Hunter and I watched video of three secondary teachers from this summer beginning to figure out how refrigerators work. Lisa and Matt (with their backs to the camera) are thinking in terms of the local energy transfers from the food to the refrigerant and from the refrigerant to the room; they are puzzling over the temperature problem, which is that in order for energy to flow, they need the refrigerant to be colder than the food, but then hotter than the room. (They aren't thinking about a compressor or an expansion valve yet.) Valentina, facing the camera, has been quiet so far. In the clip below, Lisa invites Valentina's thoughts.
Lisa: Does that - Valentina what are you thinking?
Valentina: I am thinking that I don't have a meaningful answer. So I know the energy flows from the cold to the hot. And this happened just because
Matt: Wait. [looks at Valentina.]
Valentina: Yep!
Matt. No, energy always goes from hot to cold.
Valentina: [frown]
Matt: [giggle]
Valentina: There is - inside of the refrigerator, it's colder than outside.
Matt: Yes.
Valentina: So somehow you spend energy, right, the electrical energy, to make thermal energy to go from the cold to the hot.
Lisa and Matt: [shake heads no in unison]
Valentina: Which apparently doesn't make any sense, but in this, if I will ask my students, this is what they are going to know.
Matt: Yes, I mean I think
Valentina: If it's a really warm outside, then you will hear the refrigerator [fanning herself] just working hard to make the inside much colder than the outside.
Matt: Right.
Lisa: Yeah. But it's not because energy flows from cold to hot. It's because it's hot to cold.
Valentina: Right. [flaps hand downwards] Right. But. [covers mouth]
Look at how they receive each other's statements - or don't receive them, really, because what I see is practically everybody merely reiterating their own position, not recognizing that the other person is right too. Lisa and Matt are right that locally, energy that flows from one object to another by thermal contact always flows from hot to cold. (Which law is that - the zeroth? or is that a definition of thermal energy? or thermal contact? eep.) And Valentina is right that overall, a refrigerator is a device that moves thermal energy from its cold interior to its hot exterior. But what they give each other is negation. Valentina frowns at Matt and Lisa, and Matt and Lisa shake their heads in unison at her. For whatever reason, Matt and Lisa "win" this one: Valentina stops speaking (for about 20 minutes, actually), and Matt and Lisa proceed with their analysis. But I think everyone is missing out.
Part of what bothers me is that everyone seems to be acting as if they understood the other point of view (with "right, right" and "yeah"), while also sending signals that the other point of view is repellent (no's, frowning). I think this shows a lack of congruence.
eep: if i had to choose, it would be the second law, since the entropy gained by the cold one will be greater than the entropy lost by the hot one. the zeroth one is the transitivity (and symmetry?) of the thermal equilibrium relation.
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ReplyDeleteHunter, thanks, I corrected the transcript in the post.
ReplyDeleteAfter watching this episode for the first time I was very interested in why Valentina says,
ReplyDelete'If I ask my students, this is what they are going to know.'
It seems like she is making a bid for sense-making on behalf of her students rather than herself. Why is it not OK or safe to make a bid for sense making on her own behalf?