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

In summary, the conversation discusses the concept of entropy and its relationship to reversible engines. The question is raised whether the change of entropy in a reversible engine will always be zero, and if so, does this also apply to the change of entropy in the universe. The idea of a non-reversible universe is mentioned, as well as the possibility of a reversible engine such as a Carnot engine. It is noted that in theory, a reversible engine can exist, but in practice, it is impossible due to factors such as friction.
  • #1
KFC
488
4
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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  • #2
Would this be a combustion engine?
 
  • #3
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.
 
  • #4
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.
 
  • #5
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?
 
  • #6
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.
 
  • #7
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?
 
  • #8
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...
 

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

What is a reversible engine?

A reversible engine is a theoretical concept that describes an ideal engine that is able to convert all of the energy it receives into work. This means that there is no energy lost or wasted in the process, making it 100% efficient.

How does a reversible engine work?

A reversible engine works by operating in a series of small, reversible steps. This means that each step can be easily reversed, allowing the engine to return to its initial state without any energy loss. This is in contrast to real-life engines, which operate in a series of irreversible steps and therefore are not 100% efficient.

What is entropy?

Entropy is a measure of the disorder or randomness in a system. In thermodynamics, it is often described as the measure of the amount of energy that is unavailable for work in a system. As a system becomes more disordered, its entropy increases.

How does a reversible engine relate to entropy?

A reversible engine is closely related to entropy because it operates in a way that minimizes the increase in entropy. In an ideal reversible engine, there is no increase in entropy because all of the energy is converted into work without any loss. However, in real-life engines, some energy is always lost and therefore there is an increase in entropy.

Can a reversible engine actually exist?

No, a reversible engine is a theoretical concept that cannot exist in reality. This is because it would require perfect conditions and no energy loss, which is not possible in the physical world. However, it serves as a useful model for understanding the principles of thermodynamics and the concept of entropy.

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