Lerner Physics

Electric pressure didn't make a lot of sense to most people at the end of last class period. It's not the easiest concept, and it isn't exactly analogous to fluid pressure. We looked at our results from some complicated circuits, and we kinda could explain it using pressure, but we didn't feel comfortable with it.

We went to lab to watch what happens when capacitors are charged and discharged. It was confusing at first, but then we explained it in terms of electric pressure and it started to make sense. We first modeled electric pressure with colors, using the charging and discharging diagrams from the CASTLE curriculum. But colors seemed limiting. We all knew it.

We immediately used a voltmeter to get numeric values for electric pressure. It was my favorite move of the day. We discarded the color model quickly. I like the color model of electric pressure, but I've found that when a class uses it for too long, it gets confusing. Coming up with a better model quickly made so much more sense.

We spent a lot of time today working on our model of circuits, especially around the difference between lightbulbs all in the same loop and lightbulbs each in their own loops. One group talked about how lightbulbs all in the same group are "longer." I like that metaphor, especially when I think now that lightbulbs, each in their own loops, allow "thicker" path for charge to flow. I wish I had taken pictures of the students' thinking.

We then went into lab and looked at how long bulbs and round bulbs act when put in the same circuit. It's weird! We tried to explain it with electric pressure, but we don't feel very comfortable with it. We'll spend a lot of time tomorrow trying to understand this new model of electric pressure. So far, our idea of electric pressure comes from the battery, where the battery makes positive end have more than the usual amount of charge so the negative end therefore has less charge than usual. This difference in charge density can allow a flow to occur if there' s a continuously conducting loop connecting the positive and negative end of a battery. 

Yes, I'm one of those teachers who gives a test the day before Thanksgiving. But, in my defense, the students are the ones who decided on the date.

So, instead of a typical #teach180 post, I'm going to reflect on this year so far.

I've had a great year so far teaching this class because of the students in this class. They are willing to ask questions. Most have no problem whiteboarding a problem they know they don't understand. They're willing to defend their position, even if everyone else in the class disagrees, and they only concede their point when given reasoning from the model to convince them. They work really hard and when they don't understand something, they realize it's just because they don't understand it yet. They've used their mathematical reasoning, scientific reasoning, and writing skills they've learned from other class and in other places and applied it to physics.

I couldn't ask for a better class. Even the worst of them are amazing. Even the ones who roll their eyes at my jokes or physics in general are logical, intelligent, kind human beings. Even the ones who loudly announce forty-five seconds later, as if it were new, something the rest of the class already understood are deep thinkers. Even the ones that feel like they are falling behind are still working and just feel they aren't there yet.

Yet this year, to me, is still shot through with failure on my part. One of the hardest things about teaching is realizing, every day, sometimes with just a few students, sometimes with all of them, you, as the teacher, didn't do what was best for the class.  Sometimes I find myself, most often unintentionally, guiding students in ways that won't help. Sometimes I realize too late that the way I approached the day was all wrong. I set up the experiment incorrectly. I used words that obscured, rather than clarified. I talked too much. I didn't given enough guidance. And yet, through a messy signal, I still have this class—really, all of my classes—filled with eager, thoughtful students. I give thanks for that.

We got through many labs in 70 minutes today. We quickly saw how the flow through circuits could change depending on the number of bulbs in the loops. We looked closely at what happened when we added more bulbs to a loop or adding more loops to our circuit. We also played a little bit with what wires do in a circuit. We're building up, slowly, to what happens if you have a long bulb and a round bulb in the same loop. But that's for after Thanksgiving break.

We really got to play with circuits today. We started by making sure we knew what every component was in our CASTLE bag. We're using the Modeling Instruction modified CASTLE worksheets with the V, which I love. (I don't know who created them, but thank you!) We started with a simple building of a circuit, and then we tested different items around the room. One group tested water to see whether it could complete the circuit. Then we looked at how light bulb sockets and light bulbs work, and I got to do my favorite electricity demonstration with a household light bulb and a lot of D-cell batteries. You can light the lightbulb if you string enough D-cell batteries together! But what if the filament isn't under the glass bulb...

We talked a little bit about how the compass works to show charge flow. I appealed to our understanding of volume flow rate because we have just finished the fluid model. I don't want to lean on the fluid model too much, but it helped make clear why the flow would be the same everywhere in the circuit and why the flow would start all at the same time.

We worked on some more examples of fluid mechanics, but it wasn't that groundbreaking. I let them practice how they needed to practice, which, for most of them, was alone. 

We then moved on to the beginning of electricity. In previous years, I started with electrostatics, but I wanted to go right into electric circuits this year. I feel it's more understandable and there are many more labs for electricity. We started with a great lab based off of The Private Universe video, which was made by my science methods professor from grad school. I ask them two questions: "Why are there seasons?" and "Given a lightbulb, battery, and wire, draw how you'd light a light bulb." Then I have them try it with a lightbulb, a battery, and an unbent paperclip. Many hot paperclips ensue until everyone in the class can find all the ways you can light a lightbulb with just one wire and one battery.

My eleventh graders took the practice ACT today in school, which meant I had one, solitary senior in my class for half of today. The juniors unsurprisingly came to class in a combination of gosh-that-test-was-hard and I-can't-stop-speed-reading modes. Not much was accomplished. We went over a fluid flow problem that was assigned for homework. It took a long time to whiteboard, and I wasn't going to push it today. The problem seemed relatively straightforward until we got to part d. We hadn't done a projectile problem launched at an angle before, but it was great to see the students step up to the challenge. The first board I showed above nailed it, and the second board only had one problem in the quadratic which was fixed. (The acceleration due to gravity had to be negative.) We talked about the strengths and weaknesses of each method.

We added two more assumptions to our ideal fluid model: irrotational and non-viscous This allows to focus on only a few forms of energy with fluids, and these assumptions work well for the simple situations we look at in this class. We ended with three energies we need to track at different points of a fluid: gravitational, kinetic, and interaction energy. (I'm leaving that third type of energy a mystery, but if you think enough about the water that flows slowly under your house and then streams out fast from the tap on the second floor, you can understand that the water at your tap has more gravitational energy and more kinetic energy. Where did that energy come from? What type of energy did the water lose?)

After the test on the ideal gas model, we started talking about the four assumptions of our ideal gas model. We'll assume a steady state; we're not going to talk about pipes as they fill up with water. We'll also assume incompressibility, that we can't fit more fluid into a space by applying more pressure. This means the model, from now on, doesn't include gases, but that's OK. Now we can explain why, when water comes out the tap or oil pours from a bottle, the stream gets thinner as it falls. We came up with the concept of volume flow rate, which we'll practice tomorrow.

No picture today. Today felt luxurious. The first block of 50 minutes was spent whiteboarding four problems, along with a few demonstrations of Pascal's Principle (Pascal's Demonstrator has never worked better; I've learned, through experience now, to bring students close but not too close, and to fill the demonstrator up in a translucent tub full of water) and the pressure globe. 

The second half of class was spent on clicker questions. I haven't done clicker questions in a long time. The clicker presentation was made back in '08, the first year I taught AP Physics B. I remember how difficult the questions were, how the students struggled, and how I felt helpless to help them think like a physicist. This year was so much easier, probably mostly because my students are smarter, more engaged, and more interested in thinking, with a little help from me being able to help spark those qualities and answer their questions better.