Here comes the introduction of passive components .

Capacitor are well-known passive electrical components, which can store electrical energy through rapid electric field-induced polarizations of dielectrics . When connecting into an electronic circuit, capacitor block direct current and allow alternating current to pass. 

Typically, capacitor show considerably high-power density of 100 kW/kg while relatively low energy density (<0.1 Wh/kg). For this property, capacitor have been widely used in analog filters, resonant circuits, and electric power transmission systems to control the output of electric field signals. One typical plate-type capacitor is schematically represented as Fig.  which is in the form of a multilayer structure, consisting of one dielectric layer and two conducting layers. 

Capacitor are fundamentally different from inductors in the phase angle direction. For a capacitor, the current wave is 90° before the voltage wave; for an inductor, the current wave is 90° after the voltage wave. The easiest way to remember this is in the word CIVIL (C, I before V, V before I in L), the standard symbol letter C stands for capacitor, L stands for inductor, I stands for current, and V stands for voltage.

In addition to these basic (and traditional) passive components, semiconductor diodes are also considered passive. It can be classified as a low reactance resistor, but its current resistance in one direction is much larger than the current resistance in the opposite direction. The arrow in the diode symbol is used to show the direction of low resistance current.