HR25_post - Chapter 25 Capacitance Will cover the following...

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Chapter 25 Capacitance Will cover the following topics: • Capacitance ( symbol C ) of a system of two isolated conductors Calculation of the capacitance for some simple geometries Methods of connecting capacitors (in parallel, in series) 1 Equivalent capacitance Energy stored in a capacitor Behavior of an insulator (aka dielectric ) when placed in the electric field between the plates of a capacitor Gauss’ law in the presence of dielectrics
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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: Capacitance A system of two isolated conductors separated by an insulator (e.g. air, vacuum), one with charge + q and other with –q, is known as a capacitor 2 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 q as the charge of a capacitor 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 )
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The charge q of a capacitor is proportional to the potential difference V CV 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 Capacitance (cont’d) 3 Figure below shows a less general, but more conventional arrangement called a parallel-plate capacitor consisting of the parallel conducting plates of area A separated by a distance d Electric field between plates and away from the plate edges is uniform. Close to the plate edges electric field (known as “fringe” field) becomes nonuniform
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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 Batteries 4 between two terminals of a battery is usually maintained by means of internal electrochemical reactions in which electric forces can move internal charge Batteries can be used to charge a capacitor if they both are placed in an electric circuit An electric circuit is a path through which charge can flow
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Consider electric circuit shown in the Fig. 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 When the switch S is closed, the circuit is complete – charge can flow through the switch and wires. The electric field of the Charging a Capacitor +q q 5
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