Like many of us, I use energy bar charts extensively. Though my students don't all understand every detail of them all the time, the charts are the best way I know of to get them to stop plugging numbers into equations and think for a moment about how energy is transferred. Just as "why don't you draw a free body diagram" generally gets students un-stuck on a force problem, "why don't you draw an energy bar chart" usually sets everyone on a path to success when energy is involved.
My personal touch on the energy bar chart is to insist that each bar be annotated. Even just a couple of words help, like "it's moving" under the KE bar, or "at its lowest position" when the PE bar is zero.
Still, I don't have the sense that my class internalizes the deeper meaning of the bar chart: that the total energy on the left side, plus the "work done by external forces" column, must equal the total energy on the right side. They know only because I tell them repeatedly; it seems an afterthought for my students, rather than the entire raison d'être for the chart.
How can I help my students understand intuitively that bars in the chart must be transferred among columns rather than just drawn randomly? How can I help them see that the "work done by external forces" column is the only place where it's okay to add or remove bars?
Kelly O'Shea and Chris Becke -- and I'm sure others, but these are the ones I've seen recently -- have made physical versions of the bar chart. I'll show you both...
|Physical energy bar charts from Kelly O'Shea, |
Kelly tweeted this picture. Her setup seems so simple: just a bunch of wooden blocks, pegs, and post-it labels. Simple, maybe, but elegant. Each setup is just the right size to put on a lab table for each small group to use in their problem solving. I can imagine asking each group to take two pictures of the blocks: one representing the initial energy configuration, and one representing the final energy configuration. Then I could ask something like "why are there two more blocks in the second picture than the first? Where did they come from?" And I'd expect an answer referencing work done by external forces.
|Water-based energy bar charts from Chris Becke, @beckephysics|
Chris uses water in labeled beakers to represent energy. By setting up in the front of the classroom by the sink, he can explicitly show how work done by external forces affects a system's energy status. Look at his labels; the faucet is "+work" and the drain is "–work." My students when they draw bar charts can just magically sketch a few more lines in the "Wext" column to add energy to a system. Kelly's students have to at least physically grab or throw away extra blocks. But Chris's setup requires turning on the faucet, or dumping water down the drain, in order to include work done by external forces. The meaning of "external" just got real.
I don't mean to suggest that either Kelly's or Chris's physical bar chart approach is the superior one. I'd personally use Kelly's blocks in my bog-standard style of class in which students and small groups work on predictions and experiments; I'd use Chris's faucet-based chart on occasions when I want to present demonstrations from the front of the room.
I *do* mean to suggest that if you're going to use energy bar charts as a teaching tool, I think it's worth setting up some form of physical bar chart rather than just drawing on paper. I'm gonna have to try these.