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12 February 2010

How and why to use the equation sheet

I gave a free repsonse test today.  The first problem was 1993 B2, about the electric field and potential due to point charges.  This is the easiest of such problems, I think, in the AP physics B annals.  The second problem was 2007 B2, a ranking task about circuits; this one makes my "top 5 AP physics problems of all time" list.  (That's a list I should publish some time on this blog.)  I haven't graded the third one yet, the easy 1998 problem with standing waves on a string. 

In those first two problems, at least seven off my 23 students used an incorrect equation.  Why is that so annoying?  Because the AP free response section, as well as today's test, provides an equation sheet!

Below is a note I wrote to the class's email folder.  I publish it here because I think it gives good advice about how to use the equation sheet, and perhaps implies the fundamental reason that the sheet is provided in the first place.

Look. You've got to memorize equations. We all know that. So, then, why do you even have an equation sheet?

The novice or dumb physics student hunts and pecks through the sheet. "The problem asked for a charge on the capacitor, and I know charge is Q. So, let's look for an equation with a Q in it. Here's one! ΔU = Q + W! Now I just need to plug in something or other for U and W." I know that youall aren't doing that, and I'm glad. (You would be shocked how many people are just this silly.)

However: You should never use an incorrect equation on a free response test! You've got the equation sheet right there. You should use it any time you're unsure of the exact form of an equation: "Huh, is it Q=CV, or C=QV? I don't remember. Let me look that up real quick." Or, "Is electric potential kQ/d, or d squared? I don't remember for sure, I should check."

Of course, I want you to know these things by heart. But when you know you're unsure, when you know you might have a brain fart under pressure, just use the sheet. That's what it's for.



  1. this is a general question, but do i have to simplify my answers (in terms of algebra or fractions) in order to get full credit on the ap physics c exam?

  2. You may leave unsimplified for full credit. But do be sure your answers are in terms of given variables only.

  3. thanks. so there is no point in simplifying my fractions, because it 1. takes time 2. i could simplify wrong? I'm talking specifically at this monster fraction in this problem (2007 AP C E &M #2, part a. iii.)