AC  power  supply X$_{L1}$  = 15 Ω X$_{L2}$  = 15 Ω X$_{L3}$  = 15 Ω Dielectric X Y 80  µ F  Capacitor AC generator 110V@400cps Figure 9-21.  Capacitor in an AC circuit. Figure 9-20.  Inductances in parallel. X$_{C}$ = 1 2πfC X$_{C}$ = 1 2πfC Capacitive Reactance 9-14 1.  The capacitance of parallel plates is directly  proportional to their area. A larger plate area produces  a larger capacitance, and a smaller area produces less  capacitance. If we double the area of the plates, there  is room for twice as much charge. 2.  The capacitance of parallel plates is inversely  proportional to the distance between the plates.  3.  The dielectric material effects the capacitance of  parallel plates. The dielectric constant of a vacuum is  defined as 1, and that of air is very close to 1. These  values are used as a reference, and all other materials  have values relative to that of air (vacuum). To find the capacitive reactance, the following equation: In  Figure 9-21,  a series circuit is shown in which the applied  voltage is 110 volts at 400 cps, and the capacitance of a  condenser is 80 mf. Find the capacitive reactance and the  current flow.  Where: X$_{C}$ = capacitive reactance in ohms, C = capacitance  in farads, f = frequency in cycles per second, and π = 3.1416. When an AC is applied in the circuit, the charge on the plates  constantly changes.  [Figure 9-21]  This means that electricity  must flow first from Y clockwise around to X, then from  X counterclockwise around to Y, then from Y clockwise  around to X, and so on. Although no current flows through  Capacitive reactance is a measure of how much the capacitive  circuit opposes the applied current flow. Capacitive reactance  is measured in ohms. The capacitive reactance of a circuit is  indirectly proportional to the capacitance of the circuit and  the applied frequency to the circuit. By increasing either the  capacitance or applied frequency, the capacitive reactance  decreases, and vice versa. This relationship is given as: Capacitance is a function of the physical properties of the  capacitor:  The basic unit of capacitance is the farad and is given by the  letter F. By definition, one farad is one coulomb of charge  stored with one volt across the plates of the capacitor. In  practical terms, one farad is a large amount of capacitance.  Typically, in electronics, much smaller units are used. The two  more common smaller units are the microfarad (μF), which is  10$^{-6}$ farad and the picofarad (pF), which is 10$^{-12 }$farad. the insulator between the plates of the capacitor, it constantly  flows in the remainder of the circuit between X and Y. As this  current alternates to and from the capacitor, a certain time lag  is created. When a capacitor charges or discharges through  a resistance, a certain amount of time is required for a full  charge or discharge. The voltage across the capacitor does not  change instantaneously. The rate of charging or discharging  is determined by the time constant of the circuit. This rate of  charge and discharge creates an opposition to current flow  in AC circuits known as capacitive reactance. Capacitive  reactance is symbolized by X$_{C}$ and is measured in ohms.  This characteristic of a capacitor may also create a phase shift  between voltage and current of the circuit. The phase shift  created by capacitive reactance always causes current to lead  voltage. That is, the current of a capacitive circuit reaches its  peak values before the voltage reaches peak values. Capacitance is the ability of a body to hold an electric charge.  In general, a capacitor is constructed of two parallel plates  separated by an insulator. The insulator is commonly called  the dielectric. The capacitor’s plates have the ability to store  electrons when charged by a voltage source. The capacitor  discharges when the applied voltage is no longer present and  the capacitor is connected to a current path. In an electrical  circuit, a capacitor serves as a reservoir or storehouse  for electricity. 