http://www.allaboutcircuits.com/vol_1/chpt_13/4.html The formula for series capacitance has been substituted with a formula for permittivity. Since an admin type person will likely respond to this, I'd like to ask another question about the capacitors chapter. Why doesn't the chapter talk about voltage division for capacitors in series, or current division for capacitors in parallel? This introduction to capacitors is good explanation. It's interesting to contrast it to an actual textbook. Thanks to you guys for an awesome online book.