This is a lesson on understanding what Reactance is. Electrical reactance is the resistant build up from either inductance or capacitance. Where this is used is in coils that tend to carry AC energy through them, sometimes DC but mainly through my own experience AC.
These coils tend to be used for transformers and inverters either DC-AC or AC-DC, these are the ways that we are able to gain power from a mass quantity of energy and be able to put it into small amounts of direct energy that can be used to power our phones and computers.How your average iPhone charger looks without the case:
Now when you first see it, it looks like a bunch of smashed in capacitors but if you look to the left of the picture and notice the green thing that is actually a small inductor coil that is in the first part where the AC energy goes through. Then throughout the array of capacitors and electronics it finally enters into the USB plug on the right of the picture. Same applies to your computer charger:
f= Resonant frequency=
Resonant frequency is the natural sound vibration that tends to sing throughout the coils.