Lerner Physics

"This looks like a biology worksheet."

"This looks like US History homework."

Yes, for the first time this year, I had my students do matching. Matching.

It was after we did a quick lab trying to figure out whether period or amplitude affects the speed of a wave. It wasn't a great lab. I've never gotten great results from the long springs with just pulses. But, still, it's good to play around with the wave, to see the pulse in real life, and to try to calculate its speed. 

Then I had many vocabulary words to introduce, like longitudinal wave, direction of disturbance, and trough. I wish I could get around this, but the unit of waves doesn't make coherent sense to me yet like the rest of the AP curriculum. I'm not sure what the overarching theme is. So getting through the vocabulary that isn't really that important, and doing so quickly, seemed like an OK idea.

Today was the taco party. Four years ago, one of my students left a bottle of taco sauce in my room. The bottle was brought for a fiesta in Spanish class but was never opened. The students had opened up a jar of salsa instead. So the bottle sat in my room for a while. Then I started having students turn in their labs right next to the taco sauce. Later, the taco sauce ended up on a top shelf. This year, we realized it expired in February of this year, and we planned for a taco party.

Today was the day. We had lots of sugary treats, some nacho chips and taco meat, and no plates. We managed. 

We also took a test, whiteboarded the last, relatively straightforward problems about pendulums, and played a little bit with the long slinky. We're moving on to mechanical waves tomorrow.

We worked on our graphing skills for the spring-mass system. Graphing the kinetic energy and the spring potential energy was a bit difficult. Also, finding the amplitude of the velocity-time and acceleration-time graphs requires us to go back to all our old models. 

We then talked about what restoring force means. For a restoring force situation can happen, the object must be in stable equilibrium. The time the object takes to return to equilibrium depends both on the inertia of the system and the strength of the restoring force.

Oh, we also planned for a taco party tomorrow. The taco sauce that's been in my room for three years expires tomorrow.

We started with the equation for the sinusoidal functions. We know the position-time function for the mass-spring system looks like a sine curve, and we know, from pre-cal class, that we can write a sinusoid as y = A sin (Bx + C) + D. We quickly listed what each of these four variables would do to the graph. It looked like the only one we didn't understand was B. What would affect the period of the sinusoid? We brainstormed some ideas (mass, spring constant, friction in the system, amplitude) and went into lab. 

The final model was surprising; how could it be so complicated and yet so simple? So we talked a little about and did some practice. We should be done with this unit quickly, probably by the end of tomorrow. The restoring force particle model seems so easy now that we have so many other models under our belt.

We finished magnetism today. The change in the magnetic flux can be hard to see, so I made skid some flash cards after school today. We had pictures of various situations on one set and descriptions of possible changes to the flux on the other set. One from Set A, one from Set B, then go! What is the induced current?

We then moved to a mass on a spring. The graphs were pretty, and we could explain them well.

Today was a hodgepodge of things. We started by whiteboarding what was mostly a review of right-hand rule problems, and we got to clarify what was going on. Then we did the classic "why is the object going so slowly down the pipe" demonstration. 

Then it was on to the three types of materials: ferromagnetic, paramagnetic, and diamagnetic. We, of course, had to see the levitating frog video. Then we practiced some last Faraday's law questions. 

We went slow and steady today. Magnetic flux is not an easy concept, and we want to make sure we understand it and the long, causal reasoning that goes into electromagnetic induction.

We started by debating a lot about how the flux changes. One student in particular mentioned an interesting point about what happens when you push the magnet in through the loop. Doesn't the magnetic field change direction? It took a while to clarify the question, and in that clarification, we got to talk about how the magnetic flux doesn't really have a direction. It's just positive or negative. 

We spent a lot of time talking about the reasoning we'd use to figure out the direction. We got to hear many ways to explain our thinking. At the end of class, we even got to see magnetic induction using the function generator, and the current in the second loop only happened when the current in the first loop changed. We can explain transformers and wireless charging now.

I got confused today in class.

Class started great. We discussed that the magnetic field seems to created when the electric field changes. So is an electric field created when the magnetic field changes? We had lots of loops of wires connected to sensitive voltmeter, ammeters, and galvanometers, so we could test it. We saw that the changing magnetic field does make an electric field, and we figured out it doesn't matter if the wire moves or the magnet moves.

Then, we had to figure out if we could predict the direction of the current that was induced. We tried to explain it, and I kept getting confused. It didn't seem to jibe with Lenz's Law. I couldn't figure it out. After a bit of stress sweating, I just had to let it be and teach Lenz's Law.

At the end of the day, I tried it again. What was I doing wrong? It worked perfectly. I could explain it easily. That kind of momentary brain lapse happens.

Magnetism is hard. I'm going slow this year. I want to spend time thinking about the direction of things. Three-dimensional thinking is hard, and it's a good skill to have. It's a predictor of success in engineering, so I'm willing to spend the time on it. We talked about how a mass spectrometer works. Crossed magnetic and electric fields are hard to visualize. 

We came up with various quantities that could change the magnitude of the magnetic force on a wire. We tested the number of magnets (as a proxy for the strength of the magnetic field), the length of wire, the current through the wire, and the thickness of the wire. (I liked the thickness of the wire test, but we didn't have a good way of measuring the thickness of the wire; I'll see if I can fix that for next year.) We wanted to check temperature, but I couldn't get the equipment together in such short notice. I like the fact we can test lots of different possible independent variables so that when we have our final model, we feel we've investigated everything we could.

We also spent a lot of time talking about the assessment on the electric field model. It was a tough test on a tough unit, and I have to think that some of it was my fault. How can I make the connections between field and potential really strong as well as the connections between potential and energy?