2.12 T HE P ARALLEL P LATE C APACITOR

. T HE P ARALLEL P LATE C APACITOR A parallel plate capacitor consists of two large plane parallel conducting plates separated by a small distance (Fig. . ). We first take the intervening medium between the plates to be vacuum. The effect of a dielectric medium between the plates is discussed in the next section. Let A be the area of each plate and d the separation between them. The two plates have charges Q and – Q . Since d is much smaller than the linear dimension of the plates ( d << A ), we can use the result on electric field by an infinite plane sheet of uniform surface charge density (Section . ). Plate has surface charge density σ = Q / A and plate has a surface charge density – σ . Using Eq. ( . ), the electric field in different regions is: Outer region I (region above the plate ), E σ σ ( . ) FIGURE . The parallel plate capacitor.