Extensive / Intensive variables in thermodynamics

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Main Question or Discussion Point

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.
 

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  • #2
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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.
 
  • #3
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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?
 
  • #4
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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
 

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