Physics307L F09:People/Callow/Planksummary

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Plank's Constant Summary

I worked with Johnny Gonzalez for this lab.

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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!

Links to lab notebook and manual

Link to my lab notebook

Dr. Gould's manual

Purpose of the lab

The lab has two parts. In the first part the goal is to verify properties of light, mainly that the stopping potential of an electron removed by photon is dependent only on the wavelength of a photon, and not the intensity. Raising the intensity however does increase the number of electrons removed shown in the experiment by how fast the maximum charge is reached in the apparatus.

For the second part we use some recorded values of the stopping potential for a few different wavelengths and by making a linear fit of stopping potential and frequency we are able to experimentally find plank's constant and the work function.

Summary of data

We found plank's constant to be

h = 7.18(11) * 10 − 34Js

which compared to the accepted value

h = 6.626 * 10 − 34Js

We have an error of 8% and are about 6 standard error of the means away. So we aren't in agreement with the accepted value, but we aren't too far off of agreement and we didn't have many measurements so maybe with more or better done measurements we'd be closer.

As for the work function we found it to be

W0 = − 2.53(8) * 10 − 19J

The accepted value for plank's constant was found at http://en.wikipedia.org/wiki/Planck_constant

Accepted values for work functions may be found at http://en.wikipedia.org/wiki/Work_function, but I don't know enough about the apparatus to tell which one I should use.

What I Learned

Mostly it was fun exploring why the yellow line acted up as intensity was turned down and finding a method to see why the red area had such high values. I saw that the red isn't mentioned in the lab manual and doesn't show up on http://hyperphysics.phy-astr.gsu.edu/HBASE/quantum/atspect2.html#c2 so maybe it isn't even supposed to be there. The Balmar lab also had some weird stuff going on in this area if I remember correctly.

Also for the future when measuring anything that is charging or behaves as charging did in this experiment I'll probably measure time up to some value around 90 or 95% to avoid headaches.

I want to see the problems that arise in the second order bands, which from the lab notebook there is a picture of things overlapping in the area so I'm guessing it is a similar problem to what we had trying to find values for red.

Other Notes

I'd like to thank Johnny Gonzalez for being my lab partner again and taking photos.

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