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02 June 2014

Be warned: AP Physics 1 and 2 will be far, far more difficult to teach than you think.

I'm here at the reading grading problem 7 from the very last AP Physics B exam ever.  I've been discussing with other readers how this question provides a beautiful example of the difference between AP Physics B and the new AP Physics 1 and 2 exams.  More to the point, I think this problem can provide fair warning about just how unprepared the country's students are for the verbal reasoning required on the new exams.

Look at this thin films problem via the link above.  (I can't post it directly for lawerly reasons.)  It contains five sections.  The first three ask for calculations, of frequency in air, frequency in oil, and wavelength in oil.  These are the classic AP Physics B - style questions, in which mathematical reasoning with reference to formulae is prized.  On similar problems in the past, liberal partial credit has been awarded to those who made any sort of reasonable attempt to approach the calculations.  Questions in this style -- and the liberal, somewhat-gameable partial credit -- will not exist in AP Physics 1 and 2.

The last two parts to question 7, though... they are worthy of the new exams.

Part (d) asks about the index of refraction of one of the substances.  Previous thin films questions have asked students to calculate the minimum thickness of the film; knowing how many phase changes happen, or how the phase changes might affect the result, was worth one measly point at most.  But this particular question can't be gamed in any way by referring to an equation, relevant or not.  The very nature of the question requires the student to explain that two phase changes are necessary in order to produce constructive interference; and that the only way to get a second phase change is for the bottom substance's n to be greater than the oil's n, just as the oil's n is greater than that of air.  The students themselves must generate the discussion of phase changes and interference properties, not merely take the question's cue and apply a formula.

Similarly, part (e) asks about an observation of the thin film from a different angle than straight in.  Oops -- again, no formula exists to be applied.  The only possible approach is for the student to explain -- unprompted, with no hints available from the question stem or formula sheet -- that the extra path length traveled in the oil determines which wavelength interferes constructively.  And since that path length is longer at the wider angle, the observed wavelength becomes longer, i.e. more red.

The new exams will include almost exclusively these unguided demands for description of underlying physics principles.

Currently, smart students who have been taught that physics can be gamed by plugging into formulas can earn 3s on the AP exam.  That's gonna change.  A plug and chug student can't answer these descriptive questions.  Trust me.  I've seen such questions (and the lack of reasonable answers from the vast majority of students) for years.  With few if any "calculate" questions to fall back on, there's gonna be wailing and gnashing of teeth around the country.  Not just by students, either... physics teachers who don't want to change their approach will be caught out.  They'll complain about the new exam, finding reasons that it's unfair or wrong.

Oh well.  It is incumbent on physics teachers to start working now on changing our approach to our classes.  Don't give credit on a homework or quiz question for bare calculation.  De-emphasize calculation -- make every assignment require writing in sentences.  Do experiments, and make the students explain their experimental methods and results on paper.  Don't reward correct answers unless those answers are supported by correct reasoning.

And when your students complain, just show them questions like number 7 on the 2014 exam.  It's not *your* fault that they have to explain their reasoning more thoroughly on the new exams.  It's that evil College Board.  Hide behind the monolithic corporation.  But whether you or your students like it or not, the times, they are a' changin'.