• Tags: choke, LC-oscillation, energy
• Difficulty: Moderate

A choke coil is an inductor with high inductance and negligible resistance, used to limit AC current without power loss. Unlike a resistor (which wastes energy as heat), a choke limits current via reactance $X_L$ = omega*L while dissipating (ideally) zero power. Used with fluorescent lamps and other AC devices. In a pure LC circuit (no resistance), energy oscillates between the inductor (magnetic energy = $LI^2$/2) and capacitor (electric energy = Q^$\frac{2}{2C}$) at frequency omega = 1/sqrt(LC). Total energy is constant: $LI^2$/2 + Q^$\frac{2}{2C}$ = $Q_0$^$\frac{2}{2C}$. This is analogous to a frictionless spring-mass system. The current and charge are 90 degrees out of phase. When Q is maximum, I = 0 and vice versa. In practice, some resistance always exists, causing damped oscillations.