139 This was Tuesday, 4/16. The AP class was determining the relationship between the magnetic field strength and distance from a long current-carrying wire. Almost all of my photos turned out blurry. I am not sure what I had messed up in my phone’s camera settings.



131 It’s RC circuits and capacitor discharge time in AP Physics! (Post originally meant for April 4th)

Circuit Fun

128 I had promised the AP class “Circuit Fun” on the last day before spring break. To me, this means making different circuits to light up light bulbs in various configurations!

Loop Rule

127 (Tuesday, March 26th) I love this question in Chapter 27 of Halliday, Resnick, and Walker. We spent a while in class talking about which conservation of energy loop or loops you should choose to write loop equations for in order to determine the current through resistor R. There is one loop where you can solve for the unknown with one equation. If you don’t choose that loop, you will have multiple unknowns and will need multiple loops and multiple equations to answer the question. (All batteries are ideal and have emf = 4 V and all resistors are identical and have resistance = 4 Ω.)


126 (Monday, March 25th) This video demonstrates the difficulty some groups had in collecting data from the squishy circuits lab. The conductive dough they made never gave consistent readings. When we got frustrated with trying to measure ΔV and I for their dough, we tried measuring R directly using the digital multimeters. The video shows the readings for a given chunk of dough measured in MΩ, fluctuating between 1.02 MΩ in the first frame and 0.55 MΩ in the last frame. There will be a lot of experimental uncertainty to write about in the lab report!


125 The AP class is investigating the resistivity of conductive dough. I was going to make it myself, but then I had jury duty and I had to ask the kids to make it and bring it in. Which they did! It seemed to work pretty well for some groups, but not for others. I will be experimenting on my own at home to troubleshoot some of the issues. Mostly, it had nothing to do with kids using the digital multimeters incorrectly to measure current, though if they did things wrong yesterday they may have blown a fuse and I could not tell. So that is another thing I need to check.

Pi Day!

pie120 The AP class had a test today, on potential and capacitance. Then we ate pie! The choices were: pumpkin, apple, blueberry, blackberry, chocolate, chocolate pudding, chocolate cream, and oreo/peanut butter ice cream pie!


fusion119 I have been working on solving old US Physics Team Semifinal exams with my semifinalist student. This is a scratch paper in which I was trying to figure out a solar fusion problem. Something I didn’t remember from my undergraduate education is that in addition to energy conservation, charge conservation, and momentum conservation, nuclear processes include baryon number conservation and lepton number conservation. So solving this type of problem requires remembering which particles are of which type, as well as knowing what the types of particle are. I find it a little crazy that this test for high school students involves this kind of physics, but the test is supposed to select students to compete against other students who have been picked out of their countries as the best physics students, so it has to be hard enough to winnow out the top 20 high school physicists in the country!


IMG_20130307_102401114 The AP class is making capacitors out of aluminum foil placed between the pages of books. We are hoping that the capacitance is proportional to area, inversely proportional to the distance between the plates, and we also hope to determine the dielectric constant of textbook paper.


IMG_20130228_095447110 We experimentally determined equipotential lines in AP physics today, using digital multimeters and carbon-infused conductive paper. It was a small class due to DECA, the musical, and a swim meet, so a bunch of kids will have to make it up, but that should not be too bad. I’ll be putting these pretty drawings on the bulletin board.