# SJSU EE 140 - ch24_Capacitance (21 pages)

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# ch24_Capacitance

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## ch24_Capacitance

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Pages:
21
School:
San Jose State University
Course:
Ee 140 - Principles of Electromagnetic Fields
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Chapter 24 Capacitance Dielectrics Dr Ray Kwok SJSU Capacitance Dielectrics Dr Ray Kwok Goals for Chapter 24 To consider capacitors and capacitance To study the use of capacitors in series and capacitors in parallel To determine the energy in a capacitor To examine dielectrics and see how different dielectrics lead to differences in capacitance Capacitance Dielectrics Dr Ray Kwok Keep charges apart and you get capacitance C A o A Q Q V Ed d d o parallel plates Capacitance Dielectrics Dr Ray Kwok The unit of capacitance the farad is very large Commercial capacitors for home electronics are often cylindrical from the size of a grain of rice to that of a large cigar Capacitors like those mentioned above and pictured at right are microfarad capacitors Q o A C V d o is a very small number Capacitance Dielectrics Dr Ray Kwok Applications of Capacitors e g Computers Computers use capacitors in many ways Some keyboards use capacitors at the bases of the keys When the key is pressed the capacitor spacing decreases and the capacitance increases The key is recognized by the change in capacitance Capacitance Dielectrics Dr Ray Kwok Capacitance of a Spherical Capacitor Q E 4 r o 2 Q E 4 o r 2 b Q dr Q 1 1 V E dl 2 4 o a r 2 o b a Q Q 2 o C Q 1 1 1 1 V 2 o b a b a Capacitance Dielectrics Dr Ray Kwok Capacitance of a Cylindrical Capacitor L E 2 rL o E 2 o r b dr b V E dl ln 2 o a r 2 o a C C Q V L b ln 2 o a C 2 o L b ln a 2 o L b ln a Capacitance Dielectrics Dr Ray Kwok Circuit Symbols A circuit diagram is a simplified representation of an actual circuit Circuit symbols are used to represent the various elements Lines are used to represent wires The battery s positive terminal is indicated by the longer line Capacitance Dielectrics Dr Ray Kwok Capacitors may be connected one or many at a time In series same Q V V1 V2 In parallel same V Q Q1 Q2 Capacitance Dielectrics Dr Ray Kwok Calculations regarding capacitance One step at a time More examples Capacitance Dielectrics Dr Ray

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