Day 85: Electric Field Problems and Reviewing For the Exam

In groups, we derived the equation for the electric field around a point charge. It wasn't that bad, and it let us have some really great discussion about what the variable q means in different situations. What makes q₁ different that q₂, q, and Q? It was a good discussion about what variables mean.

We also got really good at drawing the direction of electric field lines. I couldn't even think of any good questions to ask them. 

When we were reviewing for the exam, I spent a lot of time talking about conscious competence. We will go through the steps, we'll know the steps are right, and yet, still, at the end, we're sure we made a mistake. It's not uncommon. We all have our physical intuition be at odds with the models of physics all the time. But it's a new feeling for first-year physics students, so I spent some time explaining how it's normal. It's worth doing what you know is right, even if it isn't exactly what the question asks, than sit there and do nothing. Get involved with the problem. Figure out the situation really is. The more you think about a situation, the easier any question about that situation is.

I also got a very clear reminder that students don't learn at the same pace when a student asked me how to read numbers aloud that are bigger than nine thousand nine-hundred and ninety-nine. I drew this picture, and he got it almost instantly:

Day 82: The Electric Field

So, we had our electrostatic compasses, and we set them near charged objects. We saw some patterns. We quickly agreed that any positive charge would feel a force in the direction the electrostatic compass points. We could see that the direction of the force could be determined anywhere in three-dimensional space. We also could see the force would be weaker in some places and stronger in others. We then could draw these forces for some simple configurations of charges; we called these pictures of many vectors of the direction of the force the electric field. We'll refine this definition as we go on.

Day 80: Electrostatics Fun Right Before Vacation

We went over charging by friction, conduction, and induction today, and then it was time for the fun.

First, we did Fresca soda can races. Using a charge object, get a Fresca can from the start line to the finish line quickest without ever touching the cans. (Fresca cans seem to show charge separation better than all the other soda cans, for some reason. When I used to use lots of different types of cans, the team with the Fresca can always one.) It used to be that straws would work, but this year, rulers, rabbit fur, and styrofoam plates were used to accelerate the Fresca cans.

Then, we played with the van de Graaf machines. They weren't as much fun as usual; it was too humid in December in Ohio.

Day 79: Different Ways of Charging Objects

Today we learned about the various ways an object can become positively or negatively charged. We knew about friction from yesterday, but what about touching? And what about the weird way an electrophorus becomes charged? We experimented, and we used neon bulbs to be able to show which way the charge was flowing. 

Day 78: The Electric Force

How do the top tape and the bottom tape go from uncharged (or from balanced charges) to a positively-charged top tape and a negatively-charged bottom tape? We visualized that process today. We also wrapped up the last question on capacitors today, which also helped us review Kirchhoff's two laws.

Then, it was on to the electric force. I've had students do curve fits for this force before, but I didn't this year. I decided I wanted to introduce this equation quickly today, and let students tell me the similarities they saw. Boy, did they see a lot. We talked about how the force looked like the universal force of gravity. We wondered how Newton's Third Law worked with this new force. We asked about how we know there's only certain values of charge found in the universe, and doesn't that contradict what we learned about the internal structure of the proton? I had to explain that, in a proton, if you try to pull those parts apart, the force gets bigger and bigger, and it takes so much energy that it just creates a new particle. How does it create a new particle, they ask? I heard some murmurs, so when I put up the equation E = mc², I was expecting some gasps. Instead, class turned into the Jerry Springer Show for a minute. That's a great, nerdy feeling.

It's a bit of jumping around, and next year, I'll try to make it smoother, but something made me want to try teaching all the AP Physics 1 electrostatics stuff first and then delve into AP Physics 2 material. Tomorrow, we'll talk about the ways we can charge things without using friction.

Day 77: Capacitors & Sticky Tape

We were unsatisfied with our capacitor model. What makes the charges move? Why does this happen? So we made a sharp left turn into sticky tape and static electricity. We came up with experimental verification of why there are only two charges, and we even saw the electric force change based on distance. We'll get more mathematical tomorrow.

Day 75: Capacitors and Capacitance

We used the PHeT simulation to look at how capacitors act in circuits. We quickly saw relationships between charge and voltage for a given capacitor and between the physical qualities of the capacitor and its capacitance. 

This is new time in the year to talk about capacitors for me. Usually I don't introduce capacitance until we already have a detailed model for the electric force. It seems to work well, but we'll have to loop back later to talk about dielectrics.

Day 74: Capacitors in Circuits

Today was mostly spent practicing our model of internal resistance and making our models for the ammeter and voltmeter clear. But we did spend a little time figuring out how capacitors act when completely uncharged and when they've been in the circuit for a long time. We also looked at what happened if we charged a capacitor with 3 D-cells versus 6 D-cells. It seems like a lot more energy is stored when a larger potential difference is across the two terminals.