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Homework Help: Irreversible process

  1. Sep 29, 2007 #1
    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
  2. jcsd
  3. Sep 29, 2007 #2
    A reversible process does not imply that system entropy is not changing, but univers 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.
  4. Sep 29, 2007 #3
    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
  5. Sep 29, 2007 #4
    All irreversible processes have an efficiency problem: univers entropy increases, so there is some energy loss (not disappeared, just invested in increasing univers 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.
  6. Sep 30, 2007 #5

    Andrew Mason

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    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.

  7. Sep 30, 2007 #6
    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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