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C CAPACITORS

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Capacitors store electrical charge. Generally, that charge is stored physically, as equal quantities of positive and negative charge are separated on opposite faces of an insulating material. When the two faces are connected by an external current path, current flows until complete charge balance is achieved. The capacitor can then be returned to its charged state by applying voltage. | C CAPACITORS Contents Overview Electrostatic Solid-State Capacitors Electrochemical Double-Layer Capacitors Electrochemical Double-Layer Capacitors Carbon Materials Electrochemical Capacitors Ionic Liquid Electrolytes Electrochemical Hybrid Capacitors Electrochemical Metal Oxides Capacitors Electrochemical Polymer Capacitors Application Overview JR Miller JME Inc. Shaker Heights OH USA 2009 Elsevier B.V. All rights reserved. Capacitor Basics Capacitors store electrical charge. Generally that charge is stored physically as equal quantities of positive and negative charge are separated on opposite faces of an insulating material. When the two faces are connected by an external current path current flows until complete charge balance is achieved. The capacitor can then be returned to its charged state by applying voltage. Because the charge is stored physically with no chemical or phase changes taking place the process is highly reversible and the discharge-charge cycle can be repeated over and over again virtually without limit. Metrics and Definitions Charge The quantity of charge Qstored in a capacitor is equal to device voltage V times a proportionality constant C called capacitance Q cv 1 with capacitance in farads for charge Qin coulombs and voltage in volts. Charge Qin coulombs is related to the current i in amperes by Q it 2 where t is the time in seconds. Thus with charging at constant current io the capacitor voltage increases linearly with time t V C t 31 Similarly the voltage declines in a linear manner during constant current discharge. Voltage When charging a capacitor using a constant voltage source Vo the voltage increases exponentially with time v Vo t 1 exp . RsC 4 587 588 Capacitors I Overview 4 Separation d Figure 1 Simple electrostatic capacitor formed by separating two parallel conductors by a distance d. A dielectric material is commonly placed between the conductors to increase the capacitance. where Rs is the total series resistance in the .