YESTERDAY: The intro lesson on electric potential difference was a flop. I had students solve problems about moving particles in constant electric field for varying amounts of charge:
Then I asked them to generalize for any amount of charge q:
and then tried to make the connection to this new quantity called electric potential difference. Big flop. It felt abstract and contrived.
AFTER CLASS: I dug into Knight’s books and Etkina’s book to see how they approached it. Both of them did potential first (not potential difference), and took a fields approach to potential. They drew parallels between the relationship between electric force/electric field and electric potential energy/electric potential. This seemed like the most natural approach for our class, since we’ve been rocking electric fields for weeks and just finished up electric potential energy.
TODAY IN CLASS: We talked about Knight’s conceptualization:
and Etkina’s (she goes a step further and literally calls electric potential the V-field):
And if electric potential can really be treated like a field, we need a way to visualize it. But it’s not a vector, so we can’t visualize it like we do electric fields. I talked about the concept of a scalar field (or “heat map”) and showed this map of air pressure in Europe:
We discussed what the colors meant and what the lines meant. (They all had Earth Science previously, so this wasn’t entirely new to them.) Then we looked at the PhET’s Charges and Fields simulation:
I think visualizing the V-field was really key, rather than taking the “work-based” approach I had done yesterday. The picture at the beginning of this post is a summary sheet of the discussion.
The moon stops moving and begins falling towards Earth. Determine….
- Work done by Earth’s gravity
- Moon’s change in kinetic energy
- Moon’s speed when it hits Earth
After realizing that the force wasn’t constant, we decided we could split the distance between the Earth and Moon into chunks and estimate the work done for each chunk. I lead the class through a quick-and-dirty GlowScript program that could do this for us. I prettied it up to share with you here:
(It’s an interactive Trinket! Change value of N to see how the estimated values for work and impact speed change.)
I need to improve how I incorporate these programming “lead-throughs” in class. I’m thinking maybe a sheet with prompts along the way. They wouldn’t be writing the program from scratch, but the prompts would ask them for key lines of code that I would add to the program I was writing. (I’m always torn between the time needed for them to code on their own vs. the time I can do it more efficiently as a demonstration to illustrate a concept.)
And now thinking about it, it isn’t really the code syntax that’s important, but how to break down the problem into the steps of what we want the code to do for us (split distance into chunks, calculate Fg at a chunk, calculate dW at a chunk, add up the dWs, etc.). So now I think a sheet with question prompts for code is counterproductive. Maybe have them write pseudocode on whiteboards which outline the general process instead?
(Sorry for the stream of conciousness rambling.)
AP Physics C students designed a lab to test if the work done by the rubber band on the cart was equal to the resulting kinetic energy change of the cart. Since the force of the rubber band on the cart isn’t constant, the group in the picture is collecting force vs. displacement data for the rubber band so they can graph it and find the area under the graph (work). The rubber band launched the cart towards a motion detector in order to determine the resulting kinetic energy of the cart.
Since my AP Physics C students had AP Physics 1 last year, I’ve been using TIPERs as a way to reintroduce and review the AP Physics 1 topics we’ll be expanding upon this year.