User:Jacob R Jaramillo/Notebook/Notebook/Planck's constant

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Plancks Constant


SJK 15:24, 11 October 2009 (EDT)
15:24, 11 October 2009 (EDT)
Good equipment, safety, and setup. Nice figures, especially the one showing the colored spectrum.

Mercury (Hg) Vapor Light Source with Light Block, OS-9286

Specifications of Light Source:

  • Line Voltage Range: 108-132 VAC
  • Power: 125 W MAX
  • Frequency: 47-63 HZ
  • 115V

Light Aperture, AP-9369

Coupling Bar, AP-9369

Voltmeter, True RMS

One black and one red bananna cable

h/e Apparatus, AP-9369

Green Line Filter (interference filter used to transmit the 532nm band and block other wavelengths)

Yellow Line filter (interference filter used to transmit the 570nm band and block other wavelengths)

Variable Filter (100%, 80%, 60% 40% and 10% transmission outputs)


The Purpose of this lab is to measure planck's constant which is symbolized by the letter h. I will use the photoelectric effect to measure planks constant and the below formula to calculate an exact value based on my test results.


(E) energy, (h) planck's contant, (v) frequency of light, (KEmax) maximum amount of kinetic energy for an ejected electron, (Wo) is the Work function, better explained as the amount of energy required to eject an electron from the metal plane in the h/e aperatus.


Experiment conducted via instructions found in Gold Physics 307L manual[1]

  • After reviewing safety concerns (ie. light source became fairly hot and any time when dealing with a light source make certian that it isnt eye level with anyone in the lab) I turned the light source on as it requires 5 minutes to warm up.
  • Verify the batteries built into the h/e apparatus match or exceed the voltage listed on the h/e apparatus.
  • Connect the light source and the h/e apparatus with the connection arm as shown in the lab manual. Also making a crude alignment thus centering the light source emission on the aperture of the white plate.
  • Connect one end of the black banana cable to the black port (comm) on the volt meter and the other end to black port on the h/e apparatus. Then connect on end of the red banana cable to the red port (V) on teh volt meter and the other end to the red port on the h/e apparatus.


  • Turn the volt meter knob to measure Volts (V) in the DC setting.
  • Once you have allowed appropriate time for the light source to warm up, roll the light shield out of the way on the h/e apparatus thus allowing light to enter the h/e apparatus. Now adjust the position of the grating on the light source by moving it back and forth until you maintain the best possible focus on the photodiode mask.
  • Proceed to measure the stopping potential for all wavelengths in the first order and second order. To measure the stopping potential note the initial voltage, then press the PUSH TO ZERO button on the h/e apparatus to discharge any accumulated potential, then with a stop watch measure the amount of time it takes for the voltage to reach its initial point. REMEMBER: USE GREEN AND YELLOW FILTERS WHEN MEASURING GREEN AND YELLOW LIGHT AND/OR THE VARIABLE FILTER WHEN INSTRUCTED TO.


Part A

  • Select two colors and measure the stopping potential using the variable filter. Take to measurements for each wavelength and each percentage of transmission.
  • As the wavelength increases, the frequency decreases thus resulting in a longer stopping voltage. Due to the decrease in frequency, it takes longer for the electrons to be ejected from the plate. Thus lower frequencies require more time to reach the same Wo of higher frequencies.
SJK 15:22, 11 October 2009 (EDT)
15:22, 11 October 2009 (EDT)
It took me a while to realize that "Trial #1" and "Trial #2" represent seconds and not voltages. If you don't have the units in your table, you need to definitely specify elsewhere "such and such values are in seconds." Also, some description of what you were doing is necessary. A reader who knows nothing about the lab would have been pretty confused as to what these data mean. (Later note): Actually I re-read the above, and it is more clear what you are doing. Thus, really the biggest problem was not labeling the column "Time #1 (s)" which would have cleared up a lot.




Part B

  • Measure the voltage for each wavelength in the first order


Experiment #2

SJK 15:28, 11 October 2009 (EDT)
15:28, 11 October 2009 (EDT)
I think this is definitely where you ran out of time. Definitely this lab requires doing and showing the results of the linear fit to obtain h constant and the work function. I know you had extenuating circumstances, but want to make sure that you know that for sure. Also, the plots would be much clearer without the "smooth" or spline fit to the data--just showing the points as squares or circles, and then putting linear trendline through would be best.

Based on the equation E = hv, plotting E vs v will result in a slope equal to h, planck's constant.




SJK 19:14, 10 October 2009 (EDT)
19:14, 10 October 2009 (EDT)
It's desired to have the summary on a separate page from your primary notebook (which is above). More than that, though, your summaries need to provide final values with uncertainty and comparison to accepted value. I don't see a value for planck's constant here, and of course no comparison to the accepted value. Thus, it looks like perhaps you didn't do the linear fit? (Not sure yet whether you did.)

In conclusion, this lab was helpfull in spelling out exactly what planck's constant was and how easily it could be calculated. I can't even count how many times I have used planck's contant in formulas and the fact that I have the value burned into my head; however, preforming an actual experiment rather than just being told what the value is, was pretty cool. And last but not least, to mention inconsistancies, I would say that the most obvious would be my personal response time to the stop watch. Secondly, of course the voltmeter and the h/e apparatus have minimul built in tolerance that can be considered; yet, I didnt research the exact values, which would be something I would look into if I preformed this experiment again.