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Homework Help: Photoelectric homework help

  1. Nov 25, 2011 #1
    So, I got the following question.


    This is figure 13.1.


    C is the cathode material. S is the light. The voltage begins at A. The circuit carrying any current that makes it through the potential difference is everything else to the right.

    So I think, okay, to identify the cathode material I need the work function value. Not too hard.


    Note the formula I used for row 3.


    What did I do wrong? Why am I... what? These answers don't make any sense to me.



    Since the images aren't appearing, I just linked to them. Imageshack.
  2. jcsd
  3. Nov 26, 2011 #2


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    Homework Helper

    Would you please type in the problem and your attempt of solution?

  4. Nov 26, 2011 #3

    First off, apologies to the boards. I didn't look for the "homework" subforum hard enough. (Being moved on my first post is embarrassing. :shy:)

    So, here's my question, typed out.

    I have an emitter electrode of unknown composition. There is a source of controllable light to the side, pointed right at the electrode. The cathode ray created by the electrode will have to travel through a potential difference between metal plates before reaching the anode where it is converted into a current.

    My goal is to identify the emitter electrode composition. To determine this, I am given the following information.

    I can vary the wavelength of the light created by the light source. I can also measure the minimum voltage (potential difference) required to stop the cathode ray from becoming a current completely.

    If the following are the results of said experiment:
    Code (Text):
    wavelength (nm)     250 300 350 400 450 500
    minimum voltage (V) 2.70    1.90    1.30    0.80    0.50    0.10
    This is my attempt at solving it.

    (1/2)mv^2]max=eV'=(Planck's constant)(c/wavelength)-(work function)
    Where V' is the minimum voltage.

    So eV'+(work function)=(Planck's constant)(c/wavelength).

    The only property here that is of the electrode is the work function, so...

    (work function)=(Planck's constant)(c/wavelength)-(eV')

    Now, since everything here has is measured in units of electron volts, I figure I might as well cancel all the e's from both sides.

    (work function/e)=((Planck's constant)(c/wavelength)/e)-V'

    Which makes things a bit simpler for the numbers. But it doesn't matter.

    So I stuck this in Excel, hoping to come out with a single work function. But instead, my results were pretty much repeats of the minimum potential difference.

    To which I said, "This can't be right."

    (I'm only solving up to the work function in my answers, because finding the material composition from the work function is fairly trivial for this problem.)

    Thanks in advance for the help!

    (I need to learn LaTeX at some point.)
  5. Nov 26, 2011 #4


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    Homework Helper

    Your method is all right, and I do not know what your problem can be without seeing the results you got for the work function. I got values scattering between 2.2 and 2.4 eV, 2.29 in average. Have you got a table of work function values of different solids? This is rather a low value, but there are metals with work function close to that.

    Last edited: Nov 26, 2011
  6. Nov 26, 2011 #5
    This is my input into Excel. (For the work function row.)

    Code (Text):
    This is my result.

    Code (Text):
    250     300     350     400     450     500
    2.7     1.9     1.3     0.8     0.5     0.1
    -2.699999995    -1.899999996    -1.299999996    -0.799999997    -0.499999997    -0.099999998
  7. Nov 26, 2011 #6


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    Homework Helper

    Take care, the wavelengths are given in nanometers. So you need to have 250E-9,..... and so on for A1.

  8. Nov 26, 2011 #7
    Oh. Okay.

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