A parallel plate capacitor consists of two parallel metal plates each of area A separated by a distance d.
Consider a metal plate P having area A. Let positive charge Q be the charge on insulated metal plate P, when it is fully charged and V be its potential then its capacity is given by,
C1 = Q/V. …(12.46)
Now another identical metal plate P2 is kept parallel to and near metal plate P then the inner surface of metal plate P2 acquires charge -Q and the outer surface of metal plate P2 acquires charge +Q by induction. [Fig. 12.12(a)]. The induced negative charge lowers the potential of plate P. while the induced positive charge raises its potential. However negative charge being nearer to plate Pj, there is a net decrease in the potential of plate P. Now if the outer surface of P2 is earth connected, free positive charge +Q on the outer surface of P2 escapes to earth. [Fig. 12.12 (b)1. Let -V1, be electric potential due to charge -Q on plate P2.
Hence net electric potential of system is + V +
Capacity in this case, C2=Q/V-V1 …(12.47)
Thus, C2 > C1 ..(12.48)
Thus, capacity of metal plate P1 is increased by keeping another identical earth connected metal plate P2 near it.
Such an arrangement consisting of two identical conductors, one of which is positively charged and the other earth connected is called a capacitor. Since the arrangement consists of two parallel metal plates, it is called a parallel plate capacitor.