Physics307L F09:People/Martinez/Planck's Constant

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SJK 00:40, 18 December 2008 (EST)
00:40, 18 December 2008 (EST)
Good job on the data and analysis for this lab. I wasn't able to spend much time with you during the lab, so we didn't get to talk about the "green band mystery" which was the cause of your 2nd order trouble. As for your suggestion, You can definitely project the lines onto a screen, though I don't know how bright they could get. In regener, I have seen spectral lines (absorption) from sunlight spread across the wall, which is really cool. Probably the easiest way to relieve eye strain in balmer would be to use a camera.

Planck's Constant

  • This Lab was concerned with finding a value for the Planck's constant. Our Experimental procedure followed the guidelines established by Dr. Golds lab manual, lab 5. Our method followed the reasoning of thermal physicists used about the spectrum of light emitted from a black body, in order to reconcile the theoretically derived concepts that the amount would increase exponentially and the peak would occur according to Wein's law, to the experimentally observed concepts that it approached zero as the wavelength decreased and the peaks were off, Planck' set about by explaining the phenomenon using quantum mechanics. That the light comes in discrete packets of energy described by E=hν where h is the fundamental constant to be named after him. Our apparatus is a setup to measure the photoelectric effect - that incident photons of resonant energy will unbind electrons from a material and cause a current consisting of a mercury lamp coalesced into a diffraction grating by a converging lens, then the spectrum is picked up by a device that has a cathode/anode capacitor unbind electrons which cause a potential in an op-amp with infinite impedance, that picked up all the signal and read by our voltmeter. Aside from measuring the value of h and the work function W, we also went about proving that the photon theory of light was correct. We did this by comparing the voltage outputs of two experiments: first measuring the output at a single frequency of light (energy) and varying the amount of photons transmitted with a filter; then by measuring the output from several different wavelengths with the transmittance constant. We found that in fact it was the energy caused by discrete differences in the wavelength that caused the difference in potential, on the other hand because of investigations of the recharge time of the capacitor, we also showed that although the amount of light does not dictate the voltage (energy), that it does effect the current and how much electrons per time would be output.
  • wiki page for this lab Michael's Lab Notebook
  • Our data gave us two quantities(as well as an estimation of the recharge time of the capacitor which we used to justify the assumption that the current is proportional to the amount of light transmitted), the value of plank's constant and the value of the work function W. We found these quantities by using the LINEST function of Excel and using the two resulting data from the line it gave us: the slope was h/e Planck's constant over the elementary charge, and the y-intercept was W/e.

The accepted value of the constant is: 6.626068*10^-34 m^2kg/s

Our value from our calculations: 6.91(87)*10^-34 m^2kg/s SJK 00:36, 18 December 2008 (EST)
00:36, 18 December 2008 (EST)
I think you have a typo in the decimal place for your uncertainty ... should be 6.910(90), right?

Accordingly the percentage difference for our answer is: 4.297%

The Value we obtained for the work function was: 2.4(1)*10^-19 J

  • I learned a little about the circuit by which our h/e apparatus worked (by which I don't mean to be cocky as though to say I couldn't possibly learn anymore for how smart I am, rather I want to convey that this circuit was pretty over my head), and mostly as in the Balmer lab, I am very impressed that such a simple seeming device as that for the photon emission of the mercury vapor is that we can make such distinct energy levels visible, also that thing about how the UV light was visible because of the phosphorescent screen (which also made the other colors seem more purple) was interesting.
  • As for suggestions - this lab went really well so I can't complain: however, it would be cool if an apparatus like this could be used for the Balmer lab because this one was significantly easier on the eyes.