Capacitor in series? Say you have two charged capacitors in series. Zoom in on one capacitor. For this specific capacitor, the charge on the two plates will be the same in magnitude, according to my textbook.
My teacher said that the charge on the two plates won't be the same if the gap between the the plates is large. In fact, the charge on each plate in a capacitor is never the same. They're only approximated to be the same if the gap is small.
Why is the charge not the same on both plates on a capacitor in series if the gap is large?

 A: It turns out that the electric field between the capacitor plates bulges out at the edges (the "fringing field").  If the arrangement of the external conductors is not symmetrical around the two plates, some of the field lines from one plate can "escape" from the capacitor structure and wind up terminating on an external conductor.  
Since field lines terminate on charges (via Poisson's law), this asymmetry in the field implies an imbalance in the plate charges.
The wider the plate spacing (relative to the plate length and width), the more field lines can escape, and so the greater the imbalance.
Note, however, that an asymmetry in the geometry is required; otherwise, the plate charges will balance regardless of their spacing, because the field lines will be symmetrical for a symmetrical geometry.  
Thinking about this "symmetry" aspect a bit more, I see that I've left something out.  I could imagine a third electrode that was geometrically symmetrical with respect to the two plates but that nevertheless unbalanced the electric fields because its potential was set asymmetrically (e.g. a "guard ring" encircling the plates that was set to the potential of one of the plates).  Only if the third conductor's potential is set midway between those of the plates is the balance restored.  So the symmetry must extend to the voltages of the conductors and not just their geometry.
