Tankion

Meet Tankion, a capacitor and inductor pair who pass their energy back and forth.

Connect a charged capacitor to an inductor and current starts to flow. The capacitor's energy, stored in its electric field, moves into the inductor's magnetic field. Then the inductor keeps the current going and charges the capacitor the other way. The energy swaps back and forth at a natural frequency.

That frequency is set only by the inductance and capacitance. With L = 0.25 mH and C = 100 pF, f = 1 / (2π √(2.5 × 10⁻⁴ × 1.0 × 10⁻¹⁰)) ≈ 1.0 MHz, which is in the AM radio band.

In a real circuit, resistance turns some of the energy into heat on each swing, so the oscillations slowly die away unless something keeps topping them up.

Radios use this to tune in. The antenna picks up many frequencies, but the LC circuit responds strongly only to signals near its natural frequency. Turning the tuning knob changes the capacitance, which moves that frequency to pick a different station.