# Physics307L F09:People/Long/Planck's Constant

## Planck's Constant Lab Summary

This lab is designed to measure the infamous Planck's constant by taking advantage of the photoelectric effect. What my partner Tom and I did was essentially measured the stopping potential of the escaping electrons due to the incident photons of varying wavelenghts.

## Data Summary

We measured two bands of spectra from a mercury light source, I will only report the results from the first order band in my summary, our measured value for Planck's constant was:

• $7.185\times 10^{-34} Js$

The accepted value from wikipedia is:

• $6.62606896(33)\cdot 10^{-34} Js$

## Conclusion

This lab had two components to it, the first part of the experiment was essentially designed to measure the changes when varying the intensity of the light source with a reducing filter, the lab manual calls for measuring voltage rise times with a stop watch when proved to be nearly impossible, since the voltage rises exponentially, and it is also to determine when it settles on a particular value. Tom had experience with Labview and DAQ's so we decided to try to measure with that system. It was great to be able to actually get the system working as far as recording the rise time, but when it came time to fit the data to a function, it just did not seem to work and our results were sporadic and unreliable. Although I still know nothing about actually running labview, it'll be exciting to try to use it in the future with Dr. Koch and or Tom's assistance!

For experiment 2, we actually acquired some decent data, since it wasn't a time sensitive operation. We recorded data from multiple colors of the spectrum multiple times and then used excel to fit them to a line and acquire our value for Planck's constant. This lab was especially helpful to me because I feel like I really improved my skills in excel, I used the maximum likelihood formula for fitting my line (outlined here). I actually got the formula to work, and I'll include a screenshot below, because I'm that stoked about it! SJK 17:00, 30 October 2009 (EDT)
17:00, 30 October 2009 (EDT)