# Gas station without pumps

## 2014 April 26

### As expected, students did poorly on the quiz

Filed under: Circuits course — gasstationwithoutpumps @ 15:43
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I gave about the same quiz as I did last year,  changing the numbers, removing one of the harder questions, and making sure that some of the other questions reflected worked examples we had done in class. The quiz was again on the 12th day of instruction. I had intended to move it to the 10th day, but one of the students was called out of town, so I rescheduled it so that everyone could take it at the same time.

I expected similar distribution to last year’s (last year the range was 3/32 to 12/32), but was hoping for slightly better.  I saw a distinct bimodal distribution this year, with half the class getting scores from 0/33 to 6/33 and the other half getting 11/33 or 12/33. This is a little clearer distribution than last year’s, which spread the students out more uniformly. I was still hoping that some of the better students would get over half the points on the quiz, but they seemed to top out at 36%.

I worked this year’s quiz myself in about 24 minutes (which means the quiz was a little too long still—I want about a 3:1 ratio on time, and the students had only 70 minutes).

I was really depressed after last year’s quiz, because I had not been expecting such dismal performance. This year I was braced for it, but still hoping for better.  Still there were some surprises:

• There were a few questions that should have been free points (like asking for the impedance of a resistor with resistance R)—I was disappointed that some students missed even the trivial questions.
• I had a pair of questions which were identical, except that one asked for algebraic formulas for impedance and the other gave component values and asked for numbers. I put the algebraic ones first this year, so the numeric ones were just a matter of plugging the numbers into the algebraic ones (and doing a sanity check).  The algebraic ones had a mean score of 2/4 with a standard deviation of 1.2, while the numeric ones had a mean of 1.22/4 and a standard deviation of 1.2.  I had not expected a drop in performance on the numeric ones, since the received wisdom in the physics education community is that students do better with numeric examples than algebraic ones.
• No one got any points on the oscilloscope probe example, even though it was identical to an example we had worked in class.
• The average score on a load-line problem was 1/6 with a standard deviation of 1.3.  This did not look like a normal distribution, but an exponential one, with half the class getting no points.
• I had two low-pass RC filter questions. One asked for algebraic formulas; the other used the same circuit but asked for numeric answers using specific component values, voltages, and frequencies. The algebraic one was bimodal, with 2/3 of the class getting 0 and 1/3 getting the answers completely right. The numeric one was significantly worse, with only 2 out of 9 students getting any points (1/6 and 3/6).
• I asked a couple of voltage divider questions that required applying the voltage divider formula circuits in which the voltmeter was connected between two nodes, neither of which was ground.  One asked for an algebraic results (a Wheatstone bridge), the other for a numeric result (voltage across the middle resistor of three in series. Students did very poorly on both,  with only one person getting the voltage for the middle resistor (one got half credit for setting it up right, but computing wrong), and no one getting more than 1/5 for the Wheatstone bridge.

Last year I suggested several ways to handle the poor performance on the first quiz:

1. I could tell them to study and give them another quiz.  That would be totally useless, as it would just repeat the problems on this quiz.  They don’t know what it is that they need to know, and vague exhortations to study are pointless.  I don’t think the problem is lack of effort on their part, and that’s the only problem for which pep talks are a potential solution.
2. I could go over the quiz question by question, explaining how I expected students to solve them.  This is classic lecture mode and the approach I used to use. It would be easy to do, but I doubt that it would help much.  I already did an interactive lecture on the material, and another approach is now needed.
3. The students could get the quiz back and be told to go home and look up in their notes and on-line anything they did not get right.  They would find and write down the right answers, as if this were homework.  (This “quiz correction” is a standard strategy in high school teaching, but not common in college teaching.)  One difficulty here is that they might be able to find answers (say by copying from other students in the class) without understanding how to do the problems.  It is probably a better approach than yet another lecture, but I’m not sure it will work well enough.  If the students were trying to get from 80% understanding to 95%, it might be fine, but to get from 30% to 80%, something more directed is needed.  More time and open notes would help, but maybe not enough.
4. I could break them into groups and give each group a couple of the problems to work on together in class. This peer instruction technique would be a good one if about 1/2 the students were getting the problems right, but with the top of the class getting only 1/3 right, I may need to give them more guidance than just setting them loose.  For example, on some of the problems there was a fundamental misreading of the circuit schematics that was very common. I could clear up that misunderstanding in a minute or so and have them rework the problems that depended on it.  Then I could send them home to write correct solutions.
5. I could give out lots of problem sets to drill them on the material.  Of course, since it took me more than all day Sunday to make an 8-question quiz, it would take me forever to generate enough drill problems to be of any use.

