Entropy Changes in Reversible Engines: Is the Total Change Always Zero?

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Discussion Overview

The discussion revolves around the concept of entropy changes in reversible engines, specifically questioning whether the total change of entropy for such engines is always zero after completing cycles, and how this relates to the entropy of the universe. The scope includes conceptual understanding and theoretical implications in thermodynamics.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that for a reversible engine, the total change of entropy of the engine is zero after completing cycles, but question the change of entropy of the universe.
  • Others inquire whether the discussion pertains to combustion engines, indicating a lack of clarity on the type of engine being referenced.
  • A participant suggests that the change in entropy of the universe depends on whether it is considered reversible or non-reversible, citing current observations that lean towards a non-reversible scenario.
  • Some express confusion about the implications of entropy changes in the system itself, questioning if the system's entropy changes despite the engine's theoretical reversibility.
  • Concerns are raised about the practicality of achieving a truly reversible engine, with references to the Carnot engine as an ideal case and the inherent inefficiencies in real engines due to factors like friction.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the implications of entropy changes in reversible engines, with multiple competing views regarding the nature of the universe's entropy and the practicality of reversible processes in real-world engines.

Contextual Notes

Limitations include assumptions about the definitions of reversible processes and the conditions under which entropy changes are evaluated. The discussion does not resolve the complexities of these assumptions.

KFC
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I learned Thermodynamics long time ago, forgot most of the content. I just want to ask I question.

For a reversible engine, no matter how many complete cycles it done, the total change of entropy of the engine will always be ZERO, right? What about the change of entropy of the universe in this case? Also ZERO?
 
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Would this be a combustion engine?
 
p.tryon said:
Would this be a combustion engine?

I don't know, it is a conceptural question, no other detail. I read some book to review therodynamics and stat. mech. , some material said for reversible process, to complete one cycle, nothing change, the change of entropy is ZERO. Just want to confirm that.
 
KFC said:
I learned Thermodynamics long time ago, forgot most of the content. I just want to ask I question.

For a reversible engine, no matter how many complete cycles it done, the total change of entropy of the engine will always be ZERO, right? What about the change of entropy of the universe in this case? Also ZERO?

That depends no whether the universe is reversable (oscilating nuiverse) or non-reversable (heat-death). Current observations suggest that the latter is the case, so entropy just contiues increasing 'till it reaches maximum, when no more energy can flow from one place to another. At this point, the universe will be a fairly homogenous low-density cloud of energy.
 
LURCH said:
That depends no whether the universe is reversable (oscilating nuiverse) or non-reversable (heat-death). Current observations suggest that the latter is the case, so entropy just contiues increasing 'till it reaches maximum, when no more energy can flow from one place to another. At this point, the universe will be a fairly homogenous low-density cloud of energy.

I completely have no idea of this. So what about the system itself, will entropy change?
 
KFC said:
I don't know, it is a conceptural question, no other detail. I read some book to review therodynamics and stat. mech. , some material said for reversible process, to complete one cycle, nothing changes, the change of entropy is ZERO. Just want to confirm that.

This seems reasonable. If there is no change in the engine, there will be no change in entropy of the engine system. However, I am struggling to imagine an engine that does not involve some type of change.
 
p.tryon said:
This seems reasonable. If there is no change in the engine, there will be no change in entropy of the engine system. However, I am struggling to imagine an engine that does not involve some type of change.

Like carnot engine? But this is a very ideal case, textbook said there is no engine with efficiency larger than that of carnot engine, does that mean practical engine will never be reversible?
 
Yes and saying that a real engine can not be thermodynamically reversible is the same as saying it can not be 100% efficient. In theory it is possible but in practice it is impossible because in a working engine there will always be friction...etc...
 

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