A doubt from Kinetic Theory of Gases

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Discussion Overview

The discussion centers around the kinetic theory of gases, specifically the average translational kinetic energy of ideal gases and the degrees of freedom associated with gas molecules. Participants explore the implications of these concepts for different types of gases, including atomic and diatomic gases, and the conditions under which they can be considered ideal.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the relationship between average translational kinetic energy and degrees of freedom, asking if f=3 applies to all ideal gases.
  • Another participant asserts that translation has three degrees of freedom in three-dimensional space, even under certain constraints.
  • Some participants argue that ideal gas molecules lack structure and thus can only translate, supporting the notion that f=3 for ideal gases, particularly atomic gases.
  • There is a mention of more complex molecules having additional degrees of freedom due to rotational and vibrational motions, which allow for greater energy storage as temperature increases.
  • One participant expresses confusion regarding the definition of an ideal gas, suggesting that the concept is well established and typically does not include gases with f>3.
  • Another participant corrects their earlier statement about atomic gases, noting that negligible chemical interactions are necessary for them to be considered close to ideal, primarily referring to noble gases.
  • Several posts address technical issues with quoting and editing comments within the forum interface, indicating a meta-discussion about the platform itself.

Areas of Agreement / Disagreement

Participants exhibit both agreement and disagreement regarding the degrees of freedom of ideal gases and the implications of kinetic theory. While some assert that f=3 is universally applicable to ideal gases, others suggest that there may be exceptions or additional considerations for different types of gases.

Contextual Notes

There are unresolved assumptions regarding the definitions of ideal gases and the conditions under which different degrees of freedom apply. The discussion also reflects varying interpretations of the kinetic theory as it relates to different gas types.

vijayram
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I have read Average translation kinetic energy is 1/2RT per degree of freedom and Average translation kinetic energy for an ideal gases is 3/2RT.How? Does it imply f=3 for all ideal gases?
 
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There are three dimensions in our universe, so translation always has 3 degrees of freedom. Even if you constrain motion in one dimension: if the constraint is not too strong (or if you ignore quantum effects), it will have motion in that dimension.
 
Yes, exactly. The ideal gas molecule has no structure, so it can't rotate or vibrate. All it can do it move in three dimensions. f=3. Atomic gasses (at least the ones that don't make dimers) approximate this case and differ from the ideal primarily only in that they take up space reducing the available volume. More complicated molecules can vibrate and rotate and each independent motion is another degree of freedom where energy can be stored. The gas has more capacity to store energy as a function of temperature.
 
well this is weird. I keep trying to quote mfb's comment above about an ideal gas with f>3, and the app keeps quoting an entirely different post, one that doesn't even show up in the thread (on my browser anyway)

In any case, regarding the comment that an ideal gas can have degrees of freedom >3:

No volume, only perfectly elastic collisions, and no internal degrees of freedom is the definition of an ideal gas. I wouldn't be surprised if someone somewhere postulated and "an ideal diatomic gas" or similar, but in almost all contexts the meaning of "ideal gas" is extremely well established.
 
mike.Albert99 said:
Yes, exactly. The ideal gas molecule has no structure, so it can't rotate or vibrate. All it can do it move in three dimensions. f=3. Atomic gasses (at least the ones that don't make dimers) approximate this case and differ from the ideal primarily only in that they take up space reducing the available volume. More complicated molecules can vibrate and rotate and each independent motion is another degree of freedom where energy can be stored. The gas has more capacity to store energy as a function of temperature.

I know, replying to myself is goofy ...

I just wanted to correct myself in that the atoms in an atomic gasses also have to have negligible chemical interactions in order to be close to ideal. That means noble gasses for the most part.
 
I misunderstood the first post, then edited my post. I guess you opened the thread when the old text was there, but if you quote it the forum loads the current text. Ignore the old text.
mike.Albert99 said:
I just wanted to correct myself
You can edit your posts.
 
mfb said:
You can edit your posts.

Oh, that would be useful. I don't see a way to do it here (accessing by browser) do I need the app?
 
I don't know if it is possible via the app, it is certainly possible via the browser (bottom left of a post), but there is some time limit to it.
 

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