AC Components and Circuits
Section 6-2 Capacitive Reactance
Capacitive reactance is the opposition to alternating or pulsating current caused by a capacitor.
Capacitive reactance is similar to resistance in the sense that it opposes current flow. One of the major differences, however, is that the value of capacitive reactance changes with the applied frequency--resistor values do no.
- XC = Capacitive reactance in Ohms
- f = Frequency in Hertz
- C = Capacitance in Farads
|These curves show how capacitive reactance changes with frequency and the value of the capacitor, itself. |
All other things being equal, the first diagram shows that XC rises very rapidly with decreasing frequency. In fact, XC approaches infinity as the applied frequency approaches 0 Hz, or direct-current. XC approaches zero as the frequency increases.
Again, all other things being equal, the second diagram shows that larger values of capacitance lower the reactance: Very small capacitors provide high values of XC, whereas larger capacitors tend to show lower reactance.
Endless Examples & Exercises
|Given the value of an capacitor and the frequency of the applied sinusoidal waveform, calculate the amount of capacitive reactance. || |
Variations of the XC Equation
Solve for the value of the capacitor: C =
| 2pfXC |
This is a design situation where you need to select a capacitor that will provide a known amount of reactance at a certain frequency.
Given values for f and XC, calculate the value of the capacitor.
Solve for the frequency: f =
| 2pCXC |