Reversible Processes: Isothermal to Adiabatic Transition

• AirForceOne
In summary: That makes sense. In summary, the problem discussed is about a red-hot piece of iron being thrown into a lake with a lower temperature, and the book stating that this process is irreversible. The reason for this is because the heat would not spontaneously flow back from the cooler lake into the warmer iron, and a reversible process would require work to reverse it. The initial and final state variables are not the same in a reversible process, but the process can be reversed with an infinitessimal change in conditions and supplying back the work generated. A reversible physical change requires processes that are quasi-static and occur while everything is in equilibrium. Examples of non-reversible processes are the evaporation of a puddle after rain and an open bottle of carbonated
AirForceOne
The problem:
A red-hot 2.00 kg piece of iron at temperature t1=880k is thrown into a huge lake whose temperature is t2=280K. Assume the lake is so large that its temperature rise is insignificant.

The book says that this process is irreversible. Why?

I have another question. In a reversible process, are the initial and final state variables (p,v,T,Eint,S) the same? Or is does that only true for a cycle? I'm really confused haha. If all the processes in a carnot cycle are reversible, how come the temperature changes when going from the isothermal to adiabatic process, even though the process is done very slowly? How come that process is not irreversible?

Thanks.

You could not reverse the process and transfer all that heat back into the iron without doing work. IE the heat would not spontaneously flow back from the cooler lake into the warmer iron. That is why it is not reversible.

Drakkith said:
You could not reverse the process and transfer all that heat back into the iron without doing work. IE the heat would not spontaneously flow back from the cooler lake into the warmer iron. That is why it is not reversible.
Heat will not flow back from the cooler lake to the warmer iron, true. But that is not why it is not reversible. It would not flow back from the cooler lake to the warmer iron in a reversible process either. A reversible process still requires work to reverse. It is just that the amount of work is required to reverse it is the same as the amount of work generated in the forward process.

A reversible path between the two states would be achieved by connecting a Carnot heat engine between the iron and the lake until the lake and iron are at virtually the same temperature (ie. arbitrarily close to the same temperature). The work generated could be stored (say, by lifting a weight or spinning a flywheel). Then the heat could be taken out of the lake and transferred back to the iron (theoretically) by making an infinitessimal change in the relative temperatures of the iron/lake and running the heat engine in reverse as a Carnot heat pump using the stored energy. The work required for the heat pump to move that heat back would be arbitrarily close to the energy that was stored from the output of the Carnot heat engine.

AM

AirForceOne said:
I have another question. In a reversible process, are the initial and final state variables (p,v,T,Eint,S) the same? Or is does that only true for a cycle? I'm really confused haha.
The final state variables are not the same as the initial. For the iron, obviously temperature and entropy change. The temperature for the lake does not change but the entropy does (it increases).

If all the processes in a carnot cycle are reversible, how come the temperature changes when going from the isothermal to adiabatic process, even though the process is done very slowly? How come that process is not irreversible?
The temperature change is reversible because the isothermal and adiabatic processes can be reversed with an infinitessimal change in conditions and by supplying back the work that was generated in the forward process.

AM

The definition of a reversible process provides a simple test for reversibility.

A reversible process is one in which all intermediate states are equlibrium states.

Now when the hot iron is thrown into the lake is it in (thermal) equilibrium?

Whats an example of a reversible physical change not at equilibrium?

elasticities said:
Whats an example of a reversible physical change not at equilibrium?
A reversible physical change requires processes that are quasi-static. A quasi-static process means that it occurs while everything is in equilibrium (ie. out of equilibrium by an infinitessimal amount). So, there are no such examples.

AM

Andrew Mason said:
A reversible physical change requires processes that are quasi-static. A quasi-static process means that it occurs while everything is in equilibrium (ie. out of equilibrium by an infinitessimal amount). So, there are no such examples.

AM

Really?

Like a chemical process not at equilibrium would be a open can of pop...

elasticities said:
Really?

Like a chemical process not at equilibrium would be a open can of pop...
Not sure what your point is.

AM

Andrew Mason said:
Not sure what your point is.

AM

But isn't evaporation of a puddle after rain a physical change that is not at equilibrium?

Also is an open bottle of carbonated beverage (like a 2L bottle) an example of a chemical change not at equilibrium?

elasticities said:
But isn't evaporation of a puddle after rain a physical change that is not at equilibrium?

Also is an open bottle of carbonated beverage (like a 2L bottle) an example of a chemical change not at equilibrium?
Yes. But these are not reversible processes. A reversible process is one whose direction can be reversed by an infinitessimal change in conditions: ie. a process that occurs while arbitrarily close to equilibrium.

AM

1. What is a reversible process?

A reversible process is a thermodynamic process in which a system undergoes a series of changes, but at each step, it can be reversed and brought back to its original state without any changes to the surroundings. This means that the system and its surroundings are always in thermal equilibrium and there is no net change in entropy.

2. What is the difference between isothermal and adiabatic processes?

An isothermal process is one in which the temperature of the system remains constant throughout the process. This means that there is no change in internal energy, and the heat added to the system is equal to the work done by the system. On the other hand, an adiabatic process is one in which there is no heat exchange between the system and its surroundings. This means that the internal energy of the system changes due to work done on or by the system.

3. What happens during an isothermal to adiabatic transition?

During an isothermal to adiabatic transition, the temperature of the system decreases as the process becomes adiabatic. This means that there is a decrease in the internal energy of the system, and the work done by the system is greater than the heat added. As a result, the system's entropy decreases, and it moves away from thermal equilibrium.

4. What is the importance of reversible processes?

Reversible processes are important in thermodynamics because they serve as idealized models for real-world processes. They allow us to calculate the maximum work that can be obtained from a system, and they also provide a baseline for comparing the efficiency of real processes. Reversible processes also help us understand the relationship between heat, work, and changes in internal energy.

5. Can all processes be reversible?

No, not all processes can be reversible. Reversible processes are idealized models and do not occur in real-world systems. In reality, there is always some level of irreversibility due to factors such as friction, heat loss, and other dissipative processes. However, reversible processes are important as they help us understand and analyze real processes and their limitations.

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