Gas station without pumps

2016 March 30

Class topic not what was planned

Filed under: Circuits course — gasstationwithoutpumps @ 21:27
Tags: , , , ,

In my Applied Electronics for Bioengineers course, I had planned to spend the lecture time today talking about sampling and aliasing, but that is not what ended up happening.

I am making it a point to answer student questions (unless they are irrelevant) first, before doing whatever I prepared. The point of the lectures is to help students understand the reading and do the design work for the labs, and anything I have prepared is just a best guess at what the students need. Their questions address more directly what they perceive as their need.  Most of the prepared lecture material is in the book (I wrote the book based on what I have covered in lectures), so answering questions from students who have read the book and are still confused is going to be better than my repeating what is in the book.

Today students had some logistic questions about what to write up for Lab 1 (not much, it was just soldering headers onto the Teensy boards and setting up PteroDAQ—I just asked for a description of what they did, whether anything went wrong, and what they did to fix the problem) and about prelab homework for Lab 2 (do it, but don’t turn it in, it is just setting up gnuplot so that they can use it for the lab).  Those only took a couple of minutes.

The big question that diverted the entire flow of the lecture was a request for an explanation of the high-pass filter in Lab 2 that is used for recentering the function generator output at 1.65V. This lead to several things:

  • Description of block diagrams as functional blocks connected by interfaces, and why this was an important concept in engineering. Frequency and voltage information was put on the block diagram  connections.
  • Capacitor symbol and DC-blocking property of capacitors.
  • Resistor to Vref and why that would cause the output to become Vref, if there was no current through the output.
  • Back to the block diagram to add the constraint that the analog-to-digital converter on the Teensy board couldn’t take any current from its input.
  • Definition of “gain” as \frac{dV_{out}}{dV_{in}}.
  • Showing the high-pass filter Bode plot as two lines meeting at the corner frequency, and giving the corner frequency as \frac{1}{2\pi R C}, without derivation.  I promised the students that we would derive that result in a few weeks, once we’ve had complex impedance.
  • Replacing the resistor to Vref with a pair of resistors to 3.3V and Gnd.
  • Introduction of the triangular ground symbol, and rejection of the chassis ground and earth ground symbols as not relevant for the class.
  • Derivation of the voltage-divider formula from Ohm’s Law, using the important constraint that no current is taken from the output node of the voltage divider, so that the two resistors have identical currents. I had the students help with this, in order to elicit the most common mistake
  • Assertion, without derivation or explanation, that the RC time constant for the high-pass filter should treat the two resistors as being “2R” rather than “R”.

For the last couple of minutes of class, I finally got to do the demo with the homemade stroboscope and pendulum of aliasing, but it was not very effective. Even with the lights off in the classroom, there was enough light through the windows to wash out the strobe. I could not easily keep the pendulum swinging with one hand and adjust the strobe with the other.  If I do this again next year, I should make a panel with about 20 of the LED boards, for around 2.35A during the flash.  At 1.64ms for the longest flash, that’s 3.85mC, which would drain 8.2V from the 470µF capacitor, if the power supply weren’t capable of delivering that much current (but I have a 6A 9V supply, so there should be no problem delivering full power).  Hmm, maybe I should make up that panel for the Mini Maker Faire, instead of the wimpy 4-LED strobe I now have.

I’m actually pleased that I didn’t give the lecture I had planned—my book, which was based on my lectures, already covers the material adequately, and I’d much rather spend precious class time explaining the things that aren’t clear in the book.  The only way I can know what the students need to hear is for them to ask for clarification where they are confused.

2011 April 24

Lectures better than inquiry?

Filed under: Uncategorized — gasstationwithoutpumps @ 15:13
Tags: , , ,

Education Next has an article Harvard Study Shows that Lecture-Style Presentations Lead to Higher Student Achievement that weighs in on a currently controversial subject in education circles: the value of lectures. There is a longer article about the study also in Education Next: Sage on the Stage by Guido Schwerdt and Amelie C. Wuppermann, and the full report (including the actual regression models fitted) is available from the authors.

The dominant meme among recent graduates of education schools is that lecturing is dead, and that almost any other approach to teaching is better.  This is not a particularly recent meme: John Dewey argued for hand-on learning in place of lecturing over a century ago and Alison King’s article “From Sage on the Stage to Guide on the Side” was published in College Teaching, Vol. 41, 1993 and has been cited at least 186 times.   Thus the conclusion of the Harvard study, that lecturing is sometimes superior, has been greeted by many as heresy, even though the majority of teaching is still done by lecturing.

The study itself evaluated one variable: amount of time spent lecturing versus amount of time spent on in-class problem solving for middle-school students.  The output measurement was scores on standardized tests.

This was not a controlled experiment the way a drug trial might be, with different groups of students being randomly assigned to get different educational treatments.  It would be impossible to do that on a large scale, and the impossible to do a double-blind study.

Instead, the authors came up with a clever trick for using existing data from many students and still get good matching controls.  The used the TIMSS data from “6,310 students in 205 schools with 639 teachers (303 math teachers and 355 science teachers, of which 19 teach both subjects). ”  Information about classroom practices had been collected, including amount of time spent in 8 different in-class activities. On average the teachers spend about 40% of classroom time on problem-solving activities and 20% on lecturing.

One of the problems with using existing data like this is separating cause from effect—do students do better under one teaching style or is that teaching style chosen because the students are better scholars?  The clever trick here is that the students were tested in 2 different but related subjects (math and science), usually taught by two different teachers.  Thus differences in score for a single student control to a large extent for the student-specific variables that usually confound such studies.

Here is their main result:

Contrary to contemporary pedagogical thinking, we find that students score higher on standardized tests in the subject in which their teachers spent more time on lecture-style presentations than in the subject in which the teacher devoted more time to problem-solving activities.

The effect was even stronger in classes where the same group of students were together for both math and science, and stronger still among above-average students. It seems that the popular (with teachers) group work is hurting the brighter students.  The evidence that above-average students are hurt more by avoiding lectures than below-average students is only suggestive, not statistically significant—both groups are hurt by avoiding lecturing.

The authors do put in all the appropriate caveats about the limitations of their study: only 8th graders were in science and math on one particular measurement of achievement (albeit a highly respected one) and the information about pedagogical time use came from teachers’ self reports, which may have strong observer bias, given the dominant meme that one particular teaching style is preferred.

Their conclusion:

Given the limitations of the data, our finding that spending increased time on lecture-style teaching improves student test scores results should not be translated into a call for more lecture-style teaching in general. But the results do suggest that traditional lecture-style teaching in U.S. middle schools is less of a problem than is often believed.

Newer teaching methods might be beneficial for student achievement if implemented in the proper way, but our findings imply that simply inducing teachers to shift time in class from lecture-style presentations to problem solving without ensuring effective implementation is unlikely to raise overall student achievement in math and science. On the contrary, our results indicate that there might even be an adverse impact on student learning.

%d bloggers like this: