Question

In an ideal parallel $LC$ circuit, the capacitor is charged by connecting it to a $DC$ source which is then disconnected. The current in the circuit

• A. becomes zero instantaneously
• B. grows monotonically
• C. decays monotonically
• D. oscillates instantaneously

Answer 1

Answer: (D)

oscillates instantaneously

Answer 2

When capacitor is connected to a DC source and then disconnected, it gets charged and then it starts discharging through the inductor. When capacitor $C$ is fully charged to $q_0$ , the current in inductor $L$ is zero, at this instant an amount of energy $\frac{ 1}{ 2} \frac{ q_0^2 }{ C}$ is stored in electric field between the plates of the capacitor. When circuit is closed, the capacitor begins to discharge through the inductor causing current to flow. The energy or electric field between the capacitor plates has transferred to magnetic field By Lenz's law the dying magnetic field induces an emf in the inductance in the same direction as current. Hence, L-C circuit sets up oscillations.