PHY 2049: Physics II
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Tutoring Center is open in room NPB 1215,
M-F 12:00AM -4:00PM. It is free.
Hopefully all homework problems have been
solved. Please see me immediately after
the class if there is still an issue.
Exam I is on Friday May 30, 2008. It will be
here in class during class time.
PHY 2049: Physics II
An outline
 Capacitors, plate, coaxial, spherical..
 Energy, Dielectric strength, Dielectric
constant.
 Netweorks: Parallel and Series configuration.
PHY 2049: Physics II
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V  Ed 
d
o
A
C  o
d
Potential near a sheet charge
V ( z )    E.dz  Const

 V  z
o

V  V  d
o
Potential near a sheet charge
V ( z )    E.dz  Const

 V  z
o
Q
V  V 
d
o A
C  o A / d
PHY 2049: Physics II
PHY 2049: Physics II
 1
E (r )  
2o r
V r     E.dr


ln r
2o
Q
b
Va  Vb 
ln
2o h a
C  2o h ln( b / a)
PHY 2049: Physics II
q
Ek 2
r
q
V r   k
r
1 1
Va  Vb  kq  
a b
 ab 
C  4o 
  4o R
ba
PHY 2049: Physics II
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What is the capacitance of various familiar
balls.
C = 4πεoR = 10-10 R(m) F
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Basketball-it is an insulator
Van de Graff top
Earth (radius 6.4 X 106 m)
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CE = 0.71 mF
PHY 2049: Physics II
Energy stored in a capacitor
U = ½ CV2 = Q2/2C = ½ QV
 Dielectric Strength (oops I burned/broke it)
 Capacitor Enhancement by inserting a
dielectric:
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Replace εo by κεo
PHY 2049: Physics II
PHY 2049: Physics II
PHY 2049: Physics II
PHY 2049: Physics II
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Charging and discharging of capacitors.
http://edugen.wiley.com/edugen/instructor/
main.uni
PHY 2049: Physics II
25.20
PHY 2049: Physics II
Switch closed on left, capacitor C1 is being
charged
Q1  C1V  10 10V  100C
We have c2 = c3 = 20 μF in parallel.
The equivalent capacitor is then Ceq = 40
μF. When the switch is thrown to right,
the battery is disconnected, the charge is
shared between the capacitors C1 and
Ceq = 40. What is the charge on old C2
and C3? Is your answer 40 μC? What is
your answer if C2 = 10 μF and C3 = 30
μF?
V1  Veq
Q1 ' Qeq

 Q1 'Qeq  100 C
C1 Ceq
Q1 '  20C ,V1 '  2Volts
Qeq  80C ,V eq 2Volts.
PHY 2049: Physics II
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All about capacitors
Capacitance units Farad
C = 4 π κεo R
Energy stored dielectrics and strength
Parallel and series elements in a network