Energy in Electric Circuits

On Monday afternoon, Energy 2 returned to three scenarios that they had been talking about in last year's Energy 1 course: An inflated ball under water, the Gaussian Gun, and an ice pack. My group (table 3, Jim, Tim, Don) had spent less than half an hour on the scenario and when Leslie checked in with them, they had claimed that they had "figured it out." Leslie had left, and Linda Anderson came in and sat down at the table, and all of a sudden, the conversation shifted towards other issues with energy. Linda brought up the issue of energy in electric circuits. Since this is an issue that I have extensively discussed with Brian Frank in the past (an we still haven't resolved it), I was curious about what they had to say about it. Here's the little clip.




Transcript:

Linda: Electric circuits is one that I haven't quite figured out how to think about that, because there's matter and energy, it's like, you'd almost like have to have a matter and energy theater to try to separate out the idea of current and electrical energy...
Tim: [Yeah, I don't... I don't have a...
Linda: [It's a tough one, it's a tough idea.
Tim: We talk about electron migration and, you know, take it from electromagnetics [inaudible]
Linda: Mhm.
Tim: Do you still use a lot of [inaudible] from inquiry stuff [inaudible]
Don: What I do is, I, I'll...
Tim: [inaudible]
Don: Do you have a hand-crank-generator?
Tim: Not any more!
Don: I'd get one. They're like 20 bucks.
Tim: I had [inaudible]
Linda: Uh-huh.
Tim: Oh yeah! I think I do have one of those.
Don: And then, run it through a small bulb, (Linda: Uh-huh) have them crank it, they'll see the light, as they're cranking, pull the light bulb out- the resistance in the crank goes down by about 30 to 40 percent! And as you plug the light bulb back in, the resistance goes up again.
Linda: Oh that's cool!
Tim: Now stick your tongue in there. [laughs]
Linda: [laughs]
Don: It, it, it, to m... even... It am, it amazes [me! It amazes me!
Linda: [Really, that's cool, how much of a difference that is!
Don: Well, and then you start talking about work. And then you start linking the fact that power... they understand electric power
Tim: That is amazing [...]
Don: Cuz you know, they understand the power is just work over time, well, same thing with electricity! It's the same exact thing. Uhm, and, that just, um, so if you wanna... That is the most fundamental demonstration I can show someone, just to let them see that this energy transformation... is...
Tim: Cuz, [inaudible] this little plastic hand crank demo thing
Don: That's it, that's all you need. And they can feel the resistance. And then you mess with them and you, and you, and then to prove your point, you put in a broken bulb. That's the next thing I do, put a broken bulb in. And it doesn't change. And they're like, OK. Now somethin's up.
Jim: [inaudible] physics teacher [inaudible] He had a bicycle, a stationary bike with a generator, and across the handle bars, a series of light bulbs. And he showed the kids when they got on and start pedaling, they had to go faster to light up the subsequent light bulbs.
Linda: We had something like that [inaudible]
Jim: And then you had [inaudible]

The demo Don is talking about is very familiar to me, it is really an awesome way to experience electrical resistance and work with your body. Talking about embodied cognition! (Michael's AAPT talk about experiencing the tension in a taught spring as tension in your arm comes to mind)

Anyway, I'm mostly just posting this to flag the episode for further thinking in the future. I would like to think more about the connections between the mechanical resistance in the crank with the electrical resistance in the wire (light bulb), the work that you have to do while turning the crank and the energy in the circuit, and so on.

Also, at the end of the episode there seems to be an awesome shift in the participation framework. Jim is talking about the experiment with the stationary bike that is hooked up to a generator and a bunch of light bulbs. As soon as he is finished, Don turns his gaze away and toward his computer, Tim is turning his head, and Linda is also looking away. I haven't looked closely at this, but it seems like they're done talking. For real.