 If we apply an AC voltage to a coil of inductance L, then an alternating current flows through the coil. The alternating voltage changes polarity with the frequency F. With this frequency, the alternating current also changes its direction. A coil to which an alternating voltage is applied has a complex non-armic resistance which is called inductive reactance. This resistance is usually abbreviated with the letter X with a subscript L. You can easily calculate the inductive reactance. You need the AC voltage frequency F and the inductance L of the coil. X is equal to 2 times pi times F times L. Pi is a mathematical constant with a value of 3.14. The unit of inductive reactance is ohm. By the way, 2 pi F is often combined to the angular frequency omega. If you use a very high AC voltage frequency, the inductive reactance will also be very high and the coil will not allow the current to pass. 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 inductive reactance also becomes zero. The coil does not impede the current at all which is equivalent to a short circuit. As you can see from the formula, you can also use the inductance to adjust the reactance of the coil. Let's make an example. You apply 230 volts to a coil with an inductance of 500 millihenry. The applied RMS voltage has a frequency of 50 hertz. Insert inductance and frequency into the formula. The inductive reactance is 157 ohms. To determine the RMS current flowing through the coil, use the u-re formula. Instead of using the ohmic resistance R, use the inductive reactance. Rearrange the equation for current. Insert the 230 volts RMS voltage and 157 ohms. Then you get an RMS current of 1.5 amps.