Capacitors
Daniel Jensen
Purdue University
September 21, 2011
Daniel Jensen (Purdue)
Capacitors
September 21, 2011
1 / 12
Outline
1
Introduction
2
Parallel-plate Capacitor
Daniel Jensen (Purdue)
Capacitors
September 21, 2011
2 / 12
Administrative Stu
Personal Purdue website: http://web.ics.purdue.edu/∼jensend/
Homework hints 8 posted
CHIP
Homework 8 due Wednesday night
Homework 9 due Friday night
No i>clicker complaints until after exam 1
First Exam
Monday September 26th from 8-10 PM in Elliot Hall of Music
Multiple choice with about 12-17 questions
Practice exams
Questions during exams
Daniel Jensen (Purdue)
Capacitors
September 21, 2011
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Quiz Question
Two innite charged planes have charge densities +5σ and −3σ . If σ > 0,
what is the electric eld between the planes?
A) 4σ /ε0 directed to the left
B) σ / (2ε0 ) directed to the right
C) 4σ /ε0 directed to the right
D) 2σ /ε0 directed to the right
E) σ /ε0 directed to the right
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Capacitors
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Quiz Question
Exam 1 Spring 2005 - Problem 4
Two innite charged planes have charge densities +5σ and −3σ . If σ > 0,
what is the electric eld between the planes?
A) 4σ /ε0 directed to the left
B) σ / (2ε0 ) directed to the right
C) 4σ /ε0 directed to the right X
D) 2σ /ε0 directed to the right
E) σ /ε0 directed to the right
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Capacitors
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Quiz Question
Three very large non-conducting charged planes are parallel to each other
with the shown charge densities. If the plane at the bottom has a potential
of zero, what is the electric potential of the plane in the middle?
A) +σ εd0
B) −3σ εd0
C) −4σ εd0
D) −σ εd0
E) +2σ εd0
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Capacitors
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Quiz Question
Exam 1 Fall 2009 - Problem 6
Three very large non-conducting charged planes are parallel to each other
with the shown charge densities. If the plane at the bottom has a potential
of zero, what is the electric potential of the plane in the middle?
A) +σ εd0
B) −3σ εd0
C) −4σ εd0
D) −σ εd0 X
E) +2σ εd0
Daniel Jensen (Purdue)
Capacitors
September 21, 2011
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Capacitance
Denition and description
C
=
Q
∆V
Capacitance is a geometric quantity
How to measure capacitance
Place a charge on (usually) two conductors
Measure the potential drop between them
Daniel Jensen (Purdue)
Capacitors
September 21, 2011
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Quiz Question
A parallel-plate capacitor in a vacuum has a plate area of 0.2 m2 and a
plate separation of 0.1 mm. If the charge on each plate has a magnitude of
4 × 10=6 C the potential dierence across the plates is approximately:
A) 0
B) 4 × 10−2 V
C) 1 × 102 V
D) 2 × 102 V
E) 4 × 108 V
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Capacitors
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Quiz Question
Exam 1 Fall 2006 - Problem 3
A parallel-plate capacitor in a vacuum has a plate area of 0.2 m2 and a
plate separation of 0.1 mm. If the charge on each plate has a magnitude of
4 × 10=6 C the potential dierence across the plates is approximately:
A) 0
B) 4 × 10−2 V
C) 1 × 102 V
D) 2 × 102 V X
E) 4 × 108 V
Daniel Jensen (Purdue)
Capacitors
September 21, 2011
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Quiz Question
Capacitor C1 is connected alone to a battery and charged until the
magnitude of the charge on each plate is 4.0 × 10−8 C. Then it is removed
from the battery and connected to two other capacitors C2 and C3 , as
shown. The charge on the positive plate of C1 after it is attached is
reduced to 1.0 × 10−8 C. The charges on each plate of C2 and C3 are:
A)
q2
B)
q2
C)
q2
D)
q2
E)
q2
= 3.0 × 10−8 C and
= 2.0 × 10−8 C and
= 5.0 × 10−8 C and
= 3.0 × 10−8 C and
= 1.0 × 10−8 C and
Daniel Jensen (Purdue)
q3
q3
q3
q3
q3
= 3.0 × 10−8 C
= 2.0 × 10−8 C
= 1.0 × 10−8 C
= 1.0 × 10−8 C
= 3.0 × 10−8 C
Capacitors
September 21, 2011
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Quiz Question
Exam 1 Fall 2006 - Problem 4
Capacitor C1 is connected alone to a battery and charged until the
magnitude of the charge on each plate is 4.0 × 10−8 C. Then it is removed
from the battery and connected to two other capacitors C2 and C3 , as
shown. The charge on the positive plate of C1 after it is attached is
reduced to 1.0 × 10−8 C. The charges on each plate of C2 and C3 are:
A)
q2
B)
q2
C)
q2
D)
q2
E)
q2
= 3.0 × 10−8 C and
= 2.0 × 10−8 C and
= 5.0 × 10−8 C and
= 3.0 × 10−8 C and
= 1.0 × 10−8 C and
Daniel Jensen (Purdue)
q3
q3
q3
q3
q3
= 3.0 × 10−8 C X
= 2.0 × 10−8 C
= 1.0 × 10−8 C
= 1.0 × 10−8 C
= 3.0 × 10−8 C
Capacitors
September 21, 2011
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Quiz Question
A parallel plate capacitor is connected to a batter of unknown voltage for a
very long time. The capacitor is then disconnected and the separation
between its plates is increased by a factor of 3. It is then found that the
charge on the plates is ±35 µ C microcoulombs, while the nal capacitance
is 2000 × 10−12 F. What was the battery voltage?
A) 110 V
B) 70 V
C) 58 V
D) 5800 V
E) 1700 V
Daniel Jensen (Purdue)
Capacitors
September 21, 2011
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Quiz Question
Exam 1 Spring 2005 - Problem 3
A parallel plate capacitor is connected to a batter of unknown voltage for a
very long time. The capacitor is then disconnected and the separation
between its plates is increased by a factor of 3. It is then found that the
charge on the plates is ±35 µ C microcoulombs, while the nal capacitance
is 2000 × 10−12 F. What was the battery voltage?
A) 110 V
B) 70 V
C) 58 V
D) 5800 V X
E) 1700 V
Daniel Jensen (Purdue)
Capacitors
September 21, 2011
9 / 12
Quiz Question
What is the capacitance of the system shown in the gure?
d = 5.0 cm, κ1 = 4, κ2 = 6, b = 3 cm.
A
= 2 m2 ,
A) 3.01 nF
B) 2.12 nF
C) 4.57 nF
D) 1.63 nF
E) 3.54 nF
Daniel Jensen (Purdue)
Capacitors
September 21, 2011
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Quiz Question
Exam 1 Summer 2002 - Problem 12
What is the capacitance of the system shown in the gure?
d = 5.0 cm, κ1 = 4, κ2 = 6, b = 3 cm.
A
= 2 m2 ,
A) 3.01 nF
B) 2.12 nF
C) 4.57 nF
D) 1.63 nF X
E) 3.54 nF
Daniel Jensen (Purdue)
Capacitors
September 21, 2011
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Summary
Capacitance
Dielectrics
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Capacitors
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For Further Reading I
A. Author.
Handbook of Everything
Some Press, 1990.
S. Someone.
On this and that.
.
. 2(1):50100, 2000.
Journal on This and That
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Capacitors
September 21, 2011
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