I feel the same way this year about the possible teaching strategies, but this year I’m going to try a mix of methods 3 and 4, asking them to redo the quizzes at home, working with others until they are satisfied that they can now do the problems and other similar problems when asked.  I’ll have them hand it in this year as a homework, but not go over it in class until after they turn it in.  They need to take a more active role in trying to master the material, and not rely so much on my telling them what to do.

Monday we’ll cover inductors and loudspeakers, in preparation for the Tuesday measurement lab.

On Wednesday I was planning to do gnuplot analysis of the loudspeaker data, but I think I’ll keep that fairly short, so that we can get an intro to sampling and aliasing also before Thursday’s lab.  I have to decide whether to bring in my son’s stroboscope and a moving object to demonstrate aliasing.

Friday, I’ll introduce op amps, with the intent of developing the block diagram in class on Monday for a simple op amp microphone circuit for the Tuesday lab.  This weekend I need to rewrite that lab from last year—I decided last year to use the dual power supply with a center ground for their first op-amp design, rather than having them build a virtual ground (we’ll get that in the next lab assignment).

## 2014 April 25

### Ag/AgCl electrode lab went fairly well

Filed under: Circuits course — gasstationwithoutpumps @ 00:08
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Today’s electrode lab also went fairly well.  Most of the students finished on time (or nearly), though  I stayed an hour late with the singleton student—this is a lab that goes much faster if one student records data while the other reads the meters, so I again served as meter-reader for him after everyone else had left.

The group that was working the fastest today—at least 40 minutes ahead of every other group—was embarassed when I noticed that they had connected up the probes wrong on one of the multimeters, so that the voltage measurements they had been taking on that meter were just noise from floating wires. This group was the one I expected the best work from, but by not checking their wiring carefully, they went from being the fastest group to the slowest one.  I think they did make up the time and still finish on time, redoing all the measurements.

The 1 liter of each salt solution was barely enough for 5 groups for the stainless steel and Ag/AgCl electrodes.  I think that next year I’ll want 150ml per student, to be sure of having enough. I’ll probably also switch to 1M, 0.1M, 0.001M, and tap water, reducing the measurements from 5 sets to 4 sets. I wish that there were an easy way to automate the measurements, as the frequency adjustment and data reading is fairly tedious.

I spent most of this evening catching up on my e-mail and writing the quiz for tomorrow’s class.I’ll have to get in somewhat early tomorrow, to get copies of the exam printed before class.

I won’t have much free time tomorrow, as I have my weekly office hours, and tomorrow is the deadline for students declaring majors. I’ve had a steady stream of students in my office for the past couple of weeks, but I suspect that there will be a lot of people who just figure that they can drop in at the last moment.  They’ll be out of luck, because I have 4 students scheduled for half hour slots, filling my 2 office hours.

## 2014 April 24

### Stainless steel electrode analysis

Filed under: Circuits course — gasstationwithoutpumps @ 00:20
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After yesterday’s lab I wrote

I do have to get the students to start lab more efficiently. Once everyone had their setup built, they took measurements fairly quickly, but they came to lab with no schematic of their test circuit and no table set up for recording their measurements. The first hour of lab was wasted by almost everyone doing the pre-lab work that they should have done over the weekend and asked about in class on Monday.

I started out today’s lecture talking about that—how everyone was very efficient in the last hour of lab, but how it had taken them forever to set up, because they hadn’t come to lab with their schematics already drawn, ready to build, nor their data tables set up, ready to fill in.  I’ll see if they are more efficient in Thursday’s lab.

I also wrote

Tomorrow I’ll spend some time helping them write gnuplot scripts to model their impedance data.  I’m assuming it will look a lot like the data I collected last year, which means that the conventional model of polarizing electrode will not fit all that well.

I hope that we also have time for some complex impedance and voltage divider problems, so that they have a little more practice before Friday’s quiz (which I still haven’t written).

We ended up spending almost the entire hour looking at gnuplot scripting and model fitting.  In fact, the data did not fit the polarizing electrode model ($(R_{surface} || C_{surface}) + R_{bulk}$) at all well, though the students got pretty clean data from 10Hz to 300kHz. We spent some time talking about whether the model was useful, even though it was as much as 20% off, and looked at a simple power-law model that was even further off.

I showed them how to fit data for log-y plots by using fitting the log of the function to the log of the data. I don’t know how many of them got the point of that, though, as my explanation was not as clear as it might have been, and I saw some blank looks.  I’ll probably want to revisit that next week, with a clearer explanation, when we do a similar measurement lab for the loudspeaker.

