Electrical Engineering ⇒ Topic : Capacitor Store Charge

Capacitor Store Charge 

Fig.(a) shows how a capacitor stores charge when connected to a d.c. supply. The parallel plate capacitor having plates A and B is connected across a battery of V volts as shown in Fig (i). When the switch S is open as shown in Fig.(i), the capacitor plates are neutral i.e. there is no charge on the plates. When the switch is closed as shown in Fig.(ii), the electrons from plate A will be attracted by the +ve terminal of the battery and these electrons start ***accumulating on plate B. The result is that plate A attains more and more positive charge and plate B gets more and more negative charge. This action is refened to as charging a capacitor because the capacitor plates are becoming charged. This process of electron flow or charging (i.e. detaching electrons from plate A and accumulating on B) continues till p.d. across capacitor plates becomes equal to battery voltage V. When the capacitor is charged to battery voltage F, the current flow ceases as shown in Fig. (iii).If now the switch is opened as shown in Fig. (iv), the capacitor plates will retain the charges.Thus the capacitor plates which were neutral to start with now have charges on them. This shows that a capacitor stores charge. The following points may be noted about the action of a capacitor.

                                                                  figure (a)

(i) When a d.c. potential difference is applied across a capacitor, a charging current will flow until the capacitor is fully charged when the current will cease. This whole charging process takes place in a very short time, a fraction of a second. Thus a capacitor once charged,prevents the flow of direct current.

(ii) The current does not flow through the capacitor i.e. between the plates. There is only transference of electrons from one plate to the other.

(iii)When a capacitor is charged, the two plates carry equal and opposite charges (say +Q and Q). This is expected because one plate loses as many electrons as the other plate gains.Thus charge on a capacitor means charge on either plate.