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23 April 2011

2010 AP Physics B Exam Re-Redux

Times Square LED display, courtesy of
My class took the free response portion of the 2010 AP Physics B exam this week.  If you also teach AP, you're also likely completing or about to start the process of going over the 2010 exam somehow. 

By this point, I've trained my classes pretty well to focus all the way through a seven-question exam.  They know that they're only expected to get 60-65% of the points, so they relax and communicate their understanding.

Of course, that doesn't stop ME from hollering to the wall as I grade the exams.  No matter how much we discuss the issue, I still had 20-30% of my students adding electric fields as scalars on problem 3.  Point here:  I need to take my own advice, and let it go.  If I'm getting through to 70-80% of my students that electric fields are vectors, that's pretty good.  :-)

What next?  We still have two weeks until the real AP exam.  But with other AP exams cannibalizing my class attendance soon, I only have ONE week of effective teaching time remaining.  What will I do with that?

We'll do corrections to the 2010 exam, of course, and we'll keep doing practice problems from old exams.  At the end of the week, I'll give this fundamentals quiz based directly on the 2010 free response.  Take a look at the link, and feel free to use it for your class.  I've asked every recall question I can think of that relates to these questions. 

Obviously, the 2011 free response exam will be completely different.  I have no expectation that any of these same kinds of questions might show up.  However, any topic not on the free response will show up on the multiple choice.  And considering that even questions on similar topics often LOOK different to our students, I find it far more effective at this point to concentrate on fundamental facts rather than on problem-specific problem solving strategies.  

Any other thoughts about AP exam preparation?  Email me, and I'll post a Mail Time! 

19 April 2011

Electricity and magnetism fundamentals review

This quiz is available via the Google Docs link in the text
Folks nationwide are in review mode for the AP physics exams.  I find that electricity and magnetism are tough enough for first-year students when the topics are presented in isolation.  The class can, finally, get most of the facts down with enough quizzing and hammering.

However, as we review for the cumulative AP exam, students no longer have the luxury of focusing on EITHER electricity OR magnetism.  They have to deal with both topics, presented one after the other, or even in the same problem.  Aarrgh!  Suddenly I'm seing right hand rules applied to electric fields. 

Yesterday I gave a two page, cumulative fundamentals quiz on all electricity and magnetism topics.  You can access (and use!) my quiz via a google docs version.  This one is tough... my class averaged probably 16-18 out of 25 points.  Thing is, none of these problems really requires much synthesis.  These are basic facts and basic analysis which must be instinctive for anyone who claims to have a solid understanding of E&M.

How do I use this quiz?  I gave it in a 9-minute class-opening segment.  I graded it yesterday evening.  Tonight as an additional assignment, students will correct their mistakes, justifying all answers thoroughly.  Then I'll give a second, different fundamentals quiz on electricity and magnetism tomorrow.

13 April 2011

Atomic Energy Levels Exercise

Okay, I'll admit my prejudice right off the bat:  I hate teaching atomic physics.  What I love most about teaching physics, rather than math, history, or (gasp) English, is that the answer to most questions can be demonstrated unambiguously with an experiment.  We have no truck with opinions in introductory physics:  I routinely provoke an argument in class, then settle the argument with a quantitative demonstration.

With atomic physics, though, I generally cannot show a demonstration.  Even those demonstrations that I can do -- like passing light from a mercury vapor lamp through a diffraction grating to see the discrete wavelengths of light emitted -- still require multiple leaps of abstraction in order to connect the problem solving to the phenomenon.  This isn't calculating the speed of a cart.

Nevertheless, the AP curriculum requires that I teach atomic physics.  The approach I take is to avoid exposition as much as possible.  I've found analogies and cutsey trick methods of imagining "what's really happening" in an atom are useless.  Instead, I take my inspiration from Richard Feynman:  "Shut up and calculate."

I provide each student with a set of two or three energy levels from an imaginary "atom," as shown in the picture.  The energy levels in these atoms are randomly determined, and all between -0.1 eV and -6.0 eV -- each student has a different set of energy levels.  I show very briefly the equation E = hc/λ, how the energy of the photon is determined by the difference between two energy levels, and the shortcut that hc takes the value 1240 ev*nm.  From there, I let the class work through a set of questions that could be asked about such an atom:

1. Photons of what energy can be absorbed by your atom?

2. Photons of what energy can be emitted by your atom?

3. What wavelengths of electromagnetic radiation are absorbed by your atom?

4. What wavelengths of electromagnetic radiation are emitted by your atom?

5. If an electron with no initial kinetic energy is captured by your atom, what wavelengths of EM radiation will be emitted?