The problem with the conventional polarizing model is that it predicts a fairly simple Bode plot, with a constant resistance at high frequencies (Rbulk) and at very low frequencies (Rbulk + Rsurfacec. The data actually has no flat spot at frequencies as low as we could measure,
and impedance is still dropping, though slowly, at the highest frequencies we could measure.

I’m wondering whether it is worth setting up the PteroDAQ measuring system to look at much lower frequencies, to see whether impedance flattens out at a much lower frequency than the RMS voltmeters are good for.

Another possibility is try to set up a circuit to measure phase shifts, so that we can get a better idea of the complex impedance, rather than trying to infer the complex impedance from the change in its magnitude with frequency. Both of those are going to have to wait, though, as I have to write the quiz for Friday, provide feedback on some grad student papers, grade this week’s lab, grade the quizzes, and write an all-new lab handout for transimpedance amplifiers.

Actually, I’m thinking that we should do the simple one-stage or 2-stage audio amplifier first, then the transimpedance amplifier for the optical pulse monitor.  So probably this weekend will be on rewriting the audio amplifier lab.  I’m a bit torn, though, as it would be good to do the audio amplifier and power amp back-to-back, particularly if we could do the 2-stage op-amp version in one day and spread the class-D amplifier over three days.  Oh, well, I don’t need to decide until this weekend—I’ll be too busy with the quiz before them to re-order the schedule.

## 2014 April 22

### Stainless steel electrode lab went fairly well

Filed under: Circuits course — gasstationwithoutpumps @ 22:51
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Today’s electrode lab went fairly well.  This is the lab I always panic about, because the students have to move concentrated salt water (up to 1M NaCl) around the lab, and salt water and electronics equipment must not mix! I only had to chide one student once for not using the secondary containment tub, and nothing was spilled.

I had the students do more measurements this year than last, having 4 different concentrations (1M, 0.1M, 0.02M, and 0.005M NaCl) and an unknown (tap water). Last year they only had 3 known concentrations, but had to do both stainless steel and Ag/AgCl electrodes in the same lab. Most of the students finished the lab.  One group still has one set of measurements to do on Thursday, and I stayed an hour late with the singleton student—this is a lab that goes much faster if one student records data while the other reads the meters, so I served as meter-reader for him after everyone else had left.

I spent some time at the beginning of class  to teach each pair of students how to use a vernier caliper to that they could measure the dimensions of their electrodes.  I also had them measure the thickness of the electrodes using my micrometer. I’ll have them do both again (without my instructions) for the silver wire electrodes on Thursday.  Ah—I need to bring in some salt so that I can make a strong salt solution for electroplating on the chloride!  It does not need to be pure, so I won’t use up the 1M NaCl that we have for them to measure their electrodes with.

I do have to get the students to start lab more efficiently. Once everyone had their setup built, they took measurements fairly quickly, but they came to lab with no schematic of their test circuit and no table set up for recording their measurements. The first hour of lab was wasted by almost everyone doing the pre-lab work that they should have done over the weekend and asked about in class on Monday. I did insist on seeing their schematics before I would let them have any of the salt water.  Several had to redo their circuits a few times, because they made no sense (only one wire of the function generator drawn, or a circuit that did not include their current measuring resistor).

Next year, I may have to add more explicit instructions in the pre-lab to make a schematic of their test setup and a table for recording results. Tomorrow I’ll talk to them about preparing for lab so that they can start work immediately, though I don’t know if it will do any good. For next year, I should also add some discussion to the prelab about adjusting the resistor size after making a measurement.  Almost all the groups chose to use a 1Ω resistor, which means that the voltage drop across the resistor was generally quite small (1mV to 180mv).  It would be better to use a slightly larger resistor (10Ω or 100Ω) to get larger readings.  I’ll also have to tell them to set the amplitude on the waveform generator to 10v p-p, so that the signals are large enough, as the default setting when the generator is turned on is only 100mV peak-to-peak.  I think everyone got the amplitude up to at least 1v, which may be good enough.

Tomorrow I’ll spend some time helping them write gnuplot scripts to model their impedance data.  I’m assuming it will look a lot like the data I collected last year, which means that the conventional model of polarizing electrode will not fit all that well.

I hope that we also have time for some complex impedance and voltage divider problems, so that they have a little more practice before Friday’s quiz (which I still haven’t written).

Filed under: Circuits course — gasstationwithoutpumps @ 07:17
Tags: , , , , ,

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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