pay if you want something to happen quickly. As the process is more reversible, the less energy lost in entropy increase. So in terms of efficiency you would like to work always with reversible processes.
 
vivesdn said:
For the third point, I would say that for irreversible processes this equation does not apply. There is always some loss of energy when the process is not reversible: this is entropy.
In physics, laws are never broken. The first law always applies between any two equilibrium states. It does not depend upon the efficiency of the process in getting from one state to the other.

AM
 
Andrew Mason said:
In physics, laws are never broken. The first law always applies between any two equilibrium states. It does not depend upon the efficiency of the process in getting from one state to the other.

AM

OK. I must agree. The irreversible process will have a lower value for W than the corresponding reversible process between the same states (characterized by U1 and U2 ).
 

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