Gas station without pumps

2014 April 22

Electrodes and load lines

Filed under: Circuits course — gasstationwithoutpumps @ 07:17
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As planned I talked on Monday a little bit about polarizing and non-polarizing electrodes, giving them the the idea that the point of electrodes was to convert between ionic currents in solution and electron currents in wires, and that there was always a redox reaction to do the conversion.  (I did not use the term “redox” though, and I probably should have—I’ll try to work it in casually during lab today.)  I talked about three electrodes:

  • the Ag/AgCl that is used for a lot of bio research, because it is non-polarizing, works well in salt water, is generally non-toxic, and is fairly cheap.
  • stainless steel (particularly 316L), because it is commonly used in implants for its non-corroding, non-toxic properties, though it makes a polarizing electrode, which is not suitable for low-frequency measurement.
  • platinum electrode used for the hydrogen reaction that is the standard non-polarizing reference electrode (and is used in a lot of gel-electrophoresis boxes).

Although I gave the chemical reactions for Ag/AgCl (pointing out that the ion current was chloride ions) and the hydrogen reaction, I did not attempt to do so for stainless steel, because I’m still not sure which of the many oxidation reactions are relevant. I did point out that the steel is kept from rusting mainly by a chromium oxide layer on the surface, and that the same mechanism that prevents rusting also makes stainless steel a poor transducer of electron currents to ion currents.  I’m not sure I got that message across though.

I think that it may be worthwhile, either in lab today or in our data analysis on Wednesday, to mention “redox” reactions by name, and to point out more clearly that the what makes stainless steel good for implants also makes it poor for electrodes—the notion that “metal conducts” may be too strong a prior, as students are not used to thinking about the surface properties of things, but just bulk properties.

For the second half of the lecture, I introduced the notion of load lines, with open-circuit voltage VOC and short-circuit current ISC to figure out the voltage and resistance of the Thévenin equivalent of power source. I then had them work out, as a class, the Thévenin equivalent of a simple voltage divider. They got it, eventually, but I had to work through some stubborn holes in their understanding of simple circuits from physics. I think part of the problem was terminology—they apparently did not know what “short circuit” and “open circuit” meant, which I did not realize was a difficulty until near the end of the time.

I did not get the students any RC impedance or voltage divider questions to work on—I hope we have a little time for that on Wed, before Friday’s quiz. I could assign homework with voltage dividers and RC circuits, but I’m reluctant to assign homework in this class, given the amount of work expected for their lab write-ups. Several students already aren’t doing the homework I do assign—many are not even reading the lab handouts with the pre-lab assignments until just before class, when it is too late to do the work. A lot of lab time has been wasted by students trying to do the prelab work during lab.

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