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16 June 2014

Rewriting Physics B questions for Physics 2: 2014 B7, thin films

The new AP Physics 1 and 2 exams cover much of the same material that AP Physics B has covered for decades.  The style of questions will be completely different, though, requiring a tremendous amount of verbal response.  To help teachers understand how the new exam will look, the College Board has released 1.5 practice exams;* few other materials vetted by the committee are available.

* One official practice exam available to anyone who has submitted a course audit for AP Physics B or for the new courses; half a practice exam with the course description.

One of our task as AP Physics 1 and 2 teachers will be to develop a library of questions in the style of the new exams.  A great place to start is with the released Physics B exams.  But don't use them verbatim.  Rewrite the Physics B question such that calculation is minimized or eliminated; and such that students are asked to explain their understanding of each question.

As an example, take a look at problem 7 from the 2014 Physics B exam.  (You can find it at this link.)  The item presents a situation involving thin films, and then poses five different tasks.  Here is each, and how (or whether) it can be rewritten for the AP Physics 2 exam.

Part (a) is a straight-up determination of the frequency given the wavelength and speed of light.  Such straightforward "calculate this" questions will be vanishingly rare on the new exams.  I'd skip this part.

Part (b) asks for a calculation of the frequency of green light in the film; part (c) asks for a calculation of the wavelength in the film.  Think about what's important or interesting about these questions.  We're looking to see that the student recognizes that the frequency is unchanged in the film; then, we're looking to see the student use either λn = λ / n; or, we're looking to see the student use v = λf with a recalculated speed of light in the film.  So, to rewrite for physics 2, try asking a question that requires the student to know and articulate these issues.

I guess I'd make each of these parts a ranking task:  "Rank, from greatest to least, the three materials (air, oil, transparent plate) in order of the frequency of the green light in the material. If the green light has the same frequency in two or more materials, indicate so clearly in your ranking.  Justify your answer."  And again the same question for wavelength.  While the frequency question is pretty much know-it-or-don't, the justification of the wavelength ranking requires some good explanation.

Parts (d) and (e) are already exactly the kind of question that Physics 2 will be asking all over the place.*  Part (d) requires a justification that cannot be accomplished with simple reference to an equation; part (e) uses the prompts "describe" and "give an explanation for the phenomenon."  

* Although I'm told reliably that "place a check by your answer" format will be eliminated.  Part (d) would be phrased identically, but the "greater than, less than, or equal to" answer would be expected to be part of the student's prose..

If there's enough people who ask, I can write a series of suggestions about rephrasing old Physics B questions.  This approach -- determine the important conceptual element of the calculation, then ask a question that requires students to articulate that element -- should work to make any Physics B free response item into a Physics 1 or 2 problem.  


  1. Greg, I for one would love a series of suggestions to get me started until I get comfortable doing it myself. Other than the 1.5 released tests, my seminar later this summer, and your blog, there just isn't that much access to the new philosophy in practice. Thanks in advance.

  2. The thing that scares me about these new all-or-nothing conceptual problems is that they might put a kid with 70% understanding of a topic in the same boat as a kid with a 0% understanding. So many points on these new problems seem to hang on the details instead of the meat!

    Take the magnet carts on an incline for example. A huge chunk of the points rely on finding the "external force." I had to re-read the problem about 10 times to figure out what it was talking about. External force on the left cart? The right cart? The system? All forces combined? Net force? The fact is, it's not even that useful of a quantity to find (in my opinion.) Why not find all the forces on all the carts?

    Most of my kiddos this year could have solved the problem, but wouldn't have known what it was asking them to do. And our Physics B scores were pretty hot this year. Where does that leave the average physics teacher/student if even the experienced teachers are freaking out a bit?

    At least I still know how they'll ask the questions on the calc exam =)

  3. I would love to recycle the old Physics B problems into more conceptual less computational. Sounds pretty cool!