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## 16 May 2013

### 2013 AP Physics B -- my solutions, and comments

I spent some of Burrito Girl's birthday* and then much of this morning writing solutions to the 2013 AP physics B exam.  (Scroll down to find them, or do a control-F on my name.  Chris Becke beat me to posting solutions by a few hours; don't be shy about using his solutions.)  You can access my solutions at the link, but it's through "pretty good physics secure."  You must have an account with PGP.

* Burriro Girl is my wife and sidekick.  My work yesterday was interrupted with a directive that physics time had ended, and Burrito Time had arrived.

Teachers can get an account with PGP by following the instructions at the PGP-secure homepage.  Students should ask their teacher to sign up.  PGP-secure is a wonderful resource, one that teachers will often use throughout the year.  It's worth signing up.

My comments:  The first question is similar to a famous old Hewitt question, in which he asks about the water level of a lake when a freighter full with iron cannonballs throws its merchandise overboard.  Excellent question.  Questions 2-5 and 7 are straightforward and interesting.  You can add the experiment described in #3 to your repertoire of laboratory exercises.

I'll note that question 5, the thermodynamics question, uses language in anticipation of AP physics 2.  No longer will the exam say "heat added to a gas."  Instead, you'll see phrases like "energy transferred into the gas by heating."

Question 6 is difficult but excellent.  A student really has to know about magnetic fields and forces in order to handle this one.  The last part, about the emf induced in a straight wire, is something I've never before seen on AP physics B; usually we discussed emf induced in a loop of wire.  But it's straightforward enough to use BLv to get the emf.  The side of the wire with higher electric potential can be determined by applying the right-hand-rule for magnetic force to the positive charges in the wire.

Please holler at me in the comments if you find a mistake, which you surely will.  I guarantee I got a 5, but not that I got 100%.

GCJ

1. Thanks for posting them! I posted my solutions also, directly beneath yours on PGP Secure. Our answers mostly agree, except on #6. I was really surprised they didn't put a literal problem in the free response this year, as I told my students they could pretty much expect one.

1. Thanks for posting your solutions on pgp. It is always fun to look at what others have worked out. I agree that #6 eii is tricky. I think that the correct answer is that the higher potential is at the right. Positive charge naturally moves from high to low potential, so one would want to say the higher potential is on the left, but these charges are not moving naturally. An object can be forced from low to high potential, example lifting, I believe that is the case here, the positive charges are being forced by the magnetic field. Since there are more positive charges now on the right that would be the region of greater potential.