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Homework Help: Planck's Constant measured in photoelectric effect

  1. Feb 20, 2007 #1
    1. The problem statement, all variables and given/known data
    In an advanced laboratory class a student performs the photoelectric experiment. Ultraviolet light is shone on a particular metal and the stopping potential is measured at the same time. It is found that 2.61 V is needed to stop all the electrons when the wavelength of the light is 285 nm, and 4.53 V for a wavelength of 207 nm. What is the work function of the metal?


    2. Relevant equations
    [tex]hf=eV+\phi[/tex]
    Find [tex]\phi[/tex]

    3. The attempt at a solution

    (1)[tex]hf_{1}=eV_{1}+\phi[/tex]

    (2)[tex]hf_{2}=eV_{2}+\phi[/tex]

    divide 1 by 2 to get rid of h, and we get

    [tex]\frac{f_{1}}{f_{2}}=\frac{eV_{1}+\phi}{eV_{2}+\phi}[/tex]

    i can't seem to solve for [tex]\phi[/tex] and get the correct answer. i've tried doing it both algebraically(sp, sry) and with a calculator. possibly i'm putting the numbers in wrong? i have:

    [tex]e=1.6022*10^-19 C[/tex]

    [tex]f_{1}=285*10^-9 m[/tex]

    [tex]f_{2}=207*10^-9 m[/tex]

    [tex]V_{1}=2.61V[/tex]

    [tex]V_{2}=4.53V[/tex]

    where am i going wrong? please help
     
    Last edited: Feb 20, 2007
  2. jcsd
  3. Feb 20, 2007 #2
    Should you need two wavelengths to measure the work function in this experiment?
     
  4. Feb 20, 2007 #3
    my apologies, i should have added something not stated in the problem. the second part of this problem is stated:

    2. What is Planck's constant based on this measurement?

    from this i assume that h is an unknown for part 1. my reasoning is that this is an experiment, and Planck's constant is not exact; it is just supposed to be calculated from four measurements someone took in a lab.
     
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