Racism

I've been preparing for the physics C AP test partially by taking old free-response tests under simulated conditions. I've taken about 3 from the period 1999 to 2003, and I was saving the 2004 exams to take just before the real test to get the most accurate picture of my knowledge and where I need to add to it most. So, considering that the test is nearly upon me, I thought it a wise idea to take that test this evening. On the previous free-response sections I've been getting between 42 and 45 out of 45 points--outstanding results that gave me confidence. So I casually printed out the 2004 test, started the timer and breezed through the first question--an application of Gauss's Law to an infinite cylinder surrounding a line of charge--earning a full 15 points. So I moved on to the second question, involving a fairly simple RC circuit. The question was slightly odd though, giving the resistance of only one of the resistors, and asking for the voltage across the other before the branch containing the capacitor was connected. I was nervously stumped for about four minutes but then figured it out, earning an acceptable 14 out of 15 points, and finishing just as two-thirds of my time had elapsed.

And then there was the third question, a lovely example of electromagnetic induction, something that I'm usually quite good at. The problem involved an infinite wire of current I, next to which sat a rectangular loop of wire of given dimensions. The first part was rather long, asking to calculate the magnetic flux through the wire. Talk about a pain. This involved an integration of the magnetic field from Faraday's law over the dimension of the loop parallel to the wire. After an easy conceptual question, the next part asked for the current in the loop if the current in the wire was decreasing exponentially according to a given equation. By this time, even if I hadn't already made a mistake, the stupid accumulation of constants would have actually made it impossible to complete the problem on time correctly because each time I wanted to write a new line, I had to carefully keep track of about nine constants that were floating around. And finally, my personal favorite, finding the total energy dissipated by the loop, which involved integrating the expression for power dissipated from 0 to infinity, using the stupid expression with nine constants. So by the time I had finished botching this beast, I had written down about 55 constants. And since I had to write these stupid things down so many times, I gradually accumulated so many errors since I was so strapped for time, that my final answer didn't really resemble the correct answer at all by the time I arrived at the end of part d. My score? A paltrey 8 out of 15--this on a test in which you can usually get nearly half credit by simply writing down equations without evaluating anything. My final score? A lousy 38. Or at least I thought this was lousy until I looked up the average scores. On question 1: 6.34/15; question 2: 5.47/15; question 3: 3.78/15, for a grand total of 15.59/45. This test is an exercise in damage control.