## 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.