Capacitors & Capacitance Calculations
The property of capacitance that opposes current flow is exploited for the purpose of conducting signals with a higher frequency component while preventing signals of lower frequency components to pass. A common application of a capacitor is in an RF (radio frequency) circuit is where a DC bias voltage needs to be blocked from flowing through a circuit while allowing the RF signal to pass.
When used in series (left) or parallel (right) with its circuit compliment, an inductor, the inductor-capacitor combination forms a circuit that resonates at a particular frequency that depends on the values of each component. In the series circuit, the impedance to current flow at the resonant frequency is zero with ideal components. In parallel circuits (right), the impedance to current flow is infinite with ideal components.
Real-world capacitors made of physical components exhibit more than just a pure capacitance when present in an AC circuit. A common circuit simulator model is shown to the right. It includes the actual ideal capacitor with a parallel resistive component that responds to alternating current. The DC resistive component is in series with the ideal capacitor, and an inductor is connected in series with the entire assembly and represents the inductance of the component leads and plates.
Equations (formulas) for combining capacitors in series and parallel are given below. Additional equations are given for capacitors of various configurations.
As these figures and formulas indicate, capacitance is a measure of the ability of two surfaces to store an electric charge. Separated and isolated by a dielectric (insulator), a net positive charge is accumulated on one surface and a net negative charge is stored on the other surface.
In an ideal capacitor, charge would be stored indefinitely; however, real world capacitors gradually lose their charge due to leakage currents through the non-ideal dielectric.
Additionally, an inductive component is present due to metal leads (if present) and characteristics of the plate surfaces. This inductance, in combination with the capacitance, creates a resonant frequency where the capacitor looks like a pure resistance.