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30 March 2014

What do you want to know about AP Physics 1 and 2?

I haven't posted much the past few weeks because I've been writing the "teacher's manual" for the forthcoming 5 Steps to a 5: AP Physics 1.  I don't know how or where the teacher's manual will be published, but it includes lots of ideas, including a list of quantitative demonstrations.  I'm ready to get back into regular posting.  

On the weekend of April 11-13, I will be in Chicago for a meeting of AP Physics consultants.  There, we will hear the gospel of the new exams delivered unto us by official College Board representatives, people on the various development committees, etc.  

So, what questions do you have?  I can answer some right away.  But if you'll ask me now, I can soon ask directly to the people who are actually creating the exam.


19 March 2014

Mail Time: Do you teach capacitance? (Don't feel bad about leaving it out.)

Wendy Stallings writes in:

Do you avoid capacitance in your general-level class altogether, give it a passing nod, or actually cover it? I read your post from Aug 2010 about circuits, and it sounds like you ignore it altogether, but I wanted to clarify. I’ve wrestled with whether or not to include it for a few years but find the concept hard to demonstrate on a general level.  When I do teach it, I’m never satisfied with the results, but I feel bad about leaving it out.  Thoughts?

Hi, Wendy!  Let me give you a two-part answer.

To address your specific question: In general physics, I ignore capacitance altogether.  Even in the new AP Physics 1 course (which we are teaching as a broader Honors Physics 1), capacitance is not covered.  Capacitance is covered in our second-year courses, both AP Physics 2 and AP Physics C - E&M.

I don't think it's worth making general students worry about capacitance.  They get very confused, even though the concepts seem straightforward to me.  However, second year students, or those who become quite comfortable with resistors, do fine with capacitors.  They'll pick up capacitance just fine in their next physics course, whether that be with you or in college.  

To address the general principle of "feeling bad about leaving it out": My own advice is to never, ever* worry about leaving out a topic in an introductory course.  First-year physics is far more about teaching skills than about teaching content; the "Big Three" skills of quantitative, graphical, and order-of-magnitude reasoning can be taught appropriately with pretty much any combination of physics topics.

* Well, hardly ever... if you're teaching to a standardized exam like the AP or Regents, it's okay to leave out a few but not a lot of the topics on that exam.  

I understand your concern -- you hear in your mind a physics expert saying, "What?  How can you possibly claim to be giving your students a broad introduction to physics if you don't even mention capacitors, which are a fundamental and canonical topic?"  But there's an absolutely silly premise there.  Not everyone agrees what is "fundamental and canonical."  A different "expert" might argue that the lack of coverage of simple machines makes your course worthless.  Another might wail that you didn't touch relativity.  Guh... who cares.

Unless your students can drop their English, history, and foreign language courses to extend your physics class to pretty much all day, there's nothing for it.  You're NOT going to cover everything that someone, somewhere thinks is fundamental and canonical.  So freakin' what.  The only possible approach is to teach the topics that you prefer.*  As long as you're covering some sort of broad spectrum (i.e. NOT just kinematics and Newton's Laws for nine months), and as long as you're giving students plenty of practice and instruction in the Big Three skills, you're doing just fine.  Your students will be well prepared for future physics courses, whatever topics those courses cover, because they have the requisite skills; and, your students will be well educated if they choose never to study physics again.

* Or, perhaps, the topics that you don't prefer but that you have good equipment to cover.  Or the topics that are on a standardized exam.  Or whatever you personally choose to do.

18 March 2014

Bad Graphs: Everyone's students make them

What I expect a graph on a lab question to look like
Being a relatively new teacher is like being a new parent.  Everyone gives you advice, whether they have the requisite experience (or success) that would make that advice valid; everyone thinks they can do your job better than you.  Moreover, even those giving reasonable advice neglect the fact that you are likely overworked and underslept in your new position.  You're happy just to survive the next class/feeding without falling over; getting everything just right the way your mentor or your mother would do it is beyond your capability right now.

And that's fine.

