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Week 4 (Fa14)

September 23, 2014

Each semester when I look back my slides there are little gaps from days when I don’t use slides or fuzzy records about whether some things took one or two days. I’m continuing my weekly record of daily activities.

Astronomy Lecture
Looking at our first homework assignment, students are still struggling to assign map directions to celestial sphere diagrams and have trouble comparing observers at different latitudes. I went through a few sketches on the board showing an observer on Earth, drawing the corresponding horizon, and showing how to determine NSEW on the horizon. I wrote a set of review questions to be answered online which are hopefully helping.

We spent the first half of Tuesday reviewing some Moon phase concepts from last week and checking some of our results from the Cause of Moon Phases tutorial homework. This is the first tutorial in which both students in a hypothetical conversation having mistaken ideas so we talked through what was correct and what was incorrect in each student’s statement. Last 20 minutes was spent with students working on the Predicting Moon Phases tutorial about determining when different phases rise and set. Students struggled with this a lot – they weren’t drawing diagrams and weren’t sure how to determine times.

Thursday, I spent the first half of class going through some tutorial questions as a class drawing diagrams and talking about how to use an observer’s location relative to the Sun to determine approximate time of day for the observer. Students felt much more confident after this discussion and did a pretty good job finishing the tutorial on their own (although I still had to encourage students to draw diagrams for harder questions). I don’t know right now if the class discussion would have been beneficial on Tuesday, or whether starting the tutorial cold and letting students realize they had difficulties primed them to be more engaged during the discussion on Thursday. Preparation for future learning?

Astronomy Lab
This week’s lab covered angular size and parallax. We practiced holding a coin at a position where it was the same angular size as an object in the classroom and set up ratios for the distant object. This worked OK, but showed students had not engaged with the pre-lab as much as necessary. After the practice, we went outside with eclipse shades and did the same thing with a rubber band and the Sun. We also tried carrying cardboard circles to a distance at which they appeared to be the same angular size as the Sun, but those ratios tended to come out with too large of a distance to diameter ratio.

The parallax part was done inside and involved students spreading out in a right triangle and using their hands to estimate angles in the triangle and then compute distances. This was a little confusing for students who was measuring what and how this related to our conceptual introduction of parallax.

Physics 2
Monday and Wednesday we talked about the microscopic meaning of entropy working through a small toy example for which we can explicitly denote microstates. I took a homework from last year and rewrote it to be a take-home exam problem for this year. The problem has students use Excel to calculate microstates for slightly larger systems and see how the most probable state compares to the expected equilibrium condition (better agreement as the system gets larger) and also introduces how to define temperature in terms of the derivative of entropy with respect to internal energy.

We don’t really talk about macroscopic entropy. This is my decision, but I’m not sure how I feel about it. I like the microscopic entropy because I think explaining why equilibrium exists and why heat flows from hot to cold is amazing. However, macroscopic entropy would be more useful for calculations and for connecting to entropy discussions in chemistry.

Friday we talked about heat engines. We don’t calculate that much with heat engines. Mostly we write the first law of thermo for heat engines, define efficiency, and show that if a Carnot engine has constant entropy, then efficiency can be written in terms of reservoir temperatures. RealTime Physics has a nice activity on heat engines that I wanted to do, but I didn’t get organized enough to work this into lab (which I’m not teaching) and I couldn’t fit it into lecture.

Heat engines marks the end of our thermodynamics unit. I feel like this unit is under-motivated. I put a lot of work into connecting ideas and trying to make concepts feel intuitive, but I think I have a lot to do to connect more to applications and engineering aspects.

Physics 3
Monday we introduced sound waves focusing on why sound waves are longitudinal, what causes pressure variations, and why Pressure Variation vs. Position and Avg. Displacement vs. Position graphs are phase shifted from one another. We don’t do much with the speed of sound.

Wednesday we started by trying to use function generators and speakers to set up standing waves in our ear canals. This did not work too well. I didn’t give any instructions other than to slowly increase the frequency from 1 kHz to 10 kHz and see if they notice anything. These are higher frequencies than students are used to and the resonances are narrow so mostly students missed the effect. I think this works better if we talk about what happens and at what approximate frequencies before the experiment.

After the experiment, I talked about how a gas particle has no room to move if it’s next to a closed wall and hence displacement must be a node at a closed wall and built up all other boundary conditions from there. Then I had students sketch a few different standing waves for either pressure variation or displacement to check their understanding. Still some confusion about determining the harmonic number (which I refer to as simply the “first possible”, “second possible”, etc. standing wave).

In lab, we set up standing sound waves in tubes partially filled with water. This works pretty well, except that students mix up what length corresponds to the effective length (with the end effects included) and what length corresponds to the length of air-filled section of the tube.

We also looked at a video of vocal flaps vibrating to see that 1) vocal “chords” aren’t actually chords and 2) the vibration is more messy with various frequencies than you might expect. I used the video to argue that breathing in helium does not change the vibration of the flaps and so we still have unanswered questions regarding the Mythbusters video. After the tube experiment, I try to get students to listen to multiple simultaneous frequencies through a pipe for which one of the frequencies is at resonance. I still haven’t found a great way to do this. I try two speakers driven by two function generators and I try listening to a square or triangle wave through the tube and without the tube. I can tell some differences in pitch but students mostly just notice a change in volume. At the end of lab, I connected the experiment to the Mythbusters video and talked through a full explanation for why helium changes your voice. This discussion was too much of me. I need a better experiment and a unit-long structure for this question that helps students connect all the necessary concepts.


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