Lerner Physics

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?

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.

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.

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.

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.

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.

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.

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.