Solving Irreversible Processes: Q-W=U2-U1

In summary, the conversation discusses the concept of isentropic processes that are irreversible and the desirability of processes that are less irreversible. It is mentioned that for irreversible processes, the expression for the 1st law of thermodynamics may not apply and there is always some energy loss due to increasing entropy. The conversation also mentions the importance of working with reversible processes for efficiency and states that the first law of thermodynamics always applies between two equilibrium states, regardless of the efficiency of the process.
  • #1
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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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  • #2
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.
 
  • #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
 
  • #4
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.
 
  • #5
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
 
  • #6
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 ).
 

1. What does the equation Q-W=U2-U1 mean?

The equation Q-W=U2-U1 is the mathematical representation of the first law of thermodynamics, which states that the change in internal energy (U) of a system is equal to the heat (Q) added to the system minus the work (W) done by the system.

2. How is the equation Q-W=U2-U1 used in solving irreversible processes?

In solving irreversible processes, the equation Q-W=U2-U1 is used to calculate the change in internal energy of a system. This change in internal energy can then be used to determine the efficiency of the process and the amount of work that can be extracted from the system.

3. Why is the equation Q-W=U2-U1 important in thermodynamics?

The equation Q-W=U2-U1 is important in thermodynamics because it is a fundamental relationship that describes the conservation of energy in a system. It is used to analyze and understand various thermodynamic processes and systems, and is essential in the development of technologies such as heat engines and refrigeration systems.

4. Can the equation Q-W=U2-U1 be applied to all thermodynamic processes?

The equation Q-W=U2-U1 can be applied to all thermodynamic processes, both reversible and irreversible. However, in the case of irreversible processes, the equation may not accurately represent the energy changes that occur due to the presence of dissipative forces such as friction and heat transfer.

5. How do you determine the values of Q, W, U2, and U1 in the equation Q-W=U2-U1?

The values of Q, W, U2, and U1 can be determined through various experimental and analytical methods. For example, Q can be measured using calorimetry, W can be calculated by measuring the force and displacement involved in a process, and U2 and U1 can be determined by measuring the temperature and internal energy of the system before and after the process.

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