I recently wrote a post in The Basics area of this site discussing Equivalent Resistors. This is an incredibly useful tool for both simplifying circuits you are attempting to analyse and also for designing circuits which require a specific resistance which is unavailable using common components. I am happy to say there is a similar process which can be undertaken to simplify circuits made using capacitors. Further if you understand equivalent resistance you are already well on your way to applying these same tools to capacitors.
So what are the rules for reducing a capacitor circuit to an equivalent capacitance? They are the same as the rules we use for resistors however they are reversed. This means for parallel capacitors like those shown below the capacitance is the sum of the capacitors used.
And for capacitors in series the inverse of the equivalent capacitance (1/Ceq) is equal to the sum of the inverses of the capacitors (1/C1 + 1/C2 + 1/C3…).
I should say at this point there is an intuitive reason why these rules are reversed for capacitors however it relies on techniques which are slightly beyond the scope of this article. As we begin to explore phasor analysis and time dependent circuits we will return to this and develop a method which will allow us to combine capacitors, resistors and inductors into an equivalent impedance for a circuit.