Good classes are exhausting

The classes I teach each have their own personality. It's been true every semester. Some of them are sullen, clearly not wanting to be learning physics and resentful that they are being forced to do so. Some of them are studious, just-the-facts-please-ma'am types. Some of them aren't particularly interested in learning physics, but are very pleasant classes nonetheless; they will engage with me, and ask questions that, though meant jokingly, further the class. Once in a while I'll get a class that both wants to learn and will engage, which is the best kind of class. Usually, my good/bad classes are fairly evenly distributed. A goodish class and a badish class each of the two days I teach.

This semester, I have the interesting experience of having two diametrically opposed class types on the two days I teach, Tuesday and Thursday. My first class of the week is dead silent. They stare at me with dull, blank eyes. Pretty much the entire point of my particular class is to give the students a chance to ask questions and engage with the material in a way that is not possible in the larger lecture sections. I have never had a class that did not warm up after the first couple of weeks. The strange thing is that the afternoon section is almost as bad, though at least they talk enough to ask what is on the board when my handwriting slips up.

My third and fourth classes on a different day are completely different. The students will begin asking questions before class begins. They seek me in my office hours. They ask prying questions and joking questions in class. I occasionally have to ask them to be quiet so I can hear what one of them is saying, but I don't mind, since mostly they seem to be discussing the problem, double checking my math, and the like. They sometimes give me great openings for side problems. For example, this week we are covering capacitors, which is usually a fairly simple and dull topic, since we don't do anything fun with RC or LC circuits until later. Then one of my students asked, "Well, what about a flux capacitor?" After a moment of delay (since an actual flux capacitor is something else entirely), I said sure, lets find out how much capacitance it would need to have. So, starting with the "1.21 gigawatts" I led my students through a process of reasonable estimates that go into a 'back of the envelope' calculation, which is so useful in physics and engineering, and is one of those things that every talks about but you rarely get shown how to do until you are in a lab. We found a rough estimate for how many farads that thing would have to be, and came to the conclusion that it could have fit into the train of the third movie, but not the Delorean.

That's the kind of thing that I love to do. It's kind of a silly example, but that kind of engagement is what makes teaching so rewarding to me.

And apparently, exhausting. I am never tired after my first two classes. Frustrated, annoyed, wishing I could get them to talk, yes. But tired? No. After my third and fourth class though I am exhausted. Completely energized while I'm doing it, but worn out afterwards. Is it because I feel more engaged? Because I actually have to think on my feet instead of just repeating the same thing for the umpteenth time? Or is there just something inherently tiring about satisfaction?