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Homework Help: Franck hertz experiment problem

  1. Jan 31, 2013 #1
    1. The problem statement, all variables and given/known data

    a beam of electrons bombards a sample of hydrogen. through what potential difference must the electrons have been accelerated if the first line of balmer series is to be emitted

    2. Relevant equations




    3. The attempt at a solution

    i first tried to find the wavelngth corresponding to first balmer series where i put ni=2 &nf=3.. first let me know if this interpretation is correct

    then i found the wavelength l=6.563* E-7 m

    according to E=3.03*E-19J

    and finally V=E/Q=1.89 V

    but the result has to be 12.1 V

    please let me know where's the problem

  2. jcsd
  3. Jan 31, 2013 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The first line of the Balmer series is emitted when the atom goes from n = 3 to n = 2. You found the wavelength and energy of that transition, but that's not what they asked for.

    Hint: Before it can drop to a lower energy level it first must reach the higher level.
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