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Colliding hydrogen atoms find velocity to raise to N=2

  1. Mar 24, 2013 #1
    1. The problem statement, all variables and given/known data

    Two hydrogen atoms, both initially in the
    ground state, undergo a head-on collision.
    If both atoms are to be excited to the n = 2
    level in this collision, what is the minimum
    speed each atom can have before the collision?
    Answer in units of m/s

    1 mol of hydrogen is 1.008g

    2. Relevant equations

    En = K + Uelec

    For a hydrogen atom: En= 13.6/N^2

    3. The attempt at a solution

    Okay, so I know that there is a change in energy that excites the atoms to N=2. This change in energy must be from kinetic energy.

    Two atoms collide into each other. And after the collision they are raised to N=2.

    A. 2(Ei+K)=2(Ef) ;note, i used 2 because it is two atoms colliding
    The twos cancel?
    B. Ei+K=Ef
    -13.6/(1*1)+1/2(mh)v^2=-13.6/(2*2) ;mh is mass of hydrogen
    C. 10.2=1/2(mh)v^2 ;add 13.6 to both sides

    Am I correct up until here? (Part C)
    To find mass of hydrogen
    D. 1.008/6.02*10^23=1.67*10^-24

    E. 10.2=1/2(1.67*10^-24)*(v^2)
    1.22155 E 25=V^2

    Where did I mess up?
  2. jcsd
  3. Mar 24, 2013 #2
    Okay, I figured it out:


    mh is equivalent to the mass of a proton which is 938E6 MeV/C^2

    so, 20.4/938E6=v^2

    so V^2 = 2.17484E-8
    v= 1.474E-4

    You have to multiply by c, I am not exactly sure the reasoning but
    1.474E-4 * 299792458 = 44211.4107m/s

    Hope if you stumble upon this with the same problem it helps.

    Id still appreciate an explanation if anyone wants to give one.
  4. Mar 24, 2013 #3


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    First, be clear that you are using the center of mass frame. In that frame, the situation with respect to the two H is symmetric, so you can just look at one of them.

    Second, use the right units. Since you need your answer to be in MKS units, you need to convert the 13.6 eV to joules.
  5. Mar 24, 2013 #4


    User Avatar

    You are off by a factor of a million.

    Also, don't forget that in your energy balance equation the Hs still exist after the collision, so their masses are still there.
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