Chapter 25 Capacitance
25-1 WHAT IS PHYSICS?
Physics Bprovide the basic science for practical devices
designed by engineers.
Capacitor Ba device in which electrical energy B stored.
The batteries B in a camera store energy in the photoflash
unit by charging a capacitor.
The batteries B supply energy at only a modest rate B too
slowly for the photoflash unit to emit a flash of light.
25-2 Capacitance
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Chapter 25 Capacitance
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Chapter 25 Capacitance
FIG.25-3 B (a) A parallel-plate capacitor B made up of two
plates of area A separated by a distance d. The charges on
the facing plate surfaces have the same magnitude q but
opposite signs.
FIG.25-3 B (b) As the field lines show B the electric
field B the charged plates is uniform in the central
region between the plates. The field Bis not uniform
at the edges of the plates, as indicated by the
"fringing" of the field lines there.
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Chapter 25 Capacitance
Plates are conductors, B equipotential surfaces Ball
points on a plate B at the same electric potential.
The charge B q Û V B proportional to each other Ð
The proportionality constant B C B the capacitance of
the capacitor.
The value of capacitance B depends only on the
geometry of the plates and not on their B q Û V
The capacitance B measures B how much charge the
plates to produce B potential difference between them:
The greater the capacitance, B the more charge is
required.
The SI unit of capacitance B from Eq. 25-1 B coulomb
per volt B farad (F) Ð
1 farad = 1 F = 1 coulomb per volt = 1 C/V.
(25-2)
Farad B very large unit B microfarad (1 µF = 10-6 F) B Pico
farad B (1 pF = 10-12 F) B convenient units in
practice.
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Chapter 25 Capacitance
Charging a Capacitor
Charging a Capacitor B place it in an electric circuit
with a battery.
Electric circuit B a path through B charge can flow.
Battery B a device maintains B potential difference
between B capacitor terminals
FIG.25-4 (a) Battery B B switch B S Û plates
h Û l of capacitor C, connected in a circuit.
h Û l B higher Û lower B potentials B + Û 146
Chapter 25 Capacitance
FIG.25-4 (b) A schematic diagram with the
circuit elements represented by their symbols.
The circuit in Figs. B 25-4a Û b B incomplete B S B
open;
S closed, B the circuit B complete B charge B flow
through B S to the wires.
The circuit of Fig. 25-4 B completed B electrons B
driven through B the wires by an E B the battery sets up
in the wires
E B drives electrons B from capacitor plate h to the
positive terminal of the battery
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Chapter 25 Capacitance
Plate h, B losing electrons B positively charged.
The field drives B many electrons from the negative
terminal of the battery to capacitor plate l B plate l,
gaining electrons B negatively charged
When the plates B uncharged BV B between them B
zero.
The plates oppositely charged, BV increases B equals
BV between the terminals of the battery. B C B fully
charged BV Û q Brelated by Eq. 25-l.
Read carefully
PROBLEM·SOLVING TACTICS
148