1-4
Figure 1-2.—Effect of frequency on inductive reactance.
Effect of Frequency on Capacitive Reactance
In an a.c. circuit, a capacitor produces a 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 figure 1-3, 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.
