Alternating-Current Circuits in Physics

Alternating current replaces one awkward problem with a more interesting one. The answer never settles, so there is no single current to solve for, only an amplitude and a timing. Resistance alone would be dull, but coils and capacitors respond to how fast the supply changes rather than how big it is, so a coil can block a slow alternation and let a fast one through while a capacitor does the reverse. That is the whole idea behind reactance. The blocks build the toolkit in the order it is used, amplitude and root mean square values, then reactance, then the combined impedance and phase of a series circuit, then the conditions where the two reactances cancel and the circuit behaves as though the coils and capacitors were never there.