We wanted to plot the F vs. r however we noticed that based upon the free-body diagram and the triangle formed from the suspended pith ball as a charged balloon is brought near were similar. Therefore all we had to measure is the horizontal displacement from the vertical for the pith ball, d, as well as the center to center distance, r, between the charged pith ball and balloon and plot d vs. r.
We tried to take data by hand measuring but it took to long and we only got three data points. Therefore we decided to videotape the lab and use Tracker to obtain the measurements. This meant that the data acquisition was complete in only about 30 seconds. The linearized best fit was an inverse square relationship.
Hugh Ross suggested that we try his Fun Fly stick since it was more of a "point" charge. When we analyzed this data we got an inverse relationship. We couldn't figure out what we were doing wrong. Well it was a fortunate mistake. When the fly stick approached the pith it did so line an infinite line charge. The relationship in this case would not be inverse square but in fact would be an inverse relationship. Totally Cool! Last year in AP Physics we just simulated line charges with a Feinmann suggested lab of using a line of light from a computer screen. It worked but its a simulation and this is so much better since they already do the Coulomb lab. We theorized if we brought a plane charge (thick and rather large styrofoam board) we would get a constant relationship. We didn't try this since we lack the styrofoam but evidently Rob Spencer says that there is a video on line. He says that he has tried this but that it didn't come out well (its problematic). Since the pith ball has to be far from the edge of the "infinite" plane of charge and the pith ball is hanging from string, I can see how it would be problematic because the "infinite sheet might start to hit the suspension string of the pith ball and so it would have to be lowered (pith ball now closer to top edge of plane). Maybe it would be a better idea to give the pith ball the opposite charge so that it is attracted to the plane and so the plane would never approach the suspension string. Additionally the pith ball should start near the plane and move away and students need to mass the pith ball and using the free-body actually plot Fe vs. r as opposed to d vs. r. As the pith ball moves outward the angle should remain the same. Now that I think about it I think when I have my AP kids do this lab I will have them try having the pith ball oppositely charged (attracting) and not let them plot d vs. r for any.