Extensive / Intensive variables in thermodynamics

In summary: Mass, volume, internal energy, pressure, temperature. These would all be halved in a system where you take half of the moles of each element.
  • #1
fission
12
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.
 
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  • #2
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.
 
  • #3
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?
 
  • #4
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
 

What are extensive and intensive variables in thermodynamics?

In thermodynamics, extensive variables are properties that depend on the size or amount of a system, such as mass, volume, and energy. Intensive variables, on the other hand, are properties that are independent of the size or amount of a system, such as temperature, pressure, and density.

What is the difference between extensive and intensive variables?

The main difference between extensive and intensive variables is that extensive variables change with the size or amount of a system, while intensive variables remain constant regardless of the size or amount of a system. Additionally, extensive variables can be added or subtracted when combining systems, while intensive variables cannot.

Can intensive variables be converted into extensive variables?

No, intensive variables cannot be converted into extensive variables because they are fundamentally different types of properties. Intensive variables describe the state of a system, while extensive variables describe the size or amount of a system.

What is an example of an extensive variable in thermodynamics?

An example of an extensive variable is the total energy of a system. The total energy increases as the size or amount of the system increases. Another example is the volume of a gas, which also increases as the amount of gas in a system increases.

What is an example of an intensive variable in thermodynamics?

An example of an intensive variable is temperature. The temperature of a system does not change when the size or amount of the system changes. Another example is pressure, which remains constant regardless of the size or amount of a system.

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