Capacitive Reactance

In an a.c. circuit, a capacitor produces a capacitive reactance which causes the current to lead the voltage by 90 degrees. Because the capacitor "reacts" to a changing voltage, it is known as a reactive component.

The opposition a capacitor presents to a.c. is called capacitive reactance (XC). The opposition is caused by the capacitor reacting to the changing voltage of the a.c. source.

The formula for capacitive reactance is:

In contrast to the inductive reactance, this equation indicates that the CAPACITIVE REACTANCE VARIES INVERSELY WITH THE FREQUENCY. When f = 0, XC is infinite () and decreases as frequency increases.

That is, the lower the frequency, the greater the capacitive reactance; the higher the frequency, the less the reactance for a given capacitor.

As shown in the illustration below, the effect of capacitance is opposite to that of inductance. Remember, capacitance causes the current to lead the voltage by 90 degrees, while inductance causes the current to lag the voltage by 90 degrees.

Effect of frequency on capacitive reactance.

(back) (top) (next) (return to tuned circuits page)