The impedance of a component is the resistance of that component for AC voltages. The symbol for resistance is R; the symbol for impedance is X. The impedance of a capacitor is not zero; it depends on the capacity (size of the plates) and the frequency of the signal (number of polarity changes (forth and back) per second). The impedance can be calculated using the following equation.
f is the frequency in Hertz; C is the capacitance in Farad
Example: We have a 1nF capacitor and connect it to a 50Hz AC voltage source. Calculate the impedance of the capacitor.
XC = 1/(2∙π∙50∙10-9) = 3.18MΩ.