Extensive / Intensive variables in thermodynamics

AI Thread Summary
The discussion revolves around understanding intensive and extensive variables in thermodynamics, specifically in the context of halving a system. The original poster questions the classification of variables when taking half the moles of gas from an equilibrium system and placing them in a different container of the same volume. Responses clarify that this action creates a different system due to altered boundaries, emphasizing that the heuristic for categorizing variables is not meant for specific scenarios. The key takeaway is that extensive and intensive properties should be understood generically, rather than applied to unique arrangements of a system. The conversation highlights the importance of maintaining clear definitions of systems in thermodynamic studies.
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
 

Thread 'Condensate rate of water for an air conditioning cooling coil'

I need to calculate the amount of water condensed from a DX cooling coil per hour given the size of the expansion coil (the total condensing surface area), the incoming air temperature, the amount of air flow from the fan, the BTU capacity of the compressor and the incoming air humidity. There are lots of condenser calculators around but they all need the air flow and incoming and outgoing humidity and then give a total volume of condensed water but I need more than that. The size of the...
View full post »
Thread 'Why work is PdV and not (P+dP)dV in an isothermal process?'

Thread 'Why work is PdV and not (P+dP)dV in an isothermal process?'

Let's say we have a cylinder of volume V1 with a frictionless movable piston and some gas trapped inside with pressure P1 and temperature T1. On top of the piston lay some small pebbles that add weight and essentially create the pressure P1. Also the system is inside a reservoir of water that keeps its temperature constant at T1. The system is in equilibrium at V1, P1, T1. Now let's say i put another very small pebble on top of the piston (0,00001kg) and after some seconds the system...
View full post »

Thread 'Entropy and configurations of microstates'

I was watching a Khan Academy video on entropy called: Reconciling thermodynamic and state definitions of entropy. So in the video it says: Let's say I have a container. And in that container, I have gas particles and they're bouncing around like gas particles tend to do, creating some pressure on the container of a certain volume. And let's say I have n particles. Now, each of these particles could be in x different states. Now, if each of them can be in x different states, how many total...
View full post »

Similar threads

A Proof of thermodynamic stability condition
Replies
4
Views
3K
I Thermodynamic functions and state variables
Replies
8
Views
1K
I How Can Internal Energy of the Canonical Ensemble Change (Fluctuate)?
Replies
15
Views
2K
I Minimization of thermodynamic equilibrium
Replies
3
Views
2K
I Change of Variables in Thermodynamics
Replies
4
Views
2K
I Paradox: Thermodynamic equilibrium does not exist in gravitational fields
3
Replies
135
Views
6K
I Why pressure stays the same when doubling both volume and temperature?
Replies
35
Views
4K
Naive Doubt About Thermodynamic Equilibrium
Replies
10
Views
2K
What does it mean to specify the extensive state of an ideal gas?
Replies
3
Views
2K
I Concept of Thermal Equilibrium in the Context of Canonical Ensemble
Replies
5
Views
3K

Hot Threads

Recent Insights

Back
Top