Capacitor/Inductor voltage and current

We learned that the impedance of a capacitor gets shorted as frequency gets higher. Why a capaictor can be modeled as short at high frequencies?

The answer is simple as we learned in Sophomore in a university. Since voltage cannot change abruptly, if a high frequency input voltage is applied, the voltage across the capacitor cannot follow the frequency change of an input voltage signal applied.

Thus, the voltage across the capacitor eventually won't change (move) much. This voltage can be thought as ~ short!!!! if it is seen by high frequency components - NO voltage change across the capacitor! It can be modeled as a short circuit!

In a same token, static voltage (DC) in ac signal analysis also can be modeled as a short circuit!

Let's recall what we learned about Capacitor/Inductor circuit in basic circuit class.

1) Capacitor circuit

2) Inductor circuit

Similarily, (but duality to a capacitor), the current cannot change instantly. Thus, the current lags the applied voltage!!

The current through the inductor is said to be lagged the applied voltage!

(Summary)

1. Capacitor --> voltage cannot change instantly --> voltage lags current by 90 degree

2. Inductor --> current cannot change instantly --> current lags voltage by 90 degree