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17 July 2015

Rule 3 of teaching: Your students don't listen to you. (And a non-ohmic light bulb.)

Rule 3 of teaching, as described in the 5 Steps to a 5: AP Physics 1 teacher's manual:  Your students don't listen to you.  Don't worry, they don't listen to me, either.  

I hear regularly from physics and non-physics teachers fretting over the material they "cover" in class, over the precise content and activities they do.  I suggest taking a holistic view of a course as a whole, recognizing that students will rarely remember a specific classroom event more than a week or so later.  The College Board has gone over-the-top with this philosophy, prioritizing "science practices" and "big ideas" over content.  Their heart is in the right place.  An understanding of experimental physics isn't about "spit back the procedure, analysis, and results from this experiment you did six months ago."  It's more about, "here's a new situation that you've never seen; how would you answer a well-formed question with an experiment?"

More on-point, did you do an experiment measuring the resistance of a light bulb this year in AP Physics 1?  Did you show that the bulb's resistance changes depending on the voltage across it?  Did you have students design and carry out an experiment to determine whether, and to what extent, the bulb obeys Ohm's law?

Some of you are hanging your heads in shame, because you didn't -- and this very experiment showed up as free response problem 2 on the 2015 AP Physics 1 exam.  My big, friendly point is, don't worry about trying to match experiments with what might show up on the exam.  Not only is it an impossible fool's errand, but it doesn't even matter.

My class did this exact experiment in January.  I even made it what education professors would call an "open inquiry" exercise.  Toward the end of the circuits unit, during which we had always treated electronic devices as having constant resistance, I pointed out that some books suggest that a light bulb under some conditions might not obey Ohm's law.  It was each lab group's job to test the validity of those books' contention.

Oh what wonderful results we got!  Most groups figured out quickly and independently to graph voltage as a function of current.  You can see one of the graphs in the picture at the top of the post.  The curve is apparent as soon as you smack a ruler down on the page.  The slope varied from 51 ohms at about 2 V, to 77 ohms at about 8 V.  The bulb is non-ohmic, with a 30%-plus difference in resistance across a useful range of voltages.

Since we did such a good job with this experiment, one might expect that my students kicked arse on 2015 free response problem 2.  Um, nope.  My students performed, by far, worse on that problem compared to the others.  The College Board just released some class statistics, showing which quartile our students fell into on each of the free response problems.  On problems 1, 3, 4, and 5, the vast majority of my class performed in the top 25% nationally.  On problem 2, more than a third of my students were in the bottom half.  

Back in 2003, the same sort of thing happened in reverse.  I remember kicking myself because that was the first year ever when I didn't do an optics-bench-style experiment with my class; sure enough, that was the year when problem 4 was a laboratory-based optics bench question.  Turned out, though, my students did fine, indistinguishably from other years when I had sometimes done the very experiments that showed up on the exam.

The precise lab exercises you do don't matter.  And that's because of Rule 3. Don't take this rule as a complaint, or as the "get off my lawn" ramblings of an old man carrying on about the danged kids these days.  It's just a well-verified observation.  I see it as my job to be sure that my students succeed despite Rule 3.


1 comment:

  1. Greg,
    I am really glad you posted this. I read it as a critique of the supposed success of modeling instruction. Your students had all of the possible advantages that one could have going into the exam. They literally designed and performed the same experiment. So why did they not ace the question. My feeling is that exams like while the AP exams test knowledge and practice, what they really test is how a student thinks with a gun to their head. This is a learned skill that bears a closer resemblance to mastering a sport than anything academic, unless they participate in math competitions, like AMC(8,10,12). While we can do some training in the 9 months that we have them as students, it largely is dictated by their skill level when they walk into the classroom.

    For instance, I use AP exam questions for all my exams, they are timed exams, I do not address questions unless there are diagrams missing or exam errors, and I count the exam questions in proportion to the time that is allocated - 1 FRQ = 15 min, 1 MC = (45/35) min, and the exams are curved. Basically what I find is that the students who do best on the first exam tend to dominate all year and then get 5's. Now they can not accomplish this without learning the content. However I have had students who were far more dedicated, spent much more time, and consistently perform worse - sometimes catastrophically bad.

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