Impedance...?

Question

Can anyone explain why that when a capacitor and inductor with opposite impedances are placed it paralell, their effective impedance is infinte, and thus modeled as an open circuit.
I HAVE DERRIVED IT MATHEMATICALLY. BUT I CANT GRASP IT CONCEPTUALLY. LOOKING FOR A DESCRIPTIVE EXPLANATION.

Answer 1

They each carry equal current but they are 180 degrees out of phase. So draw a circuit diagram of this. At one instant the current is flowing clockwise then it reverses to counter clockwise. The current coming out of one is received by the other and vice-versa.

Answer 2

I don't know what you have derived there, but for sure it ain't correct. In an ideal LC parallel circuit there is only one frequency at which the two elements cancel: the resonance frequency. Everywhere else they don't. In addition, at resonance the system still does not act like an open circuit. An open circuit stores no energy. The LC at resonance stores C*V^2/2 because the capacitor is charged to the full voltage across the circuit when the driving voltage is max, while the current through inductor is 0 at that time.

So, no, the LC is never equivalent to an open circuit, not even when it looks like Z=infinity at its resonance point. You will notice this quite easily the first time you try to design a resonant switching converter... if you don't take the energy inside the LC into account it will show quite interesting firework effects.