Degrees of Freedom of a Diatomic Gas: 5 or 7?

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Discussion Overview

The discussion revolves around the degrees of freedom of a diatomic gas, specifically whether it is 5 or 7, depending on the consideration of vibrational energy. Participants explore the implications of including vibrational motion and the associated independent quantities that need to be specified.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants propose that the number of degrees of freedom is 5 if vibrational energy is not considered, while it increases to 7 when vibrational energy is included.
  • One participant questions what independent quantities are necessary to specify for vibrational motion, suggesting that if kinetic energy is considered, there should be 6 degrees of freedom.
  • Another participant asserts that there is only 1 degree of freedom due to vibration, indicating uncertainty about additional independent quantities.
  • A different viewpoint suggests that the second degree of freedom is related to potential energy of vibration, but questions arise regarding the applicability of potential energy in ideal gases.
  • One participant mentions that a system of particles can have a maximum of 3N degrees of freedom, proposing that 2 connected particles can have 6 degrees of freedom, broken down into spatial, vibrational, and rotational components.

Areas of Agreement / Disagreement

Participants express differing views on the number of degrees of freedom, with no consensus reached on whether it is 5 or 7, and there is ongoing uncertainty regarding the independent quantities associated with vibrational motion.

Contextual Notes

Limitations include the unclear definitions of degrees of freedom in the context of ideal gases and the unresolved nature of how potential energy applies to the discussion.

zorro
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The number of degrees of freedom of a diatomic gas is 5 if vibrational energy is not considered
However, if we consider their vibration, total number of degrees of freedom is 7.

What independent quantities do we need to specify for vibrational motion of the pair of atoms? If it is the kinetic energy of vibration about their common centre of mass, then the total no. of degrees of freedom should be 6. What is the other independent quantity?
 
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It only has 1 due to vibration, I can't think of any other.
 
I got its answer.
The second one is due to potential energy of vibration (first 1 due to kinetic energy).
But ideal gases don't have any potential energy, then how come there are 2 degrees of freedom?
 
A system of particles can have max (?) 3N degrees of freedom where N is the number of particles. In your case 2 connected particles can have 6 degrees of freedom.

Three spatial degrees, where it is in x, y, z space.

1 vibrational DOF like a spring connecting them

2 rotational degrees of freedom.
 

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