Certainly you should listen to advice from experienced teachers who have earned your respect.  But too many new teachers, like too many new parents, live in mortal fear of failing to live up to expectations.  Face it -- you're gonna screw up.  And that's okay, because every other teacher and parent in history has screwed up, too.

Exempli gratia:  I put together a series of posts about "bad graphs" at the request of numerous readers.  Some, such as the "dot-to-dot," "nonlinear axes," or "fudged line" bad graphs, represent horrid mistakes.  In my summer institutes, I am emphatic that it is our responsibility as physics teachers to do enough lab work that students don't even think to make such mistakes.  A best-fit line should be drawn properly with a ruler, as shown in the graph at the top of this post.

So are you a naughty, naughty teacher if a student makes a graph like the one below on an exam?
Look at how this student fudged his best-fit line
so that it would touch every data point.  BOUX!
Well, if at the end of the year virtually every student in your class connects data dot-to-dot or fudges his best-fit lines, then that's not a good thing.  When exactly did you do your lab work?  

Oh, that's right... in your first year, you were lucky to think up one or two good lab exercises.  And on top of everything else going on, that's all you did, just a couple of labs.  No wonder your students couldn't make an appropriate graph -- they didn't have enough practice.

So relax, and learn from the mistake.  This summer, plan some more lab exercises.  And now that you kinda know how a school year works, be sure you actually do those exercises.  Hold the students accountable for making their graphs correctly in class.  You'll be surprised at how quickly you manage to stamp out silly mistakes once you have the time and energy to focus on them.

If only one or two students out of 40 commit the sin of a bad graph under the pressure of the exam*, don't even worry about it.  You're not disappointing anyone.  Your mom isn't going to give you the look of withering scorn... she'll save that for when you listen to your pediatrician instead of to her quaint folk wisdom.

*Or because that particular student never listened, no matter how hard you tried to make him

Oh, and where did I get that bad graph?  Trimester exam last week.  My student.  One of two -- the other did the dot-to-dot thing.  Guh.  But I don't think I'm getting sacked.  Point is, it happens to all of us.

17 March 2014

Mail Time: Isolated Capacitors

Joseph Rao asks about problems with isolated* capacitors.  They usually boil down to using semi-quantitative reasoning with Q = CV.

Isolated capacitors, as opposed to capacitors in circuits with resistors.

When the capacitor remains connected to a battery, the voltage across the capacitor cannot change.  Thus any structural change in the capacitor which changes capacitance will also change the charge stored on the capacitor.  For example, doubling the distance between parallel plates halves the capacitance (by C = εA/d).  Then with voltage constant, Q = CV tells us the charge stored has also halved.

When the capacitor is not connected to any battery or external circuit, then the plates are insulated; the charge stored on those plates cannot change.  In this case, any capacitance change will change the voltage across the capacitor.  For example, doubling the distance between parallel plates again halves the capacitance; but if the capacitor isn't connected to a battery, the charge on the plates won't change, meaning that the voltage across the capacitor doubles by Q = CV.

Capacitance is a property of the capacitor's structure.  Just changing the voltage across or charge stored on a capacitor does not affect capacitance.  If we don't change the area of the plates, the distance between the plates, or the dielectric material in between the plates, the capacitance will remain unchanged.  In this case, doubling the voltage across the capacitor would double the charge stored by Q = CV at constant C.

16 March 2014

Great Link: Commentary (biting wit included!) on digital slideshows

Screenshot from Rebecca Schuman's "article" at
Physics teaching is, fundamentally, performance art.  I never use a prepared slideshow, because I want to be spontaneous in front of the class.  Even when I do the same demonstration in three different sections, I often change things up a bit each time in order to produce a fresh and entertaining performance.  In any case, students are attending my class not to see prepackaged slides, but rather for me to help them understand this daunting new subject of physics -- if they could just watch some slides and understand, then I wouldn't be worth a quarter of the money my school is paying me.

