February 10
CAPACITORS
1
A.
B.
C.
D.
E.
Great
Good
Not So Good
Bad
Don’t turn your back!

Today
◦ Examination I
◦ Complete Capacitors


Friday
DC Circuits
Play with batteries, wires and light bulbs
Watch for WebAssign
.
.
.
Next Week – More of the same
Q
C
VC
Real
World
Two pieces of metal
close to one another
but they don’t touch.
Charge has no place
to go!
Single Piece of Metal.
Not connected to
anything
+
+
+
+
+
+
+
electrons
-
electrons
+
+
+
+
+
+
+
electrons
q
+
+
+
+
+
+
+
-q
-
q
+
+
+
+
+
+
+
-q
-
Charge is the same for every capacitor in a series “string”
q
+
+
+
+
+
+
+
C1
-q
-
Wires are equipotentials … sort of.
q
+
+
+
+
+
+
+
C2
-q
-
V1
q
+
+
+
+
+
+
+
C1
-q
-
V1  V2  VT
q
+
+
+
+
+
+
+
C2
-q
-
VT
V2
V1  V2  VT
q
V1 
C1
q
V2 
C2
Important:
q
VT 
CT
q
q
q


C1 C2 CT
1
1
1


CT C1 C2
VT
QT Q1  Q2  Q3
CT 

 C1  C2  C3
VT
VT
Model
One Single
Conductor
Calculate the combined capacitance
C1=
C2=
C1||c2=
Measure=__
C1
C2
(C1+C2)series=
Determine the equivalent capacitance between A
and B for the group of capacitors in the drawing.
Let C1 = 14 µF and C2 = 5.0 µF.


A capacitor which
acquires a charge
of 1 coulomb on
each plate with
the application of
one volt is
defined to have a
capacitance of 1
FARAD
One Farad is one
Coulomb/Volt
q
C
V
or
q  CV
q
C
V
q  A   0 EA 
so
C

 0 AV
d

0 A
d
The capacitance of a
parallel plate capacitor
depends only on the
Area and separation
between the plates.
C is dependent only on
the geometry of the
device!
Coulomb 2 Coulomb 2
 0  

2
Nm
m  Joule
2
Coulomb

m  Coulomb  Volt
Coulomb Farad


m  Volt
m
and
 0  8.85  10 12 F / m  8.85 pF / m
DIELECTRIC
Some LOCAL ordering
Larger Scale Ordering
-
+
Net effect REDUCES the field
E0 
q
0 A
q  q ' E0
E

0 A

E
q
 0 A
and
q  q' 
q

C0 = Vacuum or air Value
C = With dielectric in place
C=C0