User:David K. O'Hara/Notebook/physics 307 lab/planck's constant summary

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SJK Incomplete Feedback Notice
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My feedback is incomplete on this page for two reasons. First, the value of the feedback to the students is low, given that the course is over. Second, I'm running out of time to finish grading!

This experiment was designed to find Planck's constant, one of the fundamental constants of nature. The relationship between the energy of a photon and the frequency of its electromagnetic wave was one of the truly great discoveries in science. Work that was later expanded on by Einstein's further investigation of the photoelectric effect and De Broglie's work that expanded the relationship between energy and the quantum wavelength to all particles.

In this experiment, we attempted to see how limiting the light incident on a cathode would impact the stopping potential and the time it would take an apparatus to get back to such a stopping potential. The second half of the experiment was specifically designed to find a value for planck's proportionality constant and see how close to the real thing we could get.


all calculations were performed in excel which can be found here File:Planck.xlsx or in the my lab notebook here [| lab notes]

In part A of the experiment it became clear that the stopping potential did not depend on the intensity of the light. The time it took to reach the full stopping potential did. The intensity of the light or the number of photons present, does not affect the energy of the electrons that are dislodged. The number of photons present will, however, significantly effect the number of electrons dislodged. So the more intense the light, the shorter the refresh time.

In part B, finding Planck's constant, for the first maxima I calculated a value of:

  • h = 6.99768E-34 +/- 7.43E-36 J*s
for the second maxima I calculated a value for h of:
  • h = 7.03782E-34 +/- 1.11E-35 J*s
with the accepted value of h = 6.626068 E-34 J*s

this would give me a percent error for the first maxima of 5.6 %. Not the worst data ever collected but not really the best either.

SJK 15:34, 19 December 2009 (EST)
15:34, 19 December 2009 (EST)
Excel sheet very helpful. Better than "not the worst/not the best" would be to compare with the accepted value and whether it overlapped within your range of uncertainty.


There were a couple of systemic errors in this experiment that I feel would have a large impact on the resulting data and the precision of the measurement. First, as the lab manual states there is some charge leakage in the h/e apparatus that when dealing with thousandths of a volt can't be ignored. Second, when measuring the second maxima and finding that the green filter was not stopping uv light at all it made me feel that all my stopping potentials may be slightly high due to this experimenter's inability to prevent all ambient uv light from entering the h/e apparatus. Since my value for h was higher than expected it would make sense that my Stopping Potentials were higher than they should have been, which would indicate that some higher energy electrons may have been playing havoc with my system.