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RMS Speed

  1. Oct 6, 2007 #1
    Hi and thanks for checking this out!! :)

    1. The problem statement, all variables and given/known data
    The temperature and pressure in a star's atmosphere are 1.20E6 K and 0.0400 Pa. Calculate the rms speed of free electrons (mass = 9.11 10-31 kg) there.

    2. Relevant equations

    What am I doing wrong and why do they give the pressure if that's not needed in the formula!?

    3. The attempt at a solution

    I'm using this formula:


    k=1.380658E-23 J/K
    T=1.2E6 K
    m=9.11E-31 kg

    Seems like a simple "plug and chug" type problem. My answer was 4.3E6 m/s. This is not the correct answer.
  2. jcsd
  3. Oct 6, 2007 #2

    Doc Al

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    Staff: Mentor

    That formula is incorrect. The average KE equals (3/2)kT (you must include all three translational degrees of freedom).
  4. Oct 6, 2007 #3


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    Staff: Mentor

  5. Oct 6, 2007 #4
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