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A pretty decent warm-up: Comparing & Contrasting Blackbody Radiation and Photoelectric Effect

September 19, 2014

Yesterday, we had a break from the pace in Physics II, Instead of introducing new material, we had a day to review what we had learned about blackbody radiation and the photoelectric effect. So far this semester, most have my warm-ups have been forward looking–the aim being to prepare students to maneuver more efficiently and confidently through the rest of the day. But today, the warm-up was backward-looking.

The warm-up consisted of a table I had constructed at the front board. The two columns were, “Blackbody Radiation” and “Photoelectric Effect”. The rows of the table were the following questions

– What is being emitted? Where is it being emitted from?
– What causes this emission to occur?
– What physics quantity (or quantities) characterize how strong the emissions are? (words, symbols, units, any relevant equations)
– What, if anything, does “color” have to do with it? (words, any relevant equations)
– In the situations/problems we’ve discussed, what’s typically happened to the emissions after being emissed?

Students were in groups of three and were asked to discuss each question as a group, and then to write their response on a sticky note. Groups populated the board with their responses, and were asked to look over other groups responses.

Students took to the task pretty well–seriously engaging in the questions and in the efforts to compare and contrast what’s happening in each case. What was nice is the range of “correct” answers. For example, with the photoelectric effect someone might say,”Light is emitted” or “Electromagnetic radiation is emitted” or “Photons are emitted”. For what causes this, someone might write, “Light that is incident on metal,” or “Photons colliding with electrons”. For what happens to blackbody radiation, students might say, “It spreads out and gets less intense”, or “It goes off and heats up a planet =)” While some of what students was more vague or more precise than others, very few things were way off the mark.

After they were done, I mostly just reviewed the things that students had written and helped connect their ideas to the more precise ideas and quantitative relationships they had learned. The one place we needed to talk about the most was the role that “color” played in the photoelectric effect. I asked students to talk about that one in particular back in their small groups. Back in whole class discussion, the main idea that came up was, “Bluer light tends to be more effective at ejecting electrons from the metal”. When I asked how we knew that was true, there were two different ways that students offered for making sense of this. First, was reference to observations with the PhET simulation, that we found that UV light almost always emitted electrons but red light almost never emitted electrons. I added how, when the light got even bluer, the electrons came off faster as well. Another student worked through the reasoning that bluer meant lower wavelength, and that lower wavelength meant higher frequency, and that higher frequency meant more photon energy, which meant larger lumps of energy to give to electrons” That chain of reasoning, of course, involves drawing on several different relationships and chaining them together, so I did some work to clarify that. Looking back, I could have slowed down even more on that reasoning, asking students to go through that reasoning careful amongst themselves in groups.

In general, the warm-up went well, with student engagement being high; contact with important disciplinary ideas being high; and students expressing that they felt like it was a valuable activity from which they learned a lot (and being able to articulate what they had learned). It’s a good day when you get all three of those things. I think it’s fairly easy to get either of the first two–engaged students but not with important disciplinary ideas or a lesson that should put students in contact with disciplinary ideas but they aren’t engaged. And then, even when you get both of those things occurring, it’s not necessarily true that students leave class feeling like they have learned a lot and can express what learning took place.

Two other things on my mind are these:

– The activity went well, in large part, because of the students. I more and more see how–despite the fact that I do have a strong influence on student engagement–a good classroom is really a mutual activity in which the teacher and students coordinate their activity to achieve engagement and learning (or don’t). In particular, this activity of writing on sticky-notes and putting them on the board is not something we typically do in this class, and it might have been easy for students to think it was juvenile or just weird, and disengage as a kind of passive resistance. For whatever reason, that didn’t happen. It makes me think about Dewey’s notion of submitting to an experience–letting the experience happen to you.

– Compare and contrast is not a strong part of our curriculum. Students are rarely asked to think about how ideas or phenomena are related. We just march through… one thing to the next. I’m hoping that by asking students to do more compare and contrast that students can, to a limited extent, experience a more coherent curriculum. Not sure, exactly how it will unfold, but returning the comparison contrast makes sense as next week we encounter atomic spectra and nuclear radiation. Many of the same questions above are relevant…

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