Hypothetical cyclic process- does it violate the thermodynamic laws?

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Discussion Overview

The discussion revolves around a hypothetical cyclic thermodynamic process and whether it violates the first and second laws of thermodynamics. Participants explore the implications of energy transfer, work done, and entropy changes within the context of this process.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant argues that the first law of thermodynamics is not violated because the internal energy change is zero, as calculated from the energy transfers and work done.
  • The same participant suggests that the second law is also upheld based on the Kelvin-Planck statement regarding heat transfer to a colder reservoir.
  • Another participant hints at considering the total change in entropy as a critical factor in evaluating the second law.
  • A different participant expresses uncertainty about using entropy due to its absence in their lectures but observes that the overall entropy seems to increase after each cycle.
  • One participant questions how the conclusion about increasing entropy was reached, prompting further clarification.
  • Another participant explains their reasoning regarding entropy change, noting that energy added or extracted at given temperatures contributes to the overall entropy, but expresses uncertainty about how to account for the work done in this context.

Areas of Agreement / Disagreement

Participants do not reach a consensus. There are multiple competing views regarding the implications of the cyclic process on the laws of thermodynamics, particularly concerning entropy changes.

Contextual Notes

Some participants express uncertainty about the treatment of entropy and its role in the analysis, indicating a potential limitation in their understanding of the topic as it has not been covered in their lectures.

Who May Find This Useful

This discussion may be of interest to students studying thermodynamics, particularly those exploring the implications of cyclic processes and the laws governing energy transfer and entropy.

trelek2
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Consider the following cyclic process:
Each cycle 800J of Energy is transferred from a reservoir at 800K and 600J of energy from a reservoir at 600K. 400J of heat is rejected to a reservoir at 400K and 1000J of work is done.

I think that the process doesn't violate the first or second laws of thermodynamics:
1st law: dU= dQ+dW
The internal energy of the system is constant: dU= 600+800-1000-400 = 0 so first law is not violated.
2nd law: Taking the Kelvin Planck statement: The engine not only extracts heat from a source and turns it into work but also has to pass some heat on to a colder reservoir, therefore I think that the second law is also not violated.
If I'm mistaken please correct me. If I'm not please reassure me that my logic is correct.
 
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We haven't really covered entropy in the lectures yet, so i don't think I'm ment to be using that. Nevertheless, I looked at it just now and it seems to me that the overall entropy of the system increases after each cycle, so the 2nd law is not violated, is that right?
 
trelek2 said:
it seems to me that the overall entropy of the system increases after each cycle

How did you figure that?
 
trelek2 said:
it seems to me that the overall entropy of the system increases after each cycle

How did you figure that?
 
I found that the change in entropy is equal to amount of energy added or extracted at a given temperature. So we take out 1 twice and put back in once for sure. I'm not sure what to do with the work. I thought it would be increasing entropy of the surroundings by 1000J/700K which is over 1?
 

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