1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Virial Theorem Question

  1. Oct 16, 2016 #1
    1. The problem statement, all variables and given/known data

    Using paper, pencil and the Virial theorem, calculate the position uncertainty (an estimate of the vibration amplitude) of the H atom in its ground state C-H stretching mode. In more precise language, calculate the bond length uncertainty in a C-H bond due to the C-H stretching mode. First, look up the transition energy of a C-H stretch (you can pick whichever you like, they are all the same to two significant figures). This will allow you to calculate the energy spacing between the = 0 = 1 states. Second, you can use this information to calculate the ground state energy. Next, use this information and the Virial theorem to calculate the expectation value of the ground state potential energy. Because the potential energy a depends on the square of the position it can be used to calculate the position uncertainty of the bond length. Given that the C-H bond is about 1 Å, comment on the validity of assuming a small vibration amplitude in the Harmonic Oscillator mode
    2. Relevant equations
    Transition energy=ħ ω
    ω=2πcṽ where ṽ is the wavenumber
    Ground state energy= ħ ω/2
    Virial theorem: vhat=ax^b 2<T> = b<V>


    -For the Harmonic Oscilator, <x>= 0
    variance: Δx= (<x^2>-<x>^2)^(-1/2)
    -Virial Theorem as applied to the Harmonic oscilattor says 2<KE> = 2<V>
    Then use <Etotal> = <KE> + <V>

    3. The attempt at a solution

    I am not sure about what to use as the wavenumber, I looked at the IR absorption value for the c-h stretch and used 3000cm^-1.

    = 2π(2.998*10^8m/s)*(300000m^-1)
    = 5.65*10^14s^-1

    Transition energy= ħ ω
    = 5.9551*10^-20J

    Ground state energy= ħ ω/2
    = 2.97755*10^-20J

    I am not sure about what to do after this
    Last edited: Oct 16, 2016
  2. jcsd
  3. Oct 17, 2016 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Welcome to PF!

    So far, so good. I would suggest that you continue to follow the outline given in the problem statement. What's the next step after finding the ground state energy?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted