Question

In capacitative circuit the resistance is due to

Answer 1

Capacitor:

1. A capacitor is a system of conductors and dielectrics, which can store electric charge.
2. Usually, it consists of a pair of conductors containing equal and opposite charges $\left(\pm Q\right)$ with a potential difference $\left(V\right)$ between them, which is independent of the presence of other conductors.
3. The total charge of a capacitor system is zero but by convention, the charge on the positive conductor is called the charge on the capacitor.
4. The potential difference $\left(V\right)$ between the conductors is proportional to the charge on the capacitor.
5. This can be written as $Q=CV$ where $C$ is called the capacitance of the capacitor.
6. One farad is the capacitance of a capacitor, which requires $1C$ charge to establish a potential difference of one volt across it.

Capacitive Reactance

1. Suppose an ac voltage $v=V_0\sin \omega t$ is applied across a capacitor $C$.
2. The instantaneous charge on the capacitor is $q=Cv=C{V}_0\sin \omega t$.
3. The instantaneous current is given by $i=\frac{dq}{dt}=\omega C{V}_0\cos \omega t=I_0\sin \left(\omega t+\frac{\mathrm{\pi }}{2}\right)$, where $I_0=\frac{V_0}{{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\omega C$}\right.}}$ is the peak current.
4. The quantity $\frac{1}{\omega C}$ has the dimensions of resistance and is therefore called capacitive reactance.
5. It is a measure of opposition offered by $C$ to the flow of ac current through it.
6. Note that if $C$ is large then its reactance $\frac{1}{\omega C}$ becomes small.
7. So a large capacitance offers almost a short-circuited path for ac through it but offers an almost infinite resistance to the flow of dc.
8. So a large capacitor may be used to block the dc component and allow easy flow to ac components.
9. A capacitor used in this way is called a blocking capacitor.