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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

    1/l=R(1/2^2-1/3^2)

    E=hc/l

    v=E/Q

    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

    regrds
     
  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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