Thermodynamic energy of vibration

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Discussion Overview

The discussion centers on the thermodynamic energy associated with the vibrations of bodies, specifically the implications of this energy being quantified as 1/2 kT. Participants explore whether these vibrations can be detected and the relationship between vibrational energy and heat.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that every body possesses vibrational energy quantified as 1/2 kT and questions whether this energy can be detected or ultimately manifests as heat.
  • Another participant explains that all energy related to temperature (T) is considered heat, with distinctions made regarding thermal energy in transit versus heat content. They elaborate on how energy is distributed among vibrational modes and provide examples of energy contributions from different types of motion in molecules.
  • A follow-up question asks if it is possible to detect these vibrations, seeking clarification on the relationship between detection capabilities and the classification of energy as heat.
  • A later reply distinguishes between the detectability of vibrations and their classification as heat, suggesting that detection depends on technological capabilities while the classification relates to the statistical distribution of energy across modes.

Areas of Agreement / Disagreement

Participants express differing views on the detectability of vibrational energy and its classification as heat, indicating that the discussion remains unresolved with multiple competing perspectives.

Contextual Notes

Participants reference the statistical distribution of energy and the role of technology in detection, but these aspects remain underexplored and not fully resolved within the discussion.

Boltzmann2012
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I have read that every body possesses some energy of vibration due to the thermodynamic character and it is equal to 1/2 kT. Does this mean that we can detect the vibrations of bodies?
Or ,as it is, does it ultimately end up as heat?

Regards,
Boltzmann2012
 
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All types of energy that connect to a T are considered heat (although some places will restrict "heat" to mean thermal energy in transit between objects, while others will also talk about "heat content"). The way it works is, any thermal reservoir at some T is more or less "happy" to bestow any "mode" with which it comes into contact with with kT of energy, so to know how much energy passes from the reservoir to the system when placed in thermal contact, you need only count the modes. Each vibrational mode counts as one mode, so a 3D oscillator has 3 such modes, so a total of 3kT per oscillator. Free particles only count a "half" a mode for each dimension, so 3kT/2 for a free particle in 3D. A molecule with two atoms has 3kT/2 from its free motions, and two more kT/2 from its two free rotational modes around the axes not between the atoms, for a total of 5kT/2. At very high T, it would also get kT from its vibrational mode between the atoms, for a total of 7kT/2, but you don't generally see this because at such high T, the molecule breaks up. So you see, it's all about counting modes, when you are talking about thermal energy.
 
Does that mean we can never detect these vibrations?
Please explain.

Regards,
Boltzmann2012
 
Boltzmann2012 said:
Does that mean we can never detect these vibrations?
Whether or not we can detect it, and whether or not it is considered "heat", are two very different issues. Our ability to detect just depends on the quality of our technology, whether or not it is counted as heat depends on how statistically distributed is the energy over all the accessible modes. So in some situations, we can detect heat directly, in other situations, we indirectly infer its presence.
 

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