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Saturday, 23 March 2019

Charging and Discharging of Capacitors

The process of storing electric charge in a capacitor is known as charging and the release of stored energy is known as discharging.

Charging and Discharging of Capacitor from D.C Voltage:

Let us consider a parallel plate capacitor connected to a battery through switch S. As long as the switch 'S' is in open position no current flows through the circuit and hence no charge across capacitor C.

As soon as the switch 'S' is closed there is a momentary flow of electrons from negative terminal of the battery to plate A. As there is a dielectric (insulating) medium between the plates of a capacitor these electrons can not flow through the dielectric to plate B. So these electrons will be accumulated on plate A only. At the same time the positive terminal of the battery attracts electrons from plate B leaving protons (positive charge) on plate B. Thus there will be surplus electrons on plate A and equal number of protons on plate B. So plate "A' will have negative charge and plate B will have positive charge. The opposite charges across the plates will have an associated potential difference, which is nothing but the voltage across the capacitor. This charging process continues until the capacitor voltage (Vc) equals to the battery voltage (V). During the process of charging the voltage across the capacitor Vc rises in an exponential raising fashion.

Once the capacitor voltage is equal to the applied battery voltage, the capacitor will then act as another voltage source even after the battery is removed.

The action of neutralizing the charge of a capacitor by connecting a conducting path across the capacitor is known as the discharging of a capacitor. If conducting path is provided across the capacitor, the surplus electrons will move from plate A to plate B and this will continue until the positive charge is neutralized by negative charge on the plates A and B. Then there is no net charge across the capacitor and now the capacitor is said to be completely discharged. Hence the voltage across capacitor V is zero. The discharging current is always in opposite direction to the charging current. The voltage across the capacitor discharges in an exponential decay fashion.

Charging and Discharging of Capacitor from A.C Voltage:

Let us consider a capacitor connected to a.c voltage as shown in figure (a). At point A, the capacitor starts charging and increases in the positive direction. The capacitor fully charges at point B as shown in Figure (b). As the charging voltage reaches its peak, at point B the current starts decreasing. So the charging voltage across the capacitor starts decreasing from B and is fully discharged at C as shown in Figure (c).

The same current and voltage variations are repeated during negative half cycle also. But the polarity of voltage and so the direction of current changes. There is a 900 phase difference between I and V. Capacitor again starts charging in opposite direction from C to D as shown in figure (d) and from D to E the capacitor discharges and fully discharged at E.

Charging and Discharging of Capacitor from AC Voltage
From the above discussion we can say that the capacitor charges and discharges alternatively during the complete cycle of the applied voltage. Even though no charge crosses the gap between the plates at any time, continuous alternating current flows through the circuit at all times and does not attain a steady state as in the case of application of D.C. That's why the capacitor has got the property to allow alternating current to flow through the circuit but to block or stop direct current.

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