PHY108 - Chapter 25

PHY108 - Chapter 25 - Chapter 25: Capacitors and...

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Unformatted text preview: Chapter 25: Capacitors and Dielectrics Questions to Answer o What is capacitance? o How do you determine the capacitance for a conductor or a system of conductors? o What is a parallel plate capacitor and what is its capacitance? o What is the equivalent capacitance of two capacitors connected in series? o What is the equivalent capacitance of two capacitors connected in parallel? o How does a dielectric inserted between capacitor plates change the capacitance? Why? Chapter 25 Capacitors and Dielectrics In this chapter we will describe devices called capacitors. These have the ability to: 1. Store charge and 2. Store energy Capacitors are found in practically all electronic circuits In addition we will examine what happens when we place an insulator (also known as a dielectric ) inside a capacitor (25-1) Capacitance A capacitor consists of two conductors charged to +Q and -Q separated by an insulator. We can show that for any two such conductors the charge is proportional to the potential difference: The constant of proportionality is called the capacitance and is denoted by C: Q C V = [ ] Q C= V Q Coulombs Units: C Farads V Volts = = = +Q-Q 1 2 V V V =- V 1 V 2 Q ~ V + + + + + + + + + R E Q Capacitance of an isolated conducting sphere of radius R and charge Q Potential V on the sphere: Note: C is proportional to the radius R 4 4 4 o o o Q V R Q Q C R Q V R = = = = Capacitance Symbol: C Definition: Where Q is the charge on the capacitor plates and V is the potential difference between the two plates. The ratio is a constant Q C V Q V The SI unit for the capacitance is the Farad This unit is too large for practical applications. We use instead the following subdivisions: 1 F = 10 F 1 nF = 10 F The capacitance C is a constant that depends on: 1. The shape , size, and arrangement of the plates 2. The insulator between the plates Q C V 1 (1 ) 1 C F V = In general 1 Farad is an enormous capacitance. Typical capacitances are in the microFarad, ( F) or picoFarad (pF) ranges....
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PHY108 - Chapter 25 - Chapter 25: Capacitors and...

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