Lab Notes

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  • Charge Time

It takes us three tries to get consistent increases of charge time vs decreasing intensity and to find an appropriate choice for stopping voltage for both colors. This difficulty in obtaining satisfactory results was mostly due to 1) an extra capacitance introduced by hand contact with the grounded h/e apparatus casing and 2) a failure to realign the light rays with the slit when the apparatus has moved. Cutoff voltages of 7V and 1.42V for yellow and violet light are finally chosen after realizing that the cutoff must be high enough so that a measurable charge time is required to reach it, but small enough so that it is not on the far tail of the charging scale where the voltmeter tends to jump back and forth between numbers as it slowly reaches the saturated stopping voltage.

One of the experimentalists, Mr. Darrell Bonn, noticed that the time measurements tend to be longer when his hand is left touching the h/e apparatus after pressing the discharge button. We decide to redo the measurements of 40% and 20% for yellow and are finally satisfied with the apparent linearity of our measurements. We have noticed that, according to the Dr. Gould's manual, what we see as violet the manual terms “blue”.

  • Overlap of Spectra

Darrell has the idea that his reading glasses might work as a good UV filter rather than the yellow filter (since his glasses have clear lens UV filters). When he placed a lens over the slit we did in fact witness a stopping voltage of 0.847V for the second order green band.

  • Some Data Analysis

We collected five data points at each intensity for both yellow and violet ("blue" in the manual, but it sure looks violet). I choose to represent the data by averaging the 5 times for each intensity and plotting both colors on the same axes with error bars. The MATLAB calling sequence for error bar plots is errorbar(X,Y,E) where E is a predetermined error. I choose to determine E by calculating the standard deviation of the mean for 'each' intensity so I will have to plot each point individually such that each of the 5 intensities will have its own error present (I could make just one plot with E set to the average std of the mean so that the error bars are all the same length, but what's 20 more minutes for extra accuracy?). After all this I calculate the mean of the standard deviations of the mean to provide an overall measure of error in the charge-time measurement. These procedures are undergone in the MATLAB code section entitled "Charge Time".

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