Entropy: second law for systems with zero input net energy

Click For Summary

Discussion Overview

The discussion revolves around the application of the second law of thermodynamics to systems that are not isolated but have equal energy flowing in and out, resulting in no net change in total energy. Participants explore the implications of this scenario for thermodynamic principles, particularly in relation to closed and open systems.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions whether the second law of thermodynamics can be applied to a system with equal energy influx and outflux, where total energy remains unchanged.
  • Another participant provides an example of isothermal compression of an ideal gas, noting that while work is done on the system, there is an equal outflux of heat, leading to no change in internal energy.
  • A participant acknowledges their familiarity with the example and mentions reviewing thermodynamics for applications in cosmology.
  • Clarification is sought regarding the definitions of "isolated," "closed," and "open" systems, emphasizing the differences in energy and matter exchange.
  • One participant confirms they are referring to the second law as it applies to closed systems rather than isolated ones.

Areas of Agreement / Disagreement

Participants express varying interpretations of the second law's applicability to non-isolated systems, with some agreeing on definitions of system types while others explore different scenarios without reaching a consensus.

Contextual Notes

Participants discuss the definitions of system types, which may influence their interpretations of the second law. There is an acknowledgment of the complexity involved in applying thermodynamic principles to systems with energy exchanges.

haushofer
Science Advisor
Insights Author
Messages
3,082
Reaction score
1,602
Dear all,

I'm trying to think about applying the second law of thermodynamics to a system which is not isolated, but has an energy flowing inwards and an equal (!) energy flowing outwards, such that the total energy does not change (total energy flux is zero). Can we still apply the second law in this case? And where can I find a reference (here or elsewhere) which treats this case?
 
Science news on Phys.org
If you compress an ideal gas isothermally, there is an influx of work and equal outflux of heat and the internal energy of the gas doesn't change. ( http://hyperphysics.phy-astr.gsu.edu/hbase/therm/entropgas.html )

Of course if a closed system is in thermal equilibrium with its surroundings, it's constantly exchanging molecular kinetic energy (heat) with the surroundings, but there is no net flow of heat to either direction.
 
  • Like
Likes   Reactions: haushofer
Ah, yes, of course, that's a familiar example. Thanks! My thermodynamics is a bit rusty, but I'm reviewing some for applications to cosmology (gravitating systems).
 
haushofer said:
I'm trying to think about applying the second law of thermodynamics to a system which is not isolated, but has an energy flowing inwards

Yes. Here is a place to start.
 
  • Like
Likes   Reactions: haushofer
Do we mean the same thing with terms "isolated", "closed", and "open"? An isolated system doesn't exchange either energy or matter with its surroundings. A closed system can exchange heat but not matter. An open system can exchange both heat and matter with the rest of the universe.
 
  • Like
Likes   Reactions: Chestermiller
Yes. So I'm referring to the second law for closed systems instead of isolated ones.
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
3K
  • · Replies 12 ·
Replies
12
Views
3K
Replies
10
Views
4K
  • · Replies 17 ·
Replies
17
Views
3K
  • · Replies 13 ·
Replies
13
Views
4K
  • · Replies 16 ·
Replies
16
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 13 ·
Replies
13
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K