Rebecca Schuman's digital slideshow complaining about digital slideshows -- Zen! -- makes many of my points about how (not to) teach in a much more effective way than I ever could.  Click through the whole thing -- her slideshow isn't all sarcastic "aren't professors lazy and dumb," she goes on to make useful, constructive points about how a powerpoint presentation CAN be used effectively.

And that's why I will assign all of my research students to watch Schuman's "presentation."  I struggle every year with students who don't understand that a slideshow is meant to enhance an oral presentation, not as a substitute for performance.  I regularly make students present with whiteboard-and-marker only, no slides allowed.  The bulk of my time in research physics is spent vetting presentations, explaining how a slide should include fewer words, but more pictures and graphical elements that enhance the message.  I need to find a way of delivering a painful electric shock to any student who reads more than a few words directly from a slide.

My hope is that Schuman's presentation will set the tone for this spring's research auditions, in which students must give a five-minute presentation based on their preliminary work on a USIYPT project.  Watch it for yourself -- you'll see how it can communicate appropriate use of powerpoint better than anything I could possibly say myself.

11 March 2014

Here's how we are adjusting our physics curriculum for AP Physics 1 and 2

Probably the most frequently asked question of physics teachers over the next six months will be, "How are you adjusting your curriculum to handle the new AP Physics 1 and 2 courses?"  After extensive thought and conversation, we at Woodberry finally have an answer.  I'll share our solution with you now.

But first, the critical caveat -- our solution is not "right," "best," "ideal," or even "guaranteed to be good."  I've talked to my own department, I've talked to other physics teachers around the country, I've read what the College Board has to say.  Most importantly, I and my colleagues have considered how the new AP courses fit within our school's existing structure, philosophy, and goals.  Your school can't escape the same process.  I'm letting you know our solution not because you should adapt it whole-hog, but because you can use our thoughts as one more informed idea for your own brainstorming.

To us, the purpose of the first-year course is twofold: foremost, to teach students how to learn physics at the college level, establishing habits and skills that will serve them well in any future science course.  Secondarily, we want to expose students to a survey of topics, so that those who never see physics again have a broad background in the subject, but those who move on to further study have seen as much as possible.  (After all, a physics topic isn't mastered until the third time a student sees that topic -- we want to get that first exposure in to as many things as we think reasonable.)

We very much like the new AP exams, and we want to teach to them.  We, like many folks, are faced with the issue that it is not reasonable to teach both AP Physics 1 and 2 in the same year; however, we want our first-year college-level course to be broader than just the mechanics, circuits, and waves included in AP Physics 1.  The College Board has thoughtfully designed AP Physics 1 to be limited enough in scope that there's plenty of time to integrate other topics beyond those on the exam.*

*The reason for this limited scope is that some schools are required to meet state standards that don't match those tested by the AP exam.

We will continue to teach an "Honors Physics 1" course, which will evolve to cover everything on the AP Physics 1 exam, and a bit more.  Students will be required to take the AP Physics 1 exam in May.  Our approach to Honors Physics will be to spend about two-thirds of the year teaching a variety of topics in AP Physics B style... then restarting from the beginning, demanding the deep descriptive responses that we'll see on the AP Physics 1 exams.  We'll do AP Physics 1-level laboratory work throughout, along with quantitative demonstrations and other ways of integrating experiment with problem solving.  

I'm going to rework the topic sequence from what I used to do in AP B and the previous iteration of Honors Physics.  The re-ordering groups the Three Major Mechanics Approaches of Newton's laws, energy, and momentum together; while that's nice and neat, the real reason for the sequencing listed below is that I teach seniors.  I want them to be responsible for topics beyond the AP exam early in the year, before college acceptances and senior slide.  Then the second part of the course can downplay or ignore extracurricular topics in preparation for the AP Physics 1 exam.

