In Figure -9-3. At the instant the switch is closed , the capacitor appears as a short circuit to the source voltage since it is in an uncharged state. The capacitor charges up to its peak charge during the first quarter-cycle of the ac input voltage . At the same instant, the counter emf produced by inductor because of the charging current makes it appear as an open circuit to the source as well as to the capacitor
Once the charging current reaches its peak value, there is no further change in current , and the inductor now appears as a short circuit to the capacitor , which proceeds to discharges its stored charge into the inductor. The stored electrostatic charge of the capacitor flows through the inductor and is transformed into a stored electromagnetic energy in the inductor . The discharged capacitor now appears as a short circuit across the indictor. The inductor's magnetic field collapses and transforms its energy back to recharge the capacitor
This exchange of energy from the capacitor to the inductor and then back to the capacitor is called the oscillatory current . Since the oscillatory current is contained in the parallel LC circuit, The circuit is called a tank circuit
Because the oscillatory tank circuit voltage equal and opposes the source voltage, the source current is zero, and therefore, by Ohm's Law the impedance of the tank circuit is infinite.