Matter & Interactions II, Week 3

This was the first full week of classes and I feel as though we’re dreadfully behind where we need to be.

This week was about chapter 14, electric fields and matter. I love this chapter becuase it’s mostly experimental. The experiments use plastic tape (aka sticky tape or Scotch tape). I try to let students devote as much class time as possible to these experiments becuase they can be quite engaging and lead to some excellent physics understanding.

This chapter also addresses polarization in conductors and insulators and I think this is some of the most interesting elementary physics in the course. In conductors polarization is all about mobile electrons, while in insulators it’s all about molecules. Charge distribution matters and representing these distributions with diagrams seems rather difficult for most students and I’m not sure why.

My favorite part of the chapter is the derivation of an expression for the interaction (force) experienced by a neutral atom and a charged particle. This interaction is inversely proportional to separation to the fifth power, something no one could expect (at least I couldn’t have expected it). It’s interesting to start making a catalog of separation dependencies for various electric interactions. A particle-particle (both charged) interaction depends on separation squared. A (fixed) dipole-particle (charged) interaction depends on separation cubed. Now we see a (neutral) atom-particle (charged) interaction varies inversely as separation to the fifth power. Something’s missing! Where’s the dependence upon fourth power? Is there even such an interaction? Indeed there is, and it’s quietly tucked away in problem P63 at the end of chapter 13! The interaction experienced by two fixed dipoles varies inversely with separation to the fourth power. This is a nice little progression of dependencies to be aware of and to remember. It’s a good foundation for things like the Lennard-Jones interaction, where dependencies are to the seventh and thirteenth power (that’s for the force, not the potential). In the next chapter, students will see some charge distributions that don’t vary with separation at all! How can that be? Well, one way to frame it is with symmetry. I’ll say more about that in a future post.

Oh, there’s one more interesting connection students can make in this chapter, and that in their chemistry courses they learned about various subatomic forces. Well, these forces can be traced to interactions involving dipoles and have different names depending upon the discoverer or other associated person. I think it’s important to point these connections out.

I still struggle with (lack of) student motivation, and I still feel rather stymied by it. I wish there were a way to physically force students to engage outside of the classroom but no such thing exists. Ah well…

Feedback is welcomed as always.

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