Capacitors in Series and Parallel

Capacitance depends on plate area and the gap between plates: more area or a smaller gap means more capacitance. Wire capacitors in parallel and you are effectively joining their plates into one bigger plate pair, same gap, more area. So the capacitances just add: C_eq = C₁ + C₂. Each capacitor also sees the full source voltage, so each stores its own charge independently, and the total charge is the sum.

Series is different. Put two capacitors in series and the same charge Q has to move onto every plate in the chain, because charge cannot cross the gap inside a capacitor, it only shifts off one plate and piles onto the next one through the wire. That shared Q splits the total voltage between the two capacitors, which behaves exactly like one capacitor with a wider effective gap, so capacitance drops. The combination rule matches resistors in parallel: 1/C_eq = 1/C₁ + 1/C₂, or product over sum for two capacitors. This is why series capacitance is always smaller than the smallest individual value, the opposite of what happens with resistors in series.

This topic sits right after single capacitors and charging behavior, and it feeds directly into energy calculations, since U = Q²/(2C) means rearranging the same capacitors between series and parallel changes the stored energy even though charge conservation rules differ in each case.