Lecture 9 Ch25

# Lecture 9 Ch25 - PH 222-3A Spring 2007 222 3A Capacitance...

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H 222- A Spring 2007 PH 222 3A Spring 2007 Capacitance Lecture 9 Chapter 25 (Halliday/Resnick/Walker, Fundamentals of Physics 8 th edition) 1

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Chapter 25 Capacitance In this chapter we will cover the following topics: - apacitance f a system of two isolated conductors. Capacitance C of a system of two isolated conductors. -Calculation of the capacitance for some simple geometries. -Methods of connecting capacitors (in series, in parallel). quivalent capacitance -Equivalent capacitance. -Energy stored in a capacitor. -Behavior of an insulator (a.k.a. dielectric) when placed in the electric field created in the space between the plates of a capacitor. -Gauss’ law in the presence of dielectrics. 2
A system of two isolated conductors, one with a charge Capacitance V + V - and the other - , separated by an insulator (this can be vacuum or air) is known as a "capacitor." The symbol used to indicate a c qq + apacitor is two parallel lines. We refer to the conductors as "plates." We refer to the "charge" of the capacitor as the absolute value of the arge on either plate q = charge on either plate. As shown in the figure, the charges on the capacitor plates create an electric field in the surrounding space. The electric potential C V q q = CV of the positive and negative plate are and , respectively. VV +− We use the symbol for the potential difference between e plates ( would be more appropriate) V V O the plates ( would be more appropriate). V Δ If we plot the charge as a function of we get the straight line shown in the figure. The capacitance is defined as the ratio . / q q V C V We define a capacitor of 1 F a C = SI Unit : Farad (symbol F) s one that acquires a charge = 1 C if we apply a voltage difference 1 V between its plates. qV = 3

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Parallel Plate Capacitor A parallel plate capacitor is defined as a capacitor made up from two parallel lane plates of area separated by a dis nce The electric field between the d plane plates of area separated by a distance . The electric field between the plates and away from the plate edges is Ad uniform. Close to the plates' edges the electric field (known as "fringing field") becomes nonuniform. A battery is a device that maintains a constant potential difference etween its two terminals These are ind ated in the battery symbol V Batteries + - V between its two terminals. These are indicated in the battery symbol using two parallel lines unequal in length. The longer line indicates the terminal at higher potential while the shorter line denotes _ the lower-potential terminal. 4
-q +q One method to charge a capacitor is shown in fi Wh th it h S i l d th l t i Charging a Capacitor the figure. When the switch S is closed, the electric field of the battery drives electrons from the battery negative terminal to the capacitor plate connected to - +q it (labeled " " for low). The battery positive terminal removes an equal number of electrons from e plate connected to it (labeled"" fo high) A q q the plate connected to it (labeled for high).

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## This note was uploaded on 09/26/2008 for the course PH 222 taught by Professor Mirov during the Spring '08 term at University of Alabama at Birmingham.

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Lecture 9 Ch25 - PH 222-3A Spring 2007 222 3A Capacitance...

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