I was out of the classroom today while the AP Physics 2 students completed the electrostatics unit exam. I dug through my photos and found one of a whiteboard from a few days ago that I didn’t use in that day’s post. It is an electrostatics conservation of energy problem. The group that prepared this whiteboard included an energy LOL diagram. I’m thrilled how that particular representation helps students organize their knowledge for conservation of energy problems. I can’t believe it took me so long to integrate them into my teaching!
I originally planned a lab where students would explore Millikan’s oil drop experiment by performing video analysis of a recreation of the experiment. We ended up short on time in this unit; so, I abbreviated this lab into a discussion of the historic lab and a focus on the free-body diagrams. It is a great experiment to explore at this point in the class as it reinforces concepts from fluids (density, buoyancy, and Stokes’ Law), highlights electrostatics, and sets the stage for quantum.
Continuing with the theme of having students demonstrate their understanding in multiple representations, today during peer instruction, I had a couple of questions focused on two positive point charges of equal charge, on the horizontal axis, centered on the origin. Students sketched a graph of potential vs. horizontal position and sketched a graph of electric field magnitude vs. horizontal position. Yesterday, when whiteboarding, students really struggled with this particular representation; they did much better today. My favorite feature of InfuseLearning, which we use for peer instruction, is their DrawResponse feature where students can easily sketch a graph or diagram and I can view all of them on one screen. Here is a sample:
##electrostatics ##representations ##peerinstruction ##tech
The frequency generated by the capacitance measuring circuit is between 1 and 10 kHz for the capacitances that we are measuring. At the AP Summer Institute, Patrick Polley suggested driving a speaker with this circuit. I had some time after school yesterday, so I added an NPN transistor to the circuit since the 555 doesn’t provide enough current to drive a speaker. Today at the start of class, I shared the modified circuit with my students. As students pressed on the parallel plate capacitor, the generated tone changed; it became a simple musical instrument!
Today in AP Physics 2, we discussed the results of the capacitor paradigm lab. The graphs clearly showed an inverse relationship between capacitance and distance. The plot of capacitance vs. area was linear, but not as good a fit.
After formally defining capacitance and deriving the various capacitance-related equations, I asked students a series of proportional reasoning questions:
The most thought-provoking follow-up question was after the last slide when I asked if the stored energy in the capacitor doubled, where did the energy come from?
Today, AP Physics 2 students completed the capacitor paradigm lab. The defined the purpose as to graphically and mathematically model the relationship between capacitance, area, and plate separation. They wanted to try different materials between the plates, but I only had acrylic available. I’ll try to get other materials for next year. Students used their best-fit-line equation from yesterday to measure the capacitance of the capacitors that they constructed from copper-clad circuit boards and acrylic.
Today, AP Physics 2 students quickly reconstructed the circuit from yesterday. Since the internal capacitance of this circuit is on the order of the capacitance of the parallel plate capacitors that they will build. Therefore, it is important to characterize this circuit to determine this internal capacitance. Characterizing the measurement apparatus is emerging as a theme in our labs. Students performed a similar characterization in the heat engine lab to determine the volume of the gas. In this case, students plotted capacitance vs. period and examined the intercept to determine the internal capacitance.
##electrostatics ##paradigmlab ##expdesign