The interaction between capacitance and frequency is governed by capacitive reactance, represented as XC. Reactance is the opposition to AC flow. For a capacitor: where: Capacitive reactance XC is inversely proportional to frequency f. As frequency increases, reactance decreases, allowing more AC to flow through the capacitor.
As the frequency applied to the capacitor increases, its effect is to decrease its reactance (measured in ohms). Likewise as the frequency across the capacitor decreases its reactance value increases. This variation is called the capacitor’s complex impedance.
Therefore, a capacitor connected to a circuit that changes over a given range of frequencies can be said to be “Frequency Dependant”. Capacitive Reactance has the electrical symbol “ XC ” and has units measured in Ohms the same as resistance, ( R ). It is calculated using the following formula:
In the capacitive characteristic region, the larger the capacitance, the lower is the impedance. Moreover, the smaller the capacitance, the higher is the resonance frequency, and the lower is the impedance in the inductive characteristic region. Our explanation of the frequency characteristics of capacitor impedance may be summarized as follows.
Frequency characteristics of an ideal capacitor In actual capacitors (Fig. 3), however, there is some resistance (ESR) from loss due to dielectric substances, electrodes or other components in addition to the capacity component C and some parasitic inductance (ESL) due to electrodes, leads and other components.
Capacitive reactance can be thought of as a variable resistance inside a capacitor being controlled by the applied frequency. Unlike resistance which is not dependent on frequency, in an AC circuit reactance is affected by supply frequency and behaves in a similar manner to resistance, both being measured in Ohms.