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