he area of the conductors, the greater the capacitance of the device. (Remember Section 2.5.2.2 where we dealt with a wire pair.) Here we have two conductors separated by the insulation on each wire in the pair. Likewise, the longer the wire pair, the greater the capacitance it displays. The quantity of electricity stored by a capacitor is a function of its capacitance and the emf across its terminals. It is expressed by the relationship q=EC, (A.19) where q is the quantity of electricity in coulombs, E is the emf in volts, and C is the capacitance in farads. 5 The capacitance value is a function of the dimensions of the capacitor conductor plates. In the practical world, the farad is too large a unit. As a result we usually measure capacitance in microfarads (uF) or 1 X 10-6 farads, or picofarads (pF) or 10-12 farads. Restating Eq. (A.19), we obtain q=E0EA (A.20) where EO is the permittivity constant, 8.85 X 10-12 farads/meter or 8.85 pF/m; A is the area of the plate; and E is the emf. Figure A.12 A circuit model to illustrate capacitance. Coulomb (IEEE, Ref. 2) is the unit of electric charge in SI units. The coulomb is the quantity of electric charge that passes any cross section of a conductor in 1 sec when the current is maintained at 1 A. REVIEW OF FUNDAMENTALS OF ELECTRICITY WITH TELECOMMUNICATION APPLICATIONS Substituting q from Eq. (A.19) in Eq. (A.20) and noting that the capacitance is inversely proportional to the distance, d, between the parallel plates, we have C==o(A/d) (A.21) This tells us that capacitance is a function of geometry; it is directly proportional to the area of the capacitor plates and inversely proportional to the distance between the plates. This assumes that the insulator between the plates is air. Suppose the insulator between the plates is some other material. Table A.2 lists some of the typical materials used as insulators and their dielectric constant. The capacitance increases by a numerical factor K, which is the dielectric constant. The dielectric constant of a vacuum is unit