CAPACITANCEDC CircuitsThe two conductor plates of the capacitor, shown in Figure 11a, are electrically neutral, becausethere are as many positive as negative charges on each plate. The capacitor, therefore, has nocharge.Now, we connect a batteryFigure 11 Charging a Capacitoracross the plates (Figure11b). When the switch isclosed (Figure 11c), thenegative charges on PlateA are attracted to thepositive side of the battery,while the positive chargeson Plate B are attracted tothe negative side of thebattery. This movement ofcharges will continue untilthe difference in chargebetween Plate A and PlateB is equal to the voltage ofthe battery. This is now a"charged capacitor." Capacitors store energy as an electric field between the two plates.Because very few of the chargesFigure 12 Discharging a Capacitorcan cross between the plates, thecapacitor will remain in thecharged state even if the battery isremoved. Because the charges onthe opposing plates are attractedby one another, they will tend tooppose any changes in charge. Inthis manner, a capacitor willoppose any change in voltage feltacross it.If we place a conductor across theplates, electrons will find a pathback to Plate A, and the chargeswill be neutralized again. This isnow a "discharged" capacitor (Figure 12).ES-03 Page 10 Rev. 0
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