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Homework Help: Stopping voltage.

  1. May 29, 2006 #1
    In a photoelecric experiment in which a sodium surface is used, you find a stopping potential of 1.85V for a wavelength of 300nm, and a stopping potential of 0.820V for a wavelength of 400nm. From these data find (a) a value for the Planck constant, (b) the work function for sodium, and (c) the cutoff wavelength for sodium.
    I tried solving (a) by stating the following
    h=e*lamba/c ( i let n = 1)
    And we know that K_max=e*delta V, where e is the Elementary charge, and DeltaV is the stopping potential.
    I assumed that all the Energy is transffered to K_MAX
    The units end up working out, but the solution is wrong.
    Anyone know what am i messing up on?
    The rest of the question, i believe i can get once I solve this correctly
  2. jcsd
  3. May 29, 2006 #2


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    Gold Member

    Well you're assumption was incorrect. There is a certain energy called the work function that has to be overcome before an electron can be ejected.

    [tex]E_{photon} = K + \phi [/tex]

    You can manipulate this equation to determine the equation for the kinetic energy of an ejected electron and it follows the form y=mx+b. The m will be your slope and it will actually be planck's constant
  4. May 29, 2006 #3


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    Gold Member

    m will be the slope...when x represents the frequency and y represents the KE (stopping potential * e).

    Alternatively, using the formula provided by Pengwuino for each of the two events you end up with 2 equations in 2 unknowns (h and [itex]\phi[/itex]). You know how to solve that.
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