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17 June 2010
1
Chapter 25 – Capacitance
A
capacitor
is a device in which electrical
energy can be stored
251
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2
Capacitance
Basic elements of a capacitor
252
Parallelplate capacitor
17 June 2010
3
Circuit symbols for capacitor
Examples of capacitors
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The charge
q
on the plates is found to be proportional to the potential
V
for a capacitor
q CV
=
C
is called
the capacitance
of the capacitor with a value that is
determined by
the geometry
of the capacitor
SI
unit of
C
is
the farad
1
/
farad F coulomb per volt C V
= = =
Since
the farad
is a very large unit (we show this later), it is more
practical to use
submultiples
of the farad:
1 microfarad ( 1
μ
F
= 10
6
F
)
1 nanofarad ( 1
nF
= 10
9
F
)
1 picofarad ( 1
pF
= 10
12
F
)
17 June 2010
5
Charging a Capacitor
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Calculation of Capacitance
Consider the Gaussian surface shown
0
()
EA
q
A
ε
=
Assuming
is uniform and
and
are
parallel
f
i
fi
V V E ds
−
=
−⋅
∫
E
A
.
(
)cos180
E ds
Eds
Eds
=
°=−
B
V V V Eds
+
−
=
−=
∫
Choose a path whose direction is
opposite that of the electric field,
that is, from the
negative
to the
positive
plate in the figure
25 3
E
q
A
d
E
o
∫
=
.
17 June 2010
7
Apply Equations
(A)
and
(B)
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This note was uploaded on 09/30/2010 for the course PHYS MTH 203 taught by Professor None during the Spring '10 term at American University of Sharjah.
 Spring '10
 none
 Capacitance, Energy

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