Extensive / Intensive variables in thermodynamics

Click For Summary

Discussion Overview

The discussion revolves around the classification of variables in thermodynamics as intensive or extensive, particularly in the context of halving a system. Participants explore the implications of changing system boundaries and how this affects the categorization of variables.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the classification of variables when halving a system, suggesting that if half the moles of gas are taken and placed in a different container of the same volume, the pressure should change, prompting a reevaluation of what constitutes intensive and extensive variables.
  • Another participant asserts that taking half the moles of gas creates a different system due to a change in boundaries, emphasizing that the heuristic for categorizing variables is not meant to be applied to specific scenarios.
  • A further response reiterates that the classification of extensive and intensive variables should not depend on specific arrangements and should be viewed generically.

Areas of Agreement / Disagreement

Participants express differing views on whether the proposed scenario of halving the system results in a different system, indicating a lack of consensus on the implications of changing system boundaries.

Contextual Notes

The discussion highlights the heuristic nature of defining intensive and extensive variables, suggesting that participants may be interpreting the definitions differently based on their understanding of system boundaries.

fission
Messages
12
Reaction score
0
hello everyone, recently i was studying thermodynamics and i think i got a basic doubt on what my book has to say and although i feel this is a small thing to ask but since i have no teacher with me, this is the best place i can think of.

so my book was saying about intensive and extensive variables in thermodynamics, it said to decide which variables are what half a equilibrium system and the values which do not change are intensive and the other are extensive.

so now they gave an example that if a system is halved, mass, volume, internal energy gets halved and pressure and temperature stay same, but now i have an question what if i half it in such a way that i take half moles of that gas in equilibrium and put in in different container of same volume, in such a case volume would remain same and pressure should get halved, so what are extensive and intensive variables in this case and how do we know?

thank you for reading and your replies.
 
Science news on Phys.org
fission said:
what if i half it in such a way that i take half moles of that gas in equilibrium and put in in different container of same volume
Don’t do that. That is not half of the original system. It is a different system.

The suggested exercise is not a rigorous definition, it is a simple heuristic to help you learn which quantities go in which category. Don’t stretch the heuristic. The heuristic is not referring to any specific system, it is just a way to organize your thoughts.
 
Dale said:
Don’t do that. That is not half of the original system. It is a different system.

The suggested exercise is not a rigorous definition, it is a simple heuristic to help you learn which quantities go in which category. Don’t stretch the heuristic. The heuristic is not referring to any specific system, it is just a way to organize your thoughts.
oh okay but can you please explain why it will be a different system? i mean i am taking the mass of gas from the same system, just keeping the volume same.
i am just not able to visualise it i think, can you give a basic definition of what system is and what it constitutes?
 
fission said:
can you please explain why it will be a different system?
Because you are changing the boundaries.

However, you are missing the point. This is a simple heuristic. It is not to be applied to a specific system. It is just a generic heuristic to help you organize your thoughts. Extensive and intensive do not change based on some odd arrangement of the system. Just think generically
 

Similar threads

Undergrad Thermodynamic functions and state variables
  • · Replies 8 ·
Replies
8
Views
1K
Graduate Proof of thermodynamic stability condition
  • · Replies 4 ·
Replies
4
Views
3K
Undergrad How Can Internal Energy of the Canonical Ensemble Change (Fluctuate)?
  • · Replies 15 ·
Replies
15
Views
3K
Undergrad Minimization of thermodynamic equilibrium
  • · Replies 3 ·
Replies
3
Views
2K
Undergrad Change of Variables in Thermodynamics
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad Empirical temperature scales using different thermometers
  • · Replies 7 ·
Replies
7
Views
3K
Undergrad Paradox: Thermodynamic equilibrium does not exist in gravitational fields
  • · Replies 135 ·
5
Replies
135
Views
9K
Undergrad Movable Wall in an Adiabatic System: Same Final Temperature?
  • · Replies 8 ·
Replies
8
Views
2K
Undergrad Work in hydrostatic system in cylinder with piston: P_int or P_ext?
  • · Replies 5 ·
Replies
5
Views
2K
Undergrad Questions about my Understanding of Thermodynamics and Statistical Mechanics
  • · Replies 2 ·
Replies
2
Views
3K