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.
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!
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.
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!
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.
I was at the NSTA National Conference in Chicago today. My students did something different while I was gone. My colleagues and I have been working on a series of measurement and analysis assessments. We plan to administer these to the AP Physics 1/2/C classes and track how students do throughout the year and from year to year. We defined the following standards:
- Retrieve quantitative data from an experimental source and organize it in a data table denoting; units, dependent/independent variable.
- Properly plot data and determine type of fit, fitness of fit, establish mathematical model with proper units.
- Properly analyze and make sense of the slope and intercept of the derived mathematical model.
- Apply the model to a different situation.
For the first assessment we used the Direct Measurement video of the Ping Pong Cannon.
Today was much better than yesterday. For the second year, I used the bridging activities in Randall Knight’s Five Easy Lessons for the photoelectric effect. We started with a simple resistor circuit, moved to a thermal emission diode, and finally introduced the photoelectric effect. After each, students sketched current vs. voltage graphs.
Today was not the best day of AP Physics 2. At first I considered some sort of a paradigm lab involving solar cells, but after trying various equipment and light sources, I couldn’t come up with something effective. If anyone has a good introduction activity for the photoelectric effect, please let me know!
Instead, I ended up with a series of demonstrations that I hoped would provoke good discussions. We observed various amount of current produced by a solar cell based on the intensity of the incident light. We also observed how the solar cell could like a red LED but could not light a blue LED regardless of the intensity. We observed fluorescent chalk under a UV light. We asked a bunch of questions which we will answer over the next several days. I wish I had a zinc plate to attach to an electroscope to demonstrate the photoelectric effect.
Not a great lesson, but a few good questions raised.
Today, AP Physics 2 students completed the optics unit with the geometric optics exam. Based on first impressions, we should have practice ray diagrams more than we did. It’s hard to know with the new course how much to emphasize different areas. We didn’t solve any dual-lens problems at all. Based on my interpretation of the Essential Knowledge and Learning Objective statements, I think this is a reasonable decision.
Today, I tried a new lab practicum based on The Physics Teacher article, “Determining the Thickness and Refractive Index of a Mirror.” Through careful measurements of the reflected and refracted beams and graphical analysis, students determine both the thickness of the mirror and the index of refraction. The most challenging part of the lab was keeping everything aligned and accurately measuring the angle. The use of the level app on a smartphone made measuring the angle much easier. My first class struggled to make accurate measurements as we were trying to keep the laser level and angle the mirror and screen. After watching one group during first hour try various techniques, I encouraged second hour to take a different approach. They changed the angle of the laser and left the mirror flat on the table. This approached resulted in more accurate measurements.