Day 131: Loose Ends and a New Perspective on Electric Motors

Today was the day before spring break. I had originally planned for the AP Physics 2 students to complete the quantitative photoelectric effect lab. However, I realized that a bunch of loose ends had piled up and it would probably take the entire class period to work through them. So, the lab was postponed.

After gathering feedback on yesterday’s field trip, providing feedback on recent exams, and scoring a quiz, I wanted to revisit the electric motors that we built. Students wrote paragraph-length responses explaining how their electric motor worked. I had a “bonus” prompt that asked why the battery connected to the motor would die much faster if the motor was unable to rotate than if it was rotating. Very few students could explain this phenomenon.

Today, I approached the phenomenon from a different perspective. I asked what torques are applied to the armature. Students readily identified the torques to the magnetic force and friction. I then asked if there was an unbalanced torque. Students readily answered that there was. I then ask why their motor didn’t continue to accelerate but, instead, reached a constant rotational velocity. Students didn’t know. At this point, I drew a diagram, reviewed Faraday’s Law of Induction and Lenz’ Law, and demonstrated how the electric motor reacher rotational equilibrium due to the reduced current from the back emf. Next year, I think I’ll try this approach first instead of why the battery dies so fast with a stalled motor.


Day 130: Particle Physics Masterclass at Fermilab!

Today, AP Physics 2 students ventured to Fermilab for the Particle Physics Masterclass! We decided to do the CMS J/Ψ particle Masterclass because event analysis is a good review of charged particles in magnetic fields and the resulting mass plot turns out really well.

The day started with an introduction from two Fermilab physicists and then students broken into pairs to analyze events from CMS. We had some wifi issues; so, we took over the cafeteria where there was better cell reception to create individual hotspots.


Just before lunch, we aggregated our data and produced the mass plot.

J/Psi Mass Plot from 2015 Fermilab Masterclass

Woohoo! We found the J/Ψ!

After lunch, we broken into smaller groups for tours of various parts of Fermilab. Next year, we’ll shorten lunch so we have more time for the tour.


Day 129: Mass of the Top Quark Particle Physics Preparation

Today, AP Physics 2 students complete the second activity in preparation of our visit to Fermilab: calculating the Mass of the Top Quark. I do this activity every year regardless of whether we are doing the Particle Physics Masterclass. I think it is a refreshing context in which to practice the application of the conservation of momentum in 2D. In addition, students find it fascinating how by setting c to 1, mass, energy, and momentum are numerically equivalent. Students also confront the uncertainty of how determine the angle of the momentum vector for an entire jet of particles. Between the two classes, the average mass of the top quark was calculated to be 181 GeV; a bit high.



Day 128: Rolling with Rutherford Particle Physics Preparation

This week the AP Physics 2 class is taking a break from our modern physics unit to prepare for our field trip to Fermilab to participate in a Particle Physics Masterclass. Today’s activity, Rolling with Rutherford, focused on the important role of indirect measurements in particle physics. Students roll a sphere at a line of other spheres that are obscured. Based on the ratio of collisions to rolls, students can calculate the radius of the target sphere! In previous years, we derived the necessary equation together. This year, I left it to each group. Most groups needed me to ask what effect, if any, the width of the incident sphere has on the calculation, but with that guiding question, they were able to derive the necessary equation.



The results were excellent: 1.3 cm calculated vs. 1.27 cm actual!


Day 127: Quantum Effects Whiteboarding

I really like some of the problems in Knight’s College Physics. AP Physics 2 students white boarded problem 61 in chapter 28 today. This question aligns well with the College Board’s Science Practice 7: “The student is able to connect and relate knowledge across various scales, concepts, and representations in and across domains.” This problem starts with the wavelength of a single photon emitted by an LED and asks what current is necessary to produce emitted light of the specified power.

The following whiteboard again illustrated that students haven’t yet had the opportunity to assign meaning to the slope of a stopping potential versus frequency graph for the photoelectric effect. They solved simultaneous equations instead of using the value of the slope. This year, I’m saving the quantitative photoelectric effect lab for a lab practicum. Next year, I’m considering doing the lab earlier so students have a deeper understanding of these graphs throughout the unit.


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Day 126: Quantum Effects Peer Instruction

AP Physics 2 students spent the entire class period engaged in peer instruction. I had a series of conceptual questions that covered everything from Compton Scattering to the photoelectric effect to atomic energy level diagrams to de Broglie wavelength. I didn’t realize until we were discussing the following question that, since we haven’t done the quantitative photoelectric effect lab, students don’t appreciate the significance of the slope of a stopping potential versus frequency graph. I’m looking forward to the “ah-ha” moment when they have that realization!

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Day 125: Matter Waves

Today we explored some of the more bizarre aspects of quantum physics. We revisited the double slit experiment but with electrons instead of light waves. I shared Feynman’s “Sum over Histories” model which helps that result make more sense. I referenced The Grand Design by Hawking and Mlodinow which has good explanations of both Feynman’s “Sum over Histories” model and probability in quantum theories. I believe the following graphic is from this book:


We then discussed the Heisenberg Uncertainty Principle. To provide some context to these abstract concepts, we discussed how quantum mechanical tunneling enables alpha decay and is the phenomenon that makes the scanning tunneling microscope possible. Sharing the scanning tunneling microscope was new this year and was an important authentic example for students to appreciate.