# Physics307L F09:People/Mahony/Planck

## Planck's Constant Lab Summary

In this lab, my partner Ryan and I calculated Planck's constant. We followed the instructions in Professor Gold's Manual for experiment 2, and though we intended to follow it for experiment 1 as well, we decided to use a different method involving curve fitting using labview.

For the first part of the experiment, we measured the effect different intensities of the same wavelength of light (in our case blue) had on the charge time. The second part of the experiment involved measuring the stopping potential for various wavelengths of light.

## Results

Our experiment with labview did not yield numerical results, but it showed the tendency of the stopping potential to be reached faster at higher intensities of light.

Our most accurate measurement of Planck's constant was

• $7.185(2)\cdot 10^{-34} Js$

The accepted value from wikipedia is:

• $6.62606896(33)\cdot 10^{-34} Js$

For our full calcuations and results, see the analysis section of the lab notebook.

SJK 16:52, 30 October 2009 (EDT)
16:52, 30 October 2009 (EDT)
As noted on your primary notebook page, I think you have an error in your formula which underestimated your uncertainty. It's more like 0.08E-34 uncertainty...which is still inconsistent with accepted value.

## Conclusions

Though we did not get any numerical results for how the charge time varied with intensity, it was clear from our data that the charge time decreased with higher intensity. It did not charge like an exponential, however. This decrease in charge time with an increase in intensity is in keeping with the quantum model. The higher the number of photons is, the higher the number of electrons being exciting and thus the higher the current.

The accepted value for Planck's constant did not fall within the range I calculated. Since I was off by about 500 SEM's, we must have had some large source of systematic error. Because of the simplicity of the experiment, the lack of reliance on human judgment, and my doubt that the instruments were miscalibrated, I attribute the error to other light sources that were present in the mostly dark room during the time in which we were carrying out the experiment.