What is the Entropy of a System with Doubly Degenerate Energy Level?

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Discussion Overview

The discussion revolves around the entropy of a system with a doubly degenerate energy level, specifically considering examples such as spin 1/2 particles. The scope includes theoretical considerations in statistical mechanics and the implications of degeneracy on entropy calculations.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant references an old book on Statistical Mechanics that discusses a system with one doubly degenerate energy level, seeking examples of such systems.
  • Another participant notes that real systems typically have more than two energy levels but can be approximated as two-level systems under certain conditions.
  • A participant suggests that spin 1/2 particles are a relevant example of a system with a doubly degenerate energy level in the absence of a magnetic field.
  • There is a proposal that a spin 1/2 particle, when viewed as a system, has an entropy of k ln 2 in the absence of a magnetic field, which is affirmed by another participant.

Areas of Agreement / Disagreement

Participants generally agree on the example of spin 1/2 particles having a doubly degenerate energy level and the associated entropy, but the broader applicability to other systems remains less clear.

Contextual Notes

The discussion does not resolve the broader implications of degeneracy in various systems or the conditions under which the approximations hold true.

Philip Wood
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An old book I have on elementary Statistical Mechanics (Rushbrooke) uses as an especially simple case a system with one energy level. This level is doubly degenerate. The author doesn't give an example of such a system. Can anyone think of one? And would it have entropy [itex]k\ ln 2[/itex]?

[My thoughts turn to spin 1/2 particles but I'd like comment.]
 
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Real systems tend to have more than two energy levels, but some are approximated as two level systems in certain situations.
 
Philip Wood said:
My thoughts turn to spin 1/2 particles but I'd like comment.
Yes, a spin 1/2 particle has a doubly degenerate energy level in the absence of a magnetic field.
 
DrC and TW. Many thanks for replying.

TW: would you, then, say that a spin 1/2 particle, viewed as a system, has an entropy of [itex]k\ ln2[/itex] in the absence of a magnetic field?
 
Philip Wood said:
TW: would you, then, say that a spin 1/2 particle, viewed as a system, has an entropy of k ln2 in the absence of a magnetic field?
Yes.
 

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