# Physics307L F09:People/Martin/Planck

1. Brief summary of what I did. Link the the lab manual: [1]
2. Links to all wiki pages that contain my notebook entries.

Purpose The purpose of our experiment is to determine the value of plank's constant (h). We will do this by examining the photoelectric effect and using the equation E = hf = KE + W where f is the frequency of the incident light, W is the work function (amount of energy required to eject an electron from the metal plate, h is plank's constant and KE is the max allowed kinetic energy for the ejected electrons.

Results

SJK 03:40, 11 October 2007 (CDT)
03:40, 11 October 2007 (CDT)
Great job reporting the uncertainty! This would be easier to read if you used same exponent for value and uncertainty. And also, fewer digits. e.g.:
(7.2 +/- 0.2) E-34 J s

As for taking more data, using the same methods, that would reduce your uncertainty range...but would it really reduce your systematic error?
• Planck's constant- 7.17E-34 J*s +/- 2.31E-35 J*s
• Work function- 2.15E-19 J +/- 2.3E-20 J

Are these results sastifactory, how could I improve next time?

• With the true value of h being 6.63E-34 we had a percent error of 8.14% we could have better results but having less then ten percent error is fairly good.
• I believe that the error stated above is from a systemmatic error. I think one of the systemmatic errors came from the focus of the lens, when we changed the focus of the light the voltage reading changed. The only way I can think to improve the focus factor of the lens is to put the lens on longer tracks to be assured that the correct focus of the light beam is a position that the lens can be at (on the track). It seemed that even the farthest point on the track the light was not completely focused.
• Also we could take more data next time in order to have a better representation of the mean and to decrease the size of our error bars.
• If I had more time to experiment I would try to unfocus the light a little bit to see if this "focus factor" really effected the results of our experiment.

Conclusion

SJK 03:42, 11 October 2007 (CDT)
03:42, 11 October 2007 (CDT)
Overall, excellent work, both in the lab, and in the analysis and summary!
• This data supports the idea of a quantum description of light, one where the stopping potential depends on the frequency rather then the intensity.
• But why does the time to return to the original voltage reading increase as we decrease the % of transmitted light and therefore decrease the intensity? The intensity of light effects how many electrons are displaced during the photoelectric effect not the energy that they have after displacement. Since the h/e appratus that we used the buildup of electrons to create its stopping potential the less intense the light, the less electrons displaced, the slower the buildup of the stopping potential (the voltmeter readout).