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PHY108 - Chapter 25

# PHY108 - Chapter 25 - C hapte 25 C r apacitors and Die...

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Chapter 25: Capacitors and Dielectrics Questions to Answer o What is capacitance? o How do you determinethecapacitancefor a conductor or a systemof conductors? o What is a parallel platecapacitor and what is it’s capacitance? o What is theequivalent capacitanceof two capacitors connected in series? o What is theequivalent capacitanceof two capacitors connected in parallel? o How does a dielectric inserted between capacitor plates change thecapacitance? Why?

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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

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+ + + + + + + + + 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 πε πε πε = = = =
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

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The SI unit for the capacitance is the Farad This unit is too large for practical applications. We use instead the following subdivisions: 1 μ 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, ( μ F) or picoFarad (pF) ranges.

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PHY108 - Chapter 25 - C hapte 25 C r apacitors and Die...

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