^LC oscillator or tank circuit

When a charged capacitor is connected to a pure inductor & left for discharging, electromagnetic energy oscillates between capacitor & inductor in due to the sinusoidal variation of charge accordance with the relation with q = q₀ cosωt & .

LC oscillator is equivalent to oscillations of a block connected to a spring on a smooth horizontal surface with following analogy

In actual practice the oscillations are damped as every inductor has some resistance.