6. The “Work Function” is defined as the minimum amount of energy necessary to kick an electron out of the atom from the ground state. What is the work function of your atom?

7. 7.0-eV photons are incident upon your atom. An electron in the ground state should be ejected from the atom. What will be
a. the ejected electron’s kinetic energy
b. the ejected electron’s speed
c. the incident photon’s speed

8. Assume all electrons in your atom start in the ground state. Your atom is illuminated with monochromatic 1300 nm IR radiation. What happens? (Describe any transitions that occur; if electrons are ejected from the atom, give their kinetic energy.)

9. Assume all electrons in your atom start in the ground state. Your atom is illuminated with monochromatic 620 nm red light. What happens?

10. Assume all electrons in your atom start in the ground state. Your atom is illuminated with monochromatic 200 nm UV radiation. What happens?

11. Your atom is illuminated with white light (visible wavelengths only, no IR or UV). What happens?

12. Your atom is illuminated with monochromatic 200 nm UV radiation, as in problem 10; this time, though, the radiation is more intense, i.e. the laser beam is brighter. What happens?

13. What minimum wavelength of incident light is necessary to ionize your atom?

14. Assume all electrons are initially in the ground state. 100 nm UV radiation is incident upon your atom. Electrons may be ejected from your atom. If so, they will have some kinetic energy.
a. What KE will these ejected electrons have?
b. I want to bring these electrons to rest using a potential difference. What “stopping voltage” is necessary to bring these electrons to rest?

I let the students use their texts and the 5 Steps book to try to figure these out.  I tend to help one person with each question; after that, I direct questions back to a student who has already been helped, so they teach each other. 
This exercise sure ain't perfect.  Usually, students who "get" atomic physics learn plenty from this exercise.  Those who don't "get" atomic physics from this exercise also don't "get" it through lectures or reading. 

11 April 2011

Mail Time: Electric field and potential inside a conductor

Georgian Michael Gray writes:
metal lattice representation from

I'm having a hard time with a rationale for number 19 on the 2009 MC exam. Can you help me?

Michael got a copy of the 2009 exam from last year's AP Summer Instititute.  It is also available for purchase from the College Board.  (That's why I can't post the text of the question here.)  Come to a workshop or institute, and I'll give you a copy of EVERY released exam question since 1979.
The question in question asks about the electric potential on two sides of a metal block.  The potential is V on the left side of the metal block; what is the potential on the right side?

So can I help?  Sure... but the answer depends on how deep you want to go.

At the truly introductory level, it's a fact of physics: metal objects have no electric field inside them, and are at the same electric potential everywhere. Charges can only reside on the surface of a metal object.

At the physics C level, you only need the last fact. Then you can use Gauss's law to show that a gaussian surface drawn inside the metal encloses no charge, such that there can be no electric field inside a conductor. Then potential is the spatial derivitive of electric field (i.e. voltage tells us how electric field changes). Since electric field is unchanging throughout metal, the potential must be the same everywhere but not necessarily zero.

And there's an even deeper answer, the one that discusses the periodic potential of the lattice of atoms in a metal and why electrons are free to move about the metal in the first place.  That's a graduate material science answer.

This is a HARD physics B question. I wouldn't worry if your students miss it.


08 April 2011

Loose Ends and Tidbits

from the University of Wisconsin website

Tonight my colleague Paul Vickers and I will be performing the "science demonstration" show for students who have been accepted to Woodberry for next school year.  It's a cross between a game show, improvisational comedy, and an explosion fest.  If you know of a boy between 7th and 9th grade who might be interested in coming to Woodberry Forest in a few years, stop by and see the show at 8:15.

Now's as good a time as any to post some loose ends and tidbits:

(1) I will be running three week-long summer institutes.  Please join me for one of them!

• NC State University in Raleigh, July 18-22
• UNC Charlotte in, um, Charlotte, July 25-29
• Manhattan College in, um, the Bronx*, August 1-5.

* Manhattan College is just north of the Bronx-Manhattan border.

(2) I spent a lot of time over spring break with Burrito Girl reading xkcd. Useful comics for the classroom include “Converting to Metric” and “Gravity Wells” but probably NOT “Numerical Sex Positions.”

(3) Alexis Brett has listed this blog in his “Top 100 High School Teacher Blogs.” I started writing online in 2005, before everyone and his dog had an online presence, and when there probably weren’t even 100 high school teacher blogs in existence. Alas, my “Nachoman’s Baseball” blog never acquired more than a small but devoted following.

(4) Listen in today, or any day there's a home game, to my audio broadcast of Woodberry Forest varsity baseball. 

