Physics307L F09:People/Osinski/Photoelectric

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In this experiment we apply the formula of the photoelectric effect to a series of measurements of stopping voltage resulting from the electron's emitted from a material on which light of various frequencies from an excited mercury lamp is shined in order to determine both the work function of the material and the value of Planck's constant. In addition to these measurements we also make observations of the time it takes for a capacitor in our voltage measuring apparatus to charge when light of different frequencies and intensities is shined upon the photoelectric material. A few interesting points are made about the imperfections of our measuring devises and the interference of light from higher order spectra in response to a few initially anomalous observations. The details are available in the lab notebook.


  • Charge Time

We have found that the charge time tends to decrease as a negative exponential with increasing intensity. This is to be expected of a capacitor that is charged with consecutively increasing flows of current. Our measurements of charge time ranged from 3.50s-928s for yellow light & 4.44s-19.30s for violet light spanning intensities of 20%-100% and had a standard deviation of the mean of 0.4552s.

  • Stopping Voltage vs. Frequency

All of our graphs of stopping voltage vs. frequency are quite linear, thus we can comfortably attest to the validity of the linear relationship between energy and frequency. Our reasoning is provided in Part B. as well as the end of the lab notebook.

  • Determination of h and ω_o

We took two sets of stopping voltage vs. frequency measurements for the first and second order spectra, giving us a total of four data sets. We calculated four separate values of h and ω_o, using the equation Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "":): {\displaystyle eV=h\nu-\omega_0} , along with the standard deviations of the mean for each value. Then we calculated the average of these values as a final result. Here I provide only the final results:

SJK 17:13, 2 November 2008 (EST)
17:13, 2 November 2008 (EST)
Where are your units???

h = 7.1876e-034, stdm = 3.5164e-034

ω_o = 1.6026eV, stdm = 0.6620eV

In comparison to the accepted value of h=6.626068*10^-34 our result is noticeably too large SJK 17:21, 2 November 2008 (EST)
17:21, 2 November 2008 (EST)
As noted elsewhere, your uncertainty is not calculated correctly. However, if this indeed were your error, the accepted value is easily within your uncertainty range. Therefore, the correct comparison with the accepted value would be to say that your measurements are perfectly consistent (though quite imprecise).
. I have blamed this overshoot mostly on the current leakage in the h/e apparatus and my reasoning is explained at the bottom of the lab notebook.


The overwhelming majority of the work is presented in the lab notebook.

I must admit that I am still unsure about my method for calculating the standard deviation of the mean for seems a bit large compared to the value. SJK 17:18, 2 November 2008 (EST)
17:18, 2 November 2008 (EST)
Good that you noticed this (unreasonably large slope error) I commented in your notebook, your implementation of error anlaysis aren't correct...we should talk more in person.