I've started with topics common to both the AP2 and APC exams - electrostatics, magnetism, and circuits. I've already posted algebra-based magnetism facts here, and electrostatics facts here. Then general circuits facts are here.
But I don't think I've ever posted capacitor facts. Here you go. The only Physics C extensions, then, are for RC circuits - I have facts that also talk about inductors and LR circuits, but I'll post those some other time.
Capacitors
Capacitors
store charge. The amount of charged
stored is Q = CDV.
Capacitance C is a
property of the capacitor.
In the special case of the parallel plate capacitor, its
capacitance is
The energy stored on a capacitor is 
In a circuit...
· * capacitors block all current after they’ve been
fully charged.
· * An uncharged capacitor initially will act as a
bare wire.
· * The equivalent capacitance of several parallel
capacitors is their sum.
· * Capacitors in series add inversely (like
parallel resistors) to their equivalent capacitance.
When capacitors are disconnected and reconnected, note what
stays the same:
- If a
battery is still connected, the voltage must remain the same.
- If a
battery is NOT connected, total charge must remain the same; however,
check the signs of the plates to see if some + and - charge will cancel
out.
- The
capacitance is a property of the capacitor’s structure, and will only
change if the area of the plates, the distance between the plates, or the
dielectric material is changed.
These are great! Do you have fact sheets for the other units covered in AP Physics II?
ReplyDeleteWell, um, I don't yet... you can find a bunch by googling on this blog. I have old fact sheets from physics b topics that are still useful. But I'm still working on putting together a formal fact sheet for physics 2.
ReplyDelete(The Physics 1 fact sheet is done and in the 5 steps book; the Conceptual Physics fact sheet is done, and I'll send a copy on request.)
Well, I'd say that capacitors, technically, store energy, not charge. A conventionally "charged" capacitor is, in fact, a neutral physical object (its overall charge is zero) - and herein lies the confusion of the students who, in most conventional courses, have learned basic electrostatics just before they got to the capacitors - so have a hard time understanding how a charged capacitor has zero net charge... Each plate of a capacitor does store charge, I suppose...
ReplyDeleteAm I sounding like a grouchy old man, Greg? ;-)
Boris, you’re correct, as always. I still will stick with my facts as easier to understand for a student’s first introduction to capacitors - emphasizing charge storage over energy storage helps get the idea of blocking current, and as you point out each plate does store charge. Not a grouchy old man, Boris - a technically correct one, perhaps, but never grouchy!
ReplyDelete