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The Capacitor• The basic idea is pretty simple
– Imagine you have two parallel metalplates, both of which have equal andopposite excess charges
–
Plates are separated by an insulatinglayer (air, glass, wood, etc)
• The charges would love to balance out
• Insulator blocs them (!ust as the groundblocs you from falling into the center of the
earth)
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The Capacitor• If you were to connect a resistive
wire to the plates – Charges would "ow through the wire
• Charge "ow is current
• #nergy has been released as heat
•
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The Capacitor• %emember that a voltage is the
electrical potential between twopoints in space
•
&ere, we have an imbalance of charge,and thus an electric 'eld, and thus avoltage – ield strength is dependent on number and
distribution of charges as well as materialproperties
– ield length is dependent on sie of capacitor
– Capacitor sie and material properties lumped
into single *capacitance+ C
V=Q/C
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The Capacitor
• Thus, if you connect a voltage sourceto the plates – ie charges will move to get away from
the source• Charge "ow is current
• Current will stop once charges reachequilibrium with voltage source, i-e-
• #nergy has been stored
•
. / . / . /
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The Capacitor
. / . / . /
ots of current
0ero 1C
0ero current
1C213
ots of current
&igh 1C
0ero of current
0ero 1C
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Symbol:
Units: Farads (Coulombs/Volt)
Current-Voltage relationship:
or
Note: v c must be a ontinuous !untion o! time sine the
harge stored on eah plate annot hange suddenly
Capacitor
+
v c
–
i c
dt
dvC
dt
dQi cc ==
C C
(typial range o! "alues: # pF to # F$ !or %superapa-
itors& up to a !e' F)
+
Electrolytic (polarized) capacitor
C
These have high capacitance andcannotsupport voltage drops of the wrongpolarity
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4ode 1oltage withCapacitors
+
v c
–
i c
dt
dvC
dt
dQi c
c
==
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