 If we apply an AC voltage to a capacitor of capacitance C, then an alternating current flows through the capacitor. The alternating voltage changes polarity with the frequency F. With this frequency, the alternating current also changes its direction. A capacitor to which an alternating voltage is applied has a complex non-armic resistance, which is called capacitive reactance. This resistance usually abbreviated with the letter X, with a small c. C stands for capacitor. You can easily calculate the capacitive reactance. You need the AC voltage frequency F and the capacitance C. X is equal to minus 1 over 2 times pi times F times C. Pi has the value of 3.14. The minus sign states that the alternating voltage lacks behind the alternating current. The unit of capacitive reactance is ohm. By the way, 2 pi times F is often combined to the angular frequency omega. If you use a very high AC frequency, the capacitive reactance becomes very small and the capacitor easily lets the current through. If, on the other hand, the AC voltage frequency is very low or even zero, that is if a DC voltage is applied, then the capacitive reactance becomes infinitely large. The capacitor does not let any current through. As you see, you can also use the capacitance to adjust the complex resistance of the capacitor. When we work with RMS values of voltage and current, we are usually only interested in the magnitude of the capacitor for reactance. We can therefore omit the minus sign. Let's make an example. You apply a voltage to a capacitor with a capacitance of 10 nanofarads. The 230 volts RMS voltage has a frequency of 50 hertz. Insert the values. Thus, the capacitive reactance is 318 kilo ohms. To determine the RMS current flowing through the capacitor, use the URI formula. But instead of using the ohmic resistance R, use the capacitive reactance. Rearrange for the current. Insert the 230 volts RMS voltage and 318,000 ohms. Then you get an RMS current of 0.7 milliamps.