• This lab really fits into the theme of making precision measurements, estimating uncertainties, deciding when your data is “good enough”, etc. from Autumn quarter. But here it is in Winter quarter. Not that there is anything wrong with that.
• I cast this two part lab as an exercise in developing a technique, quantifying and correcting for a known systematic effect, and characterizing how well it performs. There is NO experiment here.
• To the point above, we are starting with the understanding that we expect a specific systematic to be present in the data as a result of how the scale works. The EXPECTATION is that there will be a systematic offset in the measured force with decreasing plate separation. Day 1 is about looking for this effect and quantifying it.
• The above is an important distinction because students are supposed to compare their measurements to the EXPECTATION, which is verboten when you are doing an experiment. I.e. The theory of the force between plates of a capacitor predicts a certain relationship that the apparatus is supposed to follow and this is the metric.
• In this context, another theme of day 1 is how do you know when your data are “good enough”? For this lab students need to collect data until they see a statistically significant deviation from the theoretical prediction. We are not doing least squares fitting for this lab, so students are expected to show that on a plot of the data vs the theory, given the size of the error bars on each data point, the deviation from theory is increasingly significant as plate separation gets smaller.

The apparatus for this lab is fussy and I am not thrilled with how it was designed.

• The capacitor plates need to be clean and free of nicks. Otherwise they will start arcing too soon as their separation decreases. When an apparatus starts arcing too soon, I begin by buffing the bottom and top plates with scotchbrite pads, followed by an alcohol wipe. This works maybe 2/3 of the time. The next step is to find a large, wide and fine file to carefully file a suspected problematic plate smooth and flat. The file needs to be long and wide enough that it can be laid flat across the surface of the plate so as not to gouge it or round off the edges. Sometimes I cannot get a pair of plates to work well after these two steps, they I resort to the black magic and voodoo approach and start swapping plates to find a pair that play nice together.
• The figure of merit for how close the plates need to get is a 40g reading on the scale. 50g is better, and 30g will work, but I aim for at least 40g.
• When the plates to arc the HV jumps around and the scale can lock up. I have had to pull the power from the scales to get them to respond again.

• The three micrometer “legs” need to be adjusted carefully and in the same direction of rotation. Otherwise you get a tilt between the plates.
• This is a lab where plotting the data as you take it and evaluating where you stand in the overall process is a critical part of the learning experience.
• TAs need to be clear on which way the data should deviate from the model. Note that the force between plates is attractive, thus if you set the spacing with the voltage off, then turn on the HV, the plate spacing will decrease. Students need to be pushed to figure this out as part of evaluating whether or not they are seeing the “expected” effect.

• The goal of day 2 is to investigate the deflection of the scale plate as a function of force, develop a correction factor to be applied to the data, apply the correction factor and evaluate.
• In principle this part can be done very quickly. But not if TAs push the students to be able to explain clearly the following:
  • How does the deflection of the scale plate affect the day 1 measurements? Up, Down, Left, Right…?
  • They investigate the bias by loading the plate with masses. Tendency is to add increasing mass and measure the deflection vs mass. But the day 1 effect was the opposite, the scale plate is preloaded with the bottom plate, then it relaxes upward as the spacing gets smaller. Could there be hysteresis? Maybe students should start with all the masses on the scale and remove them one at a time to better simulate the effect from day 1?
  • How is the correction data from day 2 applied to day 1 data? This seems trivial until you start thinking carefully about it. The easy, and wrong, thing to do is to apply it different ways and see which one makes the day 1 data agree better with the theory. The correct thing to do is to understand the effects and be able to explain how the correction should be applied and WHY!
• Because students might possibly have a different scale on day 2, retaking the day 1 data would be best. I don't have them do this because it seems repetitive and tedious.
• The above point illustrates that this two day lab would be better done in one day, which was the case historically. I spread it out because I had nothing better to run for the second day. As is both days tend to be a bit short.