Electric Fields and Capacitors 97 A N 9; C 3 10 10 F; A 40 30 10 6 m 2 ; εr 5 CANddANCεεεεoror farad metre()().111118 8540520021111108304260so mm d.Ans Worked Example 3.15 Q A parallel plate capacitor consists of 11 circular plates, each of radius 25 mm, with an air gap of 0.5 mm between each pair of plates. Calculate the value of the capacitor. A N 11; r 25 10 3 m; d 5 10 4 m; ε r 1(air) ArACAnd223 23225096350 metreso, m or().()1111εεffaradthus, 8 85409635005034802340...11111111CFF or pF 348Ans 3.16 Energy Stored When a capacitor is connected to a voltage source of V volts we have seen that it will charge up until the p.d. between the plates is also Vvolts. If the capacitor is now disconnected from the supply, the charge and p.d. between its plates will be retained. Consider such a charged capacitor, as shown in Fig. 3.21 , which now has a resistor connected across its terminals. In this case the capacitor will behave as if it were a source of emf. It will therefore drive current through the resistor. In this way the stored charge will be dissipated as the excess electrons on its negative plate are returned to the positive plate. This discharge process will continue until the capacitor becomes completely discharged (both plates electrically neutral). Note that the discharge current marked on the diagram indicates conventionalcurrent ﬂ ow.