Capacitive Reactance (G5A06)
The 2019-2023 General License question pool asks about capacitive reactance:
G5A06: How does a capacitor react to AC?
A. As the frequency of the applied AC increases, the reactance decreases
B. As the frequency of the applied AC increases, the reactance increases
C. As the amplitude of the applied AC increases, the reactance increases
D. As the amplitude of the applied AC increases, the reactance decreases
Reactance is the opposition to the flow of AC in a circuit due to capacitance or inductance. Alternating current coursing back and forth through a circuit gets some push back from capacitors and inductors due to the nature of those components. We’ll take a look at inductive reactance another time, but let’s consider the interaction of AC with a capacitor to ferret out the correct response to G5A06.
Consider the construction of a capacitor. It is a pair of conductive surfaces separated by an insulating layer. The surfaces can hold charge. What happens if we apply a direct current to a capacitor in a circuit, a one-way only voltage and current? The capacitor will charge, as follows:
Upon application of voltage a sudden surge of current will flow into the capacitor as it begins to charge.
As the capacitor charges over time to the voltage level that has been applied, the current flow will reduce due to the dwindling potential difference between the capacitor and the applied voltage.
As the capacitor reaches the applied voltage level and its maximum charge capacity, the current will drop to zero.
This DC application is depicted graphically here.
Imagine that the polarity of the DC voltage is suddenly reversed across the capacitor. The charge on the capacitor will begin to deplete, with voltage dropping to zero over time. During this time current will begin to flow in the new direction, and it will flow readily, increasing until the capacitor’s charge begins to build up with the new polarity. As before, the current will be reduced as the capacitor charges up to match the applied voltage, only this time in the opposite direction as the initial application.
With AC the scenario above is replicated again and again at the frequency of the applied voltage signal. The relationship between voltage and current in a purely capacitive circuit settles rapidly into a back-and-forth pattern depicted here.
Notice that at the maximum energy storage, or voltage on the capacitor, the current is at zero whether in the positive or negative direction. In this purely capacitive circuit the current is said to lead the voltage by 90 degrees of phase angle. (See our article Complex Impedance, Part 1, for an introduction to phase angles.)
Consider now what will occur when the frequency of alternation of the applied voltage is adjusted. If the applied frequency is rapid, sufficient time may not pass for the capacitor’s voltage to build to a level equaling the applied voltage before polarity reversal. The current can swing back and forth readily, never “bogging down” due to the lack of potential difference. The AC will flow with little opposition!
However, if the frequency of alternation of the applied voltage is low, the capacitor’s charge will easily match the applied voltage each cycle and cause the current flow to reduce and cease until the polarity reverses. In this case the AC experiences significant opposition!
This effect is called capacitive reactance, and capacitive reactance decreases with increased frequency. This effect also helps to explain how a capacitor can serve as, or contribute to, a “high pass RF filter.” Higher RF frequencies may pass easily through the capacitor circuit while lower frequencies do not.
The answer to General Class question G5A06, “How does a capacitor react to AC?” is “A. As the frequency of the applied AC increases, the reactance decreases”