Resonance (RLC circuits)
Resonance is what happens when a circuit's inductor and capacitor trade energy back and forth in perfect step at one special frequency, just as a child on a swing pumps highest when pushed at the swing's natural rhythm. At that resonant frequency the inductive reactance and capacitive reactance exactly cancel, leaving the circuit purely resistive and unusually responsive to signals at that pitch.
The resonant frequency is f₀ = 1/(2π√(LC)). A series RLC circuit drops to minimum impedance at f₀ (current peaks), while a parallel RLC circuit rises to maximum impedance (current is rejected) — which is why parallel tanks are used to select a station. How sharp the peak is gets summarized by the quality factor Q: high Q means a narrow, selective response that rings for a long time; low Q means a broad, gently damped one. Resonance is the heart of radio tuning, oscillators, antennas, and the EQ filters in audio gear.
At resonance the voltage across the inductor or capacitor in a series circuit can far exceed the source voltage (by a factor of Q) — a real hazard that surprises beginners and can destroy under-rated components.