Chapter_25

# Chapter_25 - 1 In this chapter we will cover the following...

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Unformatted text preview: 1 In this chapter we will cover the following topics: Capacitance of a system of two isolated conductors. Calculation of the capacitance for some simple geometries. Methods of connecting capacitors (in series, in parallel). Equivalent capacitance. Energy stored in a capacitor. Behavior of an insulator when placed in the electric field between the plates of a capacitor. Gauss’ law in the presence of dielectrics. Chapter 25: Capacitance Capacitance 2 Capacitance Capacitance A system of two isolated (from each other and from their surroundings) conductors, one with charge + q and other with –q, form a capacitor . The two isolated conductors in a capacitor are called plates , no matter what the geometry of the capacitor is. The symbol used to represent a capacitor is: When a capacitor is charged, its plates have charges of equal magnitudes but opposite signs: +q and –q . So the net charge of capacitor is zero. However, we refer to the charge of a capacitor as being q . Because capacitor plates are conductors, they are equipotential surfaces -- all points on a plate are at the same electric potential. Moreover, there is a potential difference between the two plates. This difference is usually denoted by V (rather than ∆ V ). 3 Capacitance (cont’d) Capacitance (cont’d) The charge q of a capacitor is proportional to the potential difference V : V C q = The proportionality constant C is called the capacitance . The C depends on the geometry of the capacitor plates. The greater is capacitance the more charge is required to achieve the same potential difference. The SI unit of capacitance is farad (F): 1 F = 1 C/V The figure below shows a parallel-plate capacitor , consisting of the parallel conducting plates of area A separated by a distance d . The electric field between the plates and away from the plate edges is uniform. Close to the plate edges electric field becomes non-uniform. 4 Batteries Batteries A battery is a device that maintains a constant potential V between two terminals. These are indicated in the battery symbol using two parallel lines of unequal length. The longer line indicates the terminal at higher potential while the shorter line indicates the low potential terminal. + _ V The constant potential difference V between two terminals of a battery is usually maintained by means of internal electrochemical reactions in which electric forces can move internal charge. 5-q-q +q +q Charging a Capacitor Charging a Capacitor One way to charge a capacitor is to place it in an electric circuit with a battery. An electric circuit is a path through which charge can flow. When the switch S is open the circuit is said to be incomplete. Plates of the capacitor are uncharged, and the potential difference between them is zero....
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## This note was uploaded on 10/07/2010 for the course PHY 108 taught by Professor Iashvili during the Spring '08 term at SUNY Buffalo.

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Chapter_25 - 1 In this chapter we will cover the following...

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