Day 97: Finishing the Magnetic Force on Moving Charges

Today was mostly a long test. It happens. The unit on the electric field is the longest unit I teach, and I'm not sure how to split it up. It would make sense to start it at the beginning of a semester, but it just didn't work out that way this year. The electric field unit is definitely one I have to think more about.

We whiteboarded a few problems figuring out the direction of the magnetic force. So many questions. Some were just trying visualizing the right-hand rule, which is very difficult sometimes. The others were deep questions about the consequences of the magnetic field. Students were starting to ask questions that would lead to the Hall effect or magnetic induction, and I just had to evade the questions.

Day 96: The Magnetic Force on a Wire

I read Arons today before class and it helped. This is a lesson I need to relearn every few months.

By Arons, I mean Teaching Introductory Physics. It's so good, and so massive, that every time I dive in, I learn something new. This time, it's to think about the Newton's Third Law with magnetism. If the current pushes on the magnet, then the magnet must push on the wire. Clever! So we learned the right-hand rule for forces on wires.

But we also diverged into special relativity when a student asked about magnetic potential energy. After reasoning that the magnetic field could do no work, I said special relativity was needed to explain why it looked like the magnetic field caused the magnet to move. This video by Veritasium explains more.

Day 95: The Magnetic Field around a Wire

Yes! We're starting magnetism!

We started with a big charged plastic corrugated sheet that charges up nicely when we rub it against a certain student's curly hair. We put it next to a very strong magnet. What happened? How does that help us differentiate between magnetic interactions and electrostatic interactions?

Then, we got out the iron filings and sketched what they did near the magnets. They seemed to align to patterns we had seen before with electrostatics. There seems to be an analogy between magnetic fields and electric fields. We know, though, that any analogy isn't perfect, but it seems in our north-o-centric, up-o-centric, positive-o-centric world, we'd define the magnetic field in the direction a north pole would feel a force. (Question I didn't ask in class, but should've: where is the north pole that makes the Earth's magnetic field?)

After we sketched out some magnetic fields around magnets, we started looking at the magnetic fields around a wire. We knew there had to be a magnetic field since we saw the compass move when placed near a wire carrying a current. So we investigated it, and, in doing so, defined the right hand rule.

Day 94: Electrostatics and Conductors

Finding electric potential seems easy. Once we understand that we can add the contributions to the change in electric potential energy for our test charge like numbers since voltage is a scalar, it became easier to see. But electric field is much more difficult. How do we add all the contributions to the electric field. We had to spend some time with that one.

We also got into a long discussion about how the electric field from a long plate of charge could end up being constant, no matter how far away you were from the plate. That was a difficult one, and I don't feel my answer was the best. I have a great answer, but it uses calculus. In fact, I have more than one good answer that uses calculus. 

We finished the day talking about electrostatics and conductors, which sounds boring until you google things like Faraday cage video or lightning scars on body.

Day 93: Finishing Electric Potential

We went over our results from lab. Everyone seemed to notice that the equipotential lines between parallel plates were parallel and straight in the middle but curved towards the ends. Fringing was easy to teach because of that. We then talked about the power of voltage. Energy is easier than forces, and one student, who used forces and acceleration to solve a problem when energy made it so easy, made the point clear.

Day 92: Almost Nothing

We finished our equipotential line drawings. We also talked about how to calculate electric potential energy and electric potential.

Most of the day, though, was spent with various teachers talking to us about the science classes we could take next year.

Day 91: Visualizing Equipotential Lines

We should have done this lab yesterday. Today started with a struggle to understand how to draw equipotential lines and how equipotential lines and electric field lines interrelate. (We quickly understood that the unit for electric field strength, newtons per coulomb, were equivalent to volts per meter, but we had no idea why that was important.) It was tough. But, once we got to lab, the equipotential lines made a lot more sense. 

Day 90: Back to Potential After a Snow Day

We have a good idea of what electric field is. We've drawn a lot of pictures; we've made a mathematical model and applied it to some situations. But we don't have a good picture of electric potential energy. So we started by drawing an analogy to gravitational potential energy. We defined gravitational potential as the amount of gravitational potential energy per kilogram. We got out the contour maps and saw how these contour lines are isolines—lines of equal gravitational potential. We talked about it as a class and then in groups. We're starting to see a connection between these equipotential lines and electric field lines.

Day 89: What I Learned from Grading Exams This Year

Here's what I learned from grading exams this year:

  • When asking students to design an experiment, someone in every class will try to use every piece of equipment you offer to them to use.
  • The better the student is at mathematics, the more likely the student will overcomplicate an experiment.
  • Nobody seems to know what a motion detector is.
  • We used motion detectors on at least eight labs. I called them motion detectors every time.
  • I told my students they didn't HAVE TO simplify their answers. So they didn't. And that meant future questions got harder to do. Also, it meant that they couldn't tell how the answer to part (b) and to part (c) looked oddly the same...
  • My students really understand velocity-time graphs. I feel like they must have learned that in math class or something.
  • My students really don't like solving problems with variables. I feel like this is 100% my fault.
  • Simply restating what is in the problem is not justification.
  • In the last ten minutes of class, when students are hoping for inspiration to strike, it's fun to watch what they're doing on their papers. I saw at least two students writing and rewriting the µ's on their papers so that the descending bit was just so.
  • As a whole, my students are learning more than my students did last year, but they're not where I want them yet. I can't ask for anything more as a teacher.

Day 88: Exam Day

My AP Physics 1 and 2 students are taking their exam! Since I have them for two periods a day every day, there are two times on the exam schedule when I can give them an exam. Both times are today. I'm giving them two exams.

The first exam is the same exam I'm giving my AP Physics 1 students. My friend Linda Bugosh, who also teaches AP Physics 1, and I got together and wrote a five-question, 90-minute exam that covers what we did first semester in class: forces, energy, work, and momentum. I'm impressed at how it came out; it really gets to some of the themes of AP Physics 1 that I'm only understanding deeply as I teach the class. 

The second exam, well, I'm not as happy with it. It's 15 multiple choice and 3 free response on all the topics I did this semester with AP Physics 1 and 2 that I didn't get to in AP Physics 1. Since I didn't have anyone to collaborate with to write this exam, I don't like it as much because I haven't gone over it as closely as I have with the AP Physics 1 exam. It seemed to take the whole period, and all the students wanted to talk about it after the exam. I just wanted to use the bathroom.