Charging & Discharging of Capacitor

1. Capacitor is a device to store electric energy in the form of PD across its terminals.
2. It consists of two metallic plates having area (A) separated by small distance (d).
3. The medium between the two plates is called dielectric medium or dielectric.
4. If the dielectric medium is paper, it is called paper capacitor, if the medium is ceramic; it is called ceramic capacitor & so on. Thus the type of dielectric decides the name of capacitor.
5. The unit of capacity of a capacitor is Farad (F). But Farad is very large unit. So we use smaller units like mF, nF and pF.
6. The capacity of a capacitor is directly proportional to the area of its plates and inversely proportional to the distance between the plates: C is proportional to A/d
7. We charge a capacitor i.e. store energy in it and discharge it by using stored energy.
8. Remember that a capacitor ALWAYS charges or discharges exponentially. Thus it has exponential characteristics function during charging/discharging.
9. The capacitor (C) takes some time to charge/discharge through the resistor (R). This time (T) is given by: T=R.C
10. The charging/discharging graph is plotted with capacitor voltage versus time, as shown below. The voltage V0 is the maximum voltage across capacitor.
11. Observe the above graphs. Theoretically the capacitor charges up to 63.2% of battery voltage V and discharges up to 36.8% of its initial voltage V0.

Charing & Discharging graphs of a typical capacitor with respect to time axis
(V0 is its max. voltage)

Charing of Capacitor

Watch this video to understandd the charging process of a capacitor through a series resistor.

The simulation of charging of a capacitor through a resistor

Discharing of Capacitor

Watch this video to understand the process of discharging of a fully charged capacitor, into an LED through resistor R2. As shown in the circuit, the switch Sw2 charges the capacitor first and when Sw3 is on, it discharges into the LED.

The simulation of discharging of a capacitor through a resistor

So it can be practically proved that the total time of either charging or discharging of capacitor is approximately equal to the product of R & C i.e. T = R.C

The circuits shown above are simulated circuits and use ideal components. In our lab we tested all these circuits practically also using VTVM instead of DMM and found almost the same results. If you want to share your comments, please write it below.