Reversible Processes: Isothermal to Adiabatic Transition

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Discussion Overview

The discussion revolves around the nature of reversible and irreversible processes in thermodynamics, particularly focusing on the transition from isothermal to adiabatic processes. Participants explore the definitions, characteristics, and examples of reversible processes, as well as the implications of heat transfer between systems at different temperatures.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant questions why the process of a hot iron cooling in a lake is considered irreversible, noting that heat does not spontaneously flow from the cooler lake to the warmer iron.
  • Another participant explains that while heat will not flow back without work, this does not fully capture the reason for irreversibility, emphasizing that a reversible process would still require work to reverse.
  • A later reply suggests that a reversible path could theoretically be achieved using a Carnot heat engine and heat pump, connecting the iron and lake until they reach similar temperatures.
  • Participants discuss the nature of state variables in reversible processes, noting that while the initial and final states differ, the process can still be reversible if it involves infinitesimal changes.
  • One participant asserts that a reversible process requires all intermediate states to be equilibrium states, raising questions about the thermal equilibrium of the hot iron when introduced to the lake.
  • There is a discussion about examples of reversible physical changes and the requirement for quasi-static processes, with some participants asserting that no examples exist outside of equilibrium.
  • Another participant challenges this by suggesting examples like evaporation or an open can of soda, prompting further clarification on the definitions of reversible processes.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and examples of reversible processes, particularly regarding the conditions under which they can occur. There is no consensus on the examples provided, and the discussion remains unresolved regarding the nature of certain physical changes.

Contextual Notes

Participants highlight the importance of equilibrium states in defining reversible processes and the nuances involved in transitioning between different thermodynamic states. The discussion reflects varying interpretations of thermodynamic principles and the conditions necessary for reversibility.

AirForceOne
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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.
 
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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
 
  • #10
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?
 
  • #11
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
 

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