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Finding and approximation for Planck's constant ( H )

  1. Jun 26, 2005 #1
    I have a HSC physics assessment task (Yr 12 Australia) due in a few days where we had to take measurements of the photoelectric effect (VStop, Wave no/length, F) etc with different filters and find an approximation for H, by manipulating different equations etc.

    I already found an fairly close approximation (1.7e-34) H=6.626e-34, but i just want to no if my method was right or not.

    The work function was found by graphing Kemax against the F (Kemax = QVstop/F). A trend line was extended back to get f (3.6e-34) Approx. An approximation for H was found by getting the gradient of the slope of KEMax + W / F (2e-34). But since excel rounded it a bit i put it in my calculater (H = QVstop + W]/F) and got 1.69e-34

    So is that the rite way to do it or is there a better way
    Thnx in advance 4 ne comments
  2. jcsd
  3. Jul 4, 2005 #2
    G'day mate,
    I can't quite understand what you are getting at there, but... it seems as though you are on the right track. Also I don't know if this reply will help you in time.

    The kinetic energy of the ejected electron, KE, is equal to the energy of the initial photon, hf, minus the work function of the apparatus.


    KE = hf - W

    which I think you understand.

    Hence plotting KE vs f should yield a linear graph with gradient h and y intercept W (or -W as the case may be).

    Excel can give h simply by plotting the trendline and then "show equation". BUT this will be a horribly rounded answer, especially if you are going to use it in further calculations (which it doesn't seem as though you are). A better way of doing it is... as you may well have done ... using the regression analysis tool.

    Also don't forget to mention errors and uncertainties in your experiment. Was the filtered light perfectly monochromatic (one frequency??)
    If you can get some error bars (or even boxes) on your graph, you can get a line of minimum gradient and a line of maximum gradient. This will give you some idea of the uncertainty in your result of 1.7E-34 .
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