07 April 2011

Get your students to do your grading for you

In my honors/AP level junior/senior class, our routine is well established.  Everyone does their homework on a nightly basis, knowing that most assingments will be graded.  The class has separated, too, into some top students who get new material and difficult problems quickly, and some lower-level students who struggle with every problem.

At this point in the year, everyone still needs to be doing physics problems on a regular basis.  It's like athletics -- once a football or basketball player works himself into shape, his workout routine can relax a bit, but he still must maintain his fitness.  Even my top students must maintain their physics fitness.  The lower-end students, though, need to continue to review and develop their understanding.

In the third trimester I offer "exemptions" from nightly work to those who earn them by doing well on fundamentals quizzes, or through consistent strong homework.  (I first described the exemption process in this post.)

I've never enjoyed keeping up with the necessary grading in an advanced physics course.  Now that everyone knows how the class works, I can pawn off that drudgerous task -- to students.  Those top students (defined for me as those who earned an A or an A- for the previous trimester) don't need to do the problems every night.  So I've offered each student, in turn, the opportunity to grade problems instead of doing problems.

A student who accepts the offer to grade receives a detailed rubric, a class roster, and a set of papers.  He grades strictly according to the rubric, returning the papers before school the next day.  I glance through his work, and then return the problems.  Easy!

The student grader loves this idea -- even though grading the homework usually takes more effort than doing the nightly problems, it's different effort.  Grading to a rubric is a new and challenging skill for an otherwised unchallenged top academic dog.  These top folks don't necessarily learning anything new by doing more problems; however, they develop an entirely different perspective on physics and physics teaching by having to grade.  (And oh, the heady power... they get to take off POINTS!)

Furthermore, student graders cause the class to be more focused on their problems.  They're so used to me grading their problems that they almost don't notice where they went wrong.  But if their peer graded the assignment, they often pay more attention to common mistakes.  Discussion about common issues goes on outside of class -- and the fastest way to improving physics understanding is to engage in frequent conversation about physics.

Before you dismiss this idea out of hand, many perceived obstacles to student grading can easily be overcome.  After all, an enormous number of teachers have "student aides" who help with filing and grading.  If you're concerned about privacy, have students write numbers rather than names.  If you're worried about perceived fairness, just pledge to glance over the grading and correct any mistakes.  (My students are generally more careful graders than I am, so fairness is not an issue.)  If you're worried about cheating, I don't know what to tell you... the student who grades is himself being evaluated on how well he follows the rubric.  It is in his interest to grade honestly.  Just don't let someone who has had any honor issues be the grader.

Does anyone else have students grade nightly homework to a rubric?  Post a comment and tell us how it works for you.

01 April 2011

Corrections on a full-length free response exam

I've detailed numerous times how I handle corrections for a standard in-class test -- basically, students earn back half the credit they missed.  On AP-style tests, the half-credit-back plus the square root curve basically turns 5s into As, 4s into Bs and B+s, the 3s into B-s and Cs+s.  On regular physics quizzes, (which are easier but on which the standard for success is higher), half-credit-back makes the standard 90-80-70 scale reasonable.

But what do Ido for an EXAM -- when grades for the term have already been issued, and/or when the exam is so long that the process of corrections isn't a night's assignment, but a major undertaking?

I gave a full-length, seven-question AP exam right before spring break.  Since we returned from break, I've only assigned one homework problem each night, but I've assinged the correction of one exam problem each night as well.  Those who did well on the exam simply have less homework.  Those who didn't do so hot have the time they need to pay careful attention to the things they missed.

I count the exam corrections as a test grade for the current term.  The game I play -- and everything associated with grades is, in fact, a game -- is that everyone starts with 100% on this "test."  Students correct the lettered parts of the free response problems that they missed.*  On each problem, a solid correction earns a "minus zero."  However, small mistakes might earn "minus one" and their grade for this test is down to a 99.  A major mistake might earn -5 or even in extreme cases -10.  The final test grade counts on the standard 90-80-70 scale, which means that missing 10 points cumulatively will drop the grade by one letter.

* I generally ask students additional questions so that they're not just copying a friend's right answer.  See this post for some examples, or search under "test corrections" on this blog.

I like this game because everyone has a reasonable opportunity to earn a high test grade with merely a solid effort.  Collaboration is allowed and encouraged on test corrections (as long as no one is merely copying), so there's not reason not to get most of the answers right.  Even the students who scored well on the original exam are motivated to do corrections properly so as not to lose a lot of easy points.  And the students who need to pay attention to their mistakes generally do so.