# Gas station without pumps

## 2014 April 29

### Inductors and loudspeakers

Filed under: Circuits course — gasstationwithoutpumps @ 19:22
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On Monday I gave a little pep talk about the quiz before returning them.  I also assigned the students to redo the entire quiz  as homework due Friday, saying that we’d already gone over most of the material in class, so there wasn’t much point in my showing them again—they needed to do it themselves.  (Engineering is not a spectator sport!)

After returning the stuff I’d graded over the weekend, I talked to them about inductors doing a rather hand-wavy derivation of $V = L \frac{dI}{dt}$.  There is much more detail in this week’s lab handout, but I’ve found that students in the circuits class do not seem to be able to absorb much information in written form—a shame really, since that is how most of their future learning is going to have to happen.  I fear that most of them are going stop learning the moment they leave college, and then they’ll be stuck with obsolete knowledge and no way to remedy the problem within five years.

I also talked about loudspeakers: how they work and what the impedance vs. frequency curves look like.  They’ll be gathering data for their own loudspeakers today, so I wanted them to be aware of the existence of the resonance peak and the need to gather a lot of data around the peak in order to model it.

I did talk to them about the basic $R+ j\omega L$ curve for any inductor (with R due to the resistance of the wire), and about the resonance peak from the mass+spring harmonic oscillator that is the voice coil, cone, and suspension.  I derived the frequency of an L||C resonant circuit (by computing the impedance and seeing where it went to infinity), and gave  a rather hand-wavy explanation of the effect of adding a resistor in parallel.