My topic sequence, where an asterisk indicates material not on the AP Physics 1 exam:

Newton's Laws
*Buoyant foraces
*Electric fields and forces (just F=qE to start with)
*Magnetic Fields and forces (just F=qvB)
Uniform circular motion
Coulomb's Law
Harmonic motion

Work-energy theorem (i.e. conservation of energy)
*Ideal gas law
*Thermodynamic processes and PV diagrams

[Linear] Momentum
Impulse-momentum theorem
Collisions and conservation of linear momentum

Rotational dynamics
Rotational kinematics
Rotational inertia
Newton's second law for rotation
Rotational kinetic energy
Angular momentum and its conservation

My gut feeling is that I can get through the material listed above before the end of the second trimester, i.e. approximately March 1.  When I say the first approach is at the AP Physics B level, I don't mean the 1980s version (i.e. "shut up and calculate").  Certainly students will be asked from day one to present thoroughly annotated solutions to problems, to justify answers in words, to explain correct approaches.  But I'll save the full-fledged AP Physics 1 treatment, with quantitative-qualitative translations and paragraph responses and problems designed to take 25 minutes to answer -- for after spring break.   That's when we'll go back to the beginning of the course, simultaneously reviewing previous topics and extending my students' ability to describe, explain, argue about, and write about physics.  

The hope is that the few underclassmen in AP Physics 1, along with those who had a rigorous college-level physics course previously, can advance to AP Physics 2.  That will be a second-year course, aimed toward students who already know how to learn physics, and who thus don't need nearly the amount of guidance* that first-year college physics students do.  Since students will be familiar already with how deeply they will be asked to communicate their understanding of each topic, we can be less structured.

* i.e. whip cracking

In the summer of 2015 I'll post again to describe how our approach worked (or didn't work).  Let's hope it's the former.

06 March 2014

Make 'em work to get their test back

Back in November, I gave my freshmen a test consisting of ten"justify your answer" questions.  They did fine, generally, but I became frustrated when they did corrections.  They focused so much on figuring out why they lost points, on what they did wrong to begin with, that they couldn't redo the problems right.  In that previous post, I suggested that next time, the students would get back merely a blank copy of the test with the problems they missed circled.  Sure enough, I did exactly that last week.  It worked well, even though it was a painful 30 minute process to go through the blank tests to circle the problems that had to be redone. 

In this process, I stumbled into a couple of good teaching ideas that I'll share.

(1) David McRae, our math department chairman, caught me in the act of preparing the blank tests.  He told me of a presentation he saw at a math conference: a professor graded tests by writing detailed commentary, without writing anything at all about points gained or lost.  He tabulated grades separately, and did not (immediately) share the grades with the students.

We all know that students tend to look at their score and tune out if we don't manage the turnback of tests very carefully.  This professor took that principle to the extreme -- without scores available at all, the students could do nothing but read commentary to figure out what they did wrong.  David says he has tried this approach in his advanced courses, to very good effect (though it takes ~4 times as long to grade).  While this wasn't my approach, it's similar and interesting -- I thought I'd share.

(2) When I presented students with a blank test to correct, I did so for physics teaching reasons.  When confronted with their graded tests, students were so prejudiced by their previous responses that they had trouble redoing the problems without making the same mistakes.  The experiment bore out my hypothesis that the blank test would solve this issue.

What I hadn't counted on was the psychological effect of not returning the graded tests right away.  Many students asked, "Can I just see my original test real quick?"  They were noticeably antsy, like a Starbucks addict consigned to drink weak-sauce hotel coffee.  Test grades are a drug -- our students have been conditioned to work for them, live for them, measure a bit of their self-worth by them.  

So without any premeditated intent, I had created an amazing incentive for good test corrections.  I was holding their tests hostage!  The ransom cost: doing the problems they missed correctly.  You should have seen the relieved looks on their faces when their last corrections were approved, and they could have their tests back.  Even those who didn't do particularly well felt fine -- they now knew how to properly do every problem, which dulled the sting of a D.  They were less inclined to sour grapes ("Woah, that test was just really hard, nobody should be expected to do that.") and more into self reflection ("D'oh, I really knew how to do a bunch of those problems I missed, so I should have had a B+.")