Capacitor potential difference convention in the algebra-based intro physics classroom

6698583231_2e44f43993_zI have long felt kind of “eh” about teaching about capacitors. Okay, so they store charge, that’s great. But I’ve come to realize that they, like so many subtopics in intro physics (okay, really, all of them), offer students a chance to make sure they understand one or more core principles. For capacitors, those are electric field, electric potential difference, and electric potential energy. However, they still have their sticky points, especially when it comes to conventions in textbook.

Today in a review session some questions came up about capacitors. The conundrum landed upon potential difference as defined by V-final – V-initial or just the absolute value of the difference (or something else)? Here are the notes I wrote up for my students. Note that we are using Knight’s College Physics. It all comes down to delta-V overall being the expected difference of the final and initial conditions, while delta-V for a capacitor being defined as potential as V+ – V- (or difference of potential at positive side of the capacitor  and the potential at the negative side).



Image Creative Commons licensed / Flickr user Arduino Labs