EGR 1011 Capacitors Chapter 12

EGR 1012 Capacitance – the ability of a component to store energy by accumulating charge A capacitor is a circuit component designed to store charge Practical applications with capacitors: Camera flash – Charges up and then quickly discharges Power storage – Solar collectors charge up capacitors so that energy can be used after dark Definition

EGR 1013 Capacitor Construction 2 Plates Separated by a Dielectric

EGR 1014 Variable Capacitors –Interleaved-Plate Capacitors

EGR 1015 Fixed Value Capacitors Polarized Electrolytic Capacitors Most electrolytic capacitors are polarized

EGR 1016 Capacitance Amount of charge that a capacitor can store per unit volt applied where C = the capacitance of the component, in Coulombs per Volt defined as Farad (F) [C] = [Q]/[V]=C/V = F. Q = the total charge stored by the component V= the voltage across the capacitor corresponding to the value of Q

EGR 1017 Capacitance Examples C =

EGR 1018 Unit of Measure – farad (F) = 1 coulomb per volt (C/V) Typical ranges –Most capacitors fall in the picofarad (pF) to microfarad (  F) range –Tolerance Usually fairly poor Variable capacitors used where exact values required

EGR 1019 Physically large capacitors usually have their values printed directly on the case Smaller capacitors are generally labeled using a code: –2-digit code: the number represents the value of the component in pF Example: 15 = 15 pF –3-digit code: the code is interpreted like the first three digits of a resistor code Example: 473 = 47 x 10 3 pF = 47 nF –The numbers 6 and 7 are not used as multiplier values –The numbers 8 and 9 are decoded as follows: 8 = 0.01 and 9 = 0.1 Example: 158 = 0.15 pF Capacitor Value Codes

EGR Physical Characteristics of Capacitors where C = the capacity of the component, in farads (F)  = permittivity of the dielectric A= the area of either plate, in square meters (m 2 ) d= the distance between the plates, in meters (m) A d What are the units of  ?

EGR Comparison to Resistance For resistance, R =  L/A For capacitance, C =  A/d As  increases, R increases; as  increases, C increases As L increases, R increases; as d increases, C decreases As A increases, R decreases; as A increases, C increases

EGR Permittivity Permittivity of a capacitor dielectric is  =  o x  r - Permittivity of a vacuum:  o = 8.85x F/m MULTIPLIED BY - The relative permittivity of the material  r e.g.: Material  r air1 paper2.5 mica5 glass7.5

EGR Team Activity 1 If you have a capacitor with the following parameters, what is its capacitance? Plate cross-sectional area = 1cm 2 Dielectric material = air distance between plates = 2cm What happens to the capacitance if you change the dielectric to oil and the distance between plates to 1cm? For the original dielectric material and plate distance, what would the cross-sectional area need to be to create a 1 F capacitor?

EGR Series Capacitors Where C T = the total series capacitance C n = the highest-numbered capacitor in the circuit

EGR Team Activity 2 Determine the total capacitance

EGR Parallel Capacitors where C n = the highest-numbered capacitor in the parallel circuit A1A1 A2A2

EGR Team Activity 3 Determine the total capacitance

EGR Demonstration ?parent=capacitor.htm&url= gnet.fsu.edu/electromag/java/capacitor/

EGR Relationship between Capacitor Voltage and Current Capacitor Current where i= the instantaneous value of capacitor current C = the capacity, in farads = the instantaneous rate of change in capacitor voltage i + _ vcvc

EGR Team Activity 4 If the voltage across a 2  F capacitor is what is the current through the capacitor?