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19 March 2014

Mail Time: Do you teach capacitance? (Don't feel bad about leaving it out.)

Wendy Stallings writes in:

Do you avoid capacitance in your general-level class altogether, give it a passing nod, or actually cover it? I read your post from Aug 2010 about circuits, and it sounds like you ignore it altogether, but I wanted to clarify. I’ve wrestled with whether or not to include it for a few years but find the concept hard to demonstrate on a general level.  When I do teach it, I’m never satisfied with the results, but I feel bad about leaving it out.  Thoughts?

Hi, Wendy!  Let me give you a two-part answer.

To address your specific question: In general physics, I ignore capacitance altogether.  Even in the new AP Physics 1 course (which we are teaching as a broader Honors Physics 1), capacitance is not covered.  Capacitance is covered in our second-year courses, both AP Physics 2 and AP Physics C - E&M.

I don't think it's worth making general students worry about capacitance.  They get very confused, even though the concepts seem straightforward to me.  However, second year students, or those who become quite comfortable with resistors, do fine with capacitors.  They'll pick up capacitance just fine in their next physics course, whether that be with you or in college.  

To address the general principle of "feeling bad about leaving it out": My own advice is to never, ever* worry about leaving out a topic in an introductory course.  First-year physics is far more about teaching skills than about teaching content; the "Big Three" skills of quantitative, graphical, and order-of-magnitude reasoning can be taught appropriately with pretty much any combination of physics topics.

* Well, hardly ever... if you're teaching to a standardized exam like the AP or Regents, it's okay to leave out a few but not a lot of the topics on that exam.  

I understand your concern -- you hear in your mind a physics expert saying, "What?  How can you possibly claim to be giving your students a broad introduction to physics if you don't even mention capacitors, which are a fundamental and canonical topic?"  But there's an absolutely silly premise there.  Not everyone agrees what is "fundamental and canonical."  A different "expert" might argue that the lack of coverage of simple machines makes your course worthless.  Another might wail that you didn't touch relativity.  Guh... who cares.

Unless your students can drop their English, history, and foreign language courses to extend your physics class to pretty much all day, there's nothing for it.  You're NOT going to cover everything that someone, somewhere thinks is fundamental and canonical.  So freakin' what.  The only possible approach is to teach the topics that you prefer.*  As long as you're covering some sort of broad spectrum (i.e. NOT just kinematics and Newton's Laws for nine months), and as long as you're giving students plenty of practice and instruction in the Big Three skills, you're doing just fine.  Your students will be well prepared for future physics courses, whatever topics those courses cover, because they have the requisite skills; and, your students will be well educated if they choose never to study physics again.

* Or, perhaps, the topics that you don't prefer but that you have good equipment to cover.  Or the topics that are on a standardized exam.  Or whatever you personally choose to do.

1 comment:

  1. As a professor who teaches applied circuits to bioengineers, I really want students to have seen capacitors and RC time constants. I can live without them understanding magnetic fields and inductors, useful as those are, but capacitance is really fundamental.

    This year, because of scheduling constraints, I had to reduce the prereqs so that they may be taking the E&M course the same quarter as applied circuits, so seeing capacitance in high school (at the very least Q=CV, preferably also C proportional to area and inversely to separation) would be very valuable. I don't need I = C dV/dT, because I rederive that in class, but I don't have enough time to teach Q=CV as well.