Physics GRE Q: 3D Harmonic Oscillator & Thermal Equilibrium

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Discussion Overview

The discussion revolves around a question from the Physics GRE concerning the average total energy of a three-dimensional harmonic oscillator in thermal equilibrium. Participants explore concepts related to the equipartition theorem and degrees of freedom, while seeking clarification on the correct answer and underlying principles.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant states that the correct answer to the GRE question is D, but expresses uncertainty about how to arrive at that conclusion.
  • Another participant emphasizes the need for the original poster to show their work and thoughts to facilitate assistance.
  • Some participants reference the equipartition theorem, suggesting that each degree of freedom contributes (1/2)kT to the energy.
  • There is a discussion about the number of degrees of freedom, with one participant questioning why there would be 6 degrees of freedom in this context.
  • Another participant notes that a one-dimensional harmonic oscillator has one degree of freedom and its total energy is expressed as (1/2)kx^2.
  • One participant provides a link to the equipartition theorem for further reference.
  • A later reply discusses the average total energy expression in relation to degrees of freedom and the Hamiltonian for n-dimensional systems.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the number of degrees of freedom or the correct average total energy. Multiple competing views remain regarding the application of the equipartition theorem and the interpretation of the problem.

Contextual Notes

There are unresolved assumptions regarding the definition of degrees of freedom in the context of the harmonic oscillator and the implications for the average total energy calculation.

daveyman
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I'm studying for the Physics GRE and I came across this question. The correct answer is D, but I'm not sure quite how to do it. Any help would be greatly appreciated!

A three-dimensional harmonic oscillator is in thermal equilibrium with a temperature reservoir at temperature T. The average total energy of the oscillator is

(A) (1/2)kT
(B) kT
(C) (3/2)kT
(D) 3kT
(E) 6kT
 
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You need to show work in order to get help here. What have you tried so far? What concepts, etc. apply? What are your thoughts?
 
Last edited:
I'm really not sure where to start. I guess I'll just post this in a different part of the forum then.
 
I'm studying for the Physics GRE and I came across this question. The correct answer is D, but I'm not sure quite how to do it. Any help would be greatly appreciated!

A three-dimensional harmonic oscillator is in thermal equilibrium with a temperature reservoir at temperature T. The average total energy of the oscillator is

(A) (1/2)kT
(B) kT
(C) (3/2)kT
(D) 3kT
(E) 6kT
 
Well the equipartition theorem says that each Degree Of Freedom = 1/2KT. Can you think of why there would be 6 DOF?
 
That's exactly what I was thinking. It seems like the problem basically states that there are 3 degrees of freedom, which would yield an answer of (3/2)kT. However, the answer guide clearly says that the answer is 6kT (this is from an official GRE practice test).

I can't think of why there would be 6 degrees of freedom...
 
Ok let's go back to the 1 dimensional harmonic oscillator... How would you write its total energy? How many DOF is that?
 
Total energy for a 1-dimensional harmonic oscillator is (1/2)kx^2, right? And wouldn't that just be one degree of freedom?
 
Total energy of a 1D SHO is (1/2)kx^2 + (1/2)mv^2...
 
  • #11
daveyman said:
I'm studying for the Physics GRE and I came across this question. The correct answer is D, but I'm not sure quite how to do it. Any help would be greatly appreciated!

A three-dimensional harmonic oscillator is in thermal equilibrium with a temperature reservoir at temperature T. The average total energy of the oscillator is

(A) (1/2)kT
(B) kT
(C) (3/2)kT
(D) 3kT
(E) 6kT

Acording to the Equipartition Theorem, there is a kt/2 contribution to the energy from each degree of quadratic freedom in the Hamiltonian. In equation form, the average total energy is <E> = skT, where s is the degrees of freedom.

What is the expression for the Hamiltonian for any n-dimensional 1-particle system?
 

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