[Explained] Why Inductor block AC and Capacitor block DC?

There is a very important difference between the property of an inductor and a capacitor in that an Inductor block AC but pass DC whereas a capacitor block DC but pass AC. Now, the question is why and how they block or pass AC and DC? In this article, we are going to discuss about it.

Why Inductor Block AC?

The answer to this question can be given in many ways. Let's find the answer from the below equation. We know that an inductor has inductive reactance property by which it opposes the flow of current through it.

The equation of inductive reactance is, Xl = 2πfL

You can see in the above equation, frequency is there. So when we apply AC supply to the inductor, it will oppose the flow of current because as the AC has some frequency, there will be inductive reactance occur.

The opposition by the inductor due to the inductive reactance property is proportional to the supply frequency which means if the supply frequency increases the opposition also be increased. For this reason, an inductor can totally block the very high-frequency AC.

Also, Remember that An inductor can pass alternating current (AC) signals within a certain frequency range. The behavior of an inductor in an AC circuit depends on its inductance (L) and the frequency of the AC signal. At low frequencies, the reactance of an inductor (Xl) is relatively low, allowing AC current to flow through it with minimal opposition. 

Why Inductor Pass DC?

Now if we apply DC supply to an inductor, the inductive reactance will be zero because the frequency of DC is zero.

So, as the inductive reactance is zero it will not oppose the flow of current. So the inductor passes the DC current, of course, it will offer very small resistance to the flow of current which depends upon the internal resistance of the inductor material.

Why Capacitor Block DC?

Here also we will find the answer from the equation of the capacitor which is capacitive reactance. Capacitive Reactance is the property of the capacitor by virtue of which it opposes the flow of current.

The equation of capacitive reactance is, Xc = 1 / 2πfC

So, according to the above equation if we apply DC supply to the capacitor the capacitive reactance will be indeterminate(we can say very high) because the frequency of DC is zero.

So as the capacitive reactance is very high, the current cannot flow through the capacitor.

Also, Remember that In a DC circuit, once a capacitor is fully charged or discharged, it acts as an open circuit, effectively blocking the flow of DC current. This is because a capacitor stores electrical charge on its plates, and once the plates reach their maximum charge or are fully discharged, the capacitor does not allow further current to pass through.

However, during the transient phase when a DC voltage is initially applied or removed, a capacitor behaves as a temporary conductor. It allows current to flow for a short period until the voltage across the capacitor plates reaches the same level as the source or the voltage drops to zero.

Why Capacitor pass AC?

Now if we apply an AC supply to the capacitor, it will pass because AC has a frequency. 

The blocking property of the capacitor is inversely proportional to the frequency which means if the supply frequency increases, the capacitive reactance decreased so the capacitor will pass more current.

Read Also:

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