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PHYSICS 632 SUMMER 2008
9:00 – 10:50 Room 203
Electricity, Magnetism
and Light
Richard A. Lindgren, Office Room 302
Text: Halliday, Resnick, and Walker, 7’th edition, Extended Version,
Starting with Chapter 21 on Charge.
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Your Goals and Interest
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get a degree
crossover teaching
fill in knowledge gaps
review, learn new teaching ideas
peer learning
modeling
inquiry learning
group learning
new experiments and demos
computer technology
solidify concepts
learn how to do problems.
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Assignments
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See syllabus for daily homework assignments using WebAssign.
Work on problems in recitation after lecture.
Discuss with TA’s and others in the apartments at night. Due at 8:00 am
next day.
Algebra, trigonometry, unit vectors and vectors, derivatives and
integration review.
How do you improve problem-solving skills. Lots of practice and more
practice.
Quizzes on second three Tuesdays and final on WebAssign, 30%
conceptual questions, 70% problem oriented.
There will also be 4 homework assignments due in August and
September.
Final exam at the end of September on WebAssign.
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Class Organization
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Cartoon
Demonstrations
Lecture and discussion
Experiments and demos with explanations
Hand-worked problems on Elmo
Clicker/Polling problems
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References and other Texts
• Calculus-Based Physics II (download)
– Schnick
– http://www.anselm.edu/internet/physics/cbphysics/downloadsII.ht
ml
• Physics for Scientists and Engineers, 6th Edition
– Serway, Jewett
• Physics for Scientists and Engineers, 3rd Edition
– Fishbane, Gasiorowicz, Thornton
• Physics for Scientists & Engineers, 4th Edition
– Giancoli
• Principals of Electricity
– Page, Adams
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Lecture 1 Charge Ch. 21
• Electrostatics: study of electricity when the charges are not in motion. Good place to
start studying E&M because there are lots of demonstrations.
• Cartoon - Charge is analogous to mass
• Demo - Large Van de Graaff
• Math Review and Tooling up
• Topics
–What is electric charge? Point objects, Size. Atomic model
–Methods of charging objects. Friction,Contact, Induction, Machines
–Instruments to measure charge
–Quantization of charge and conservation of charge
–Coulombs Law and examples
–Principle of superposition and examples
• Demos
• Elmo
–Two pennies
–Three charges in a plane
–Hanging path ball
• Polling
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Charged Hair Van de Graaff Demo
• Need a female teacher to come forward.
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How does this gadget produce a mini-lightning bolt?
What upward forces are keeping your hair up?
How are these forces produced?
Why do the hair strands spread out from each other?
Why do they spread out radially from the head?
Is hair a conductor or insulator? How can we find out? Does it depend if
is wet or dry.
• To understand what is going on we need to know about charge and we
need a model of electricity.
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• “In the matter of physics, the first lessons should
contain nothing but what is experimental and
interesting to see. A pretty experiment is in itself
more valuable than 20 formulae.” Albert Einstein
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Charge
• What is charge?
• How do we visualize it.
• We only know charge exists because in experiments electric forces cause
objects to move.
• Charge is analogous to mass in mechanics. We know how it behaves, but we
don’t know what it really is. The same is true for charge..
• Need more experiments to get the point across.
• UVa Electrostatics Kit
(http://people.virginia.edu/~ral5q/classes/phys632/summer08/lecture_1.html)
• Rub a Balloon
• 2 x 4 on glass.
• Need a model
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More on charging using the UVa
Electrostatic Kit
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QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Bohr Model
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3 protons, 3 neutrons
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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More complicated than just protons,
neutrons, and electrons, but this will
suffice.
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Electron: Considered a point object with radius less than 10-18 meters with electric
charge e= -1.6 x 10 -19 Coulombs (SI units) and mass me= 9.11 x 10 - 31 kg
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Proton: It has a finite size with charge +e, mass mp= 1.67 x 10-27 kg and with radius
– 0.805 +/-0.011 x 10-15 m scattering experiment
– 0.890 +/-0.014 x 10-15 m Lamb shift experiment
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Neutron: Similar size as proton, but with total charge = 0 and mass mn=
– Positive and negative charges exists inside the neutron
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Pions: Smaller than proton. Three types: + e, - e, 0 charge and radius
– 0.66 +/- 0.01 x 10-15 m
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Quarks: Point objects. Confined to the proton and neutron,
– Not free
– Proton (uud) charge = 2/3e + 2/3e -1/3e = +e
– Neutron (udd) charge = 2/3e -1/3e -1/3e = 0
– An isolated quark has never been found
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• Normally atoms are in the lowest energy state. This means that
the material is electrically neutral. You have the same number of
electrons as protons in the material.
• How do we change this?
• How do we add more electrons than protons or remove
electrons?
• There are several different ways.
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3
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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More on our Model of electricity
Consider solid material like a piece of copper wire. The proton core is
fixed in position in a lattice like structure. In a conductor, the valence electrons
are free to move about. How many electrons are free to move about?
1cm long and a radius of 0,005 cm
Copper (Face Centered Cube) - good conductor
Copper atom:
Z=29(protons), N= 34(neutrons),
29 Electrons
Carbon or diamond - poor conductor
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Summary Comments
•Silk(+) on teflon(-)
•Silk (-) on acrylic (+)
•Wood doesn’t charge
•Charged objects always attract neutral objects
•Show Triboelectric series
•Not only chemical composition important, structure of surface is
important - monolayer of molecules involved, quantum effect.
(nanotechnology)
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Triboelectric series
http://www.sciencejoywagon.com/physicszone/lesson/07elecst/static/triboele.htm
Positive (Lose electrons easily)
Air
Human Hands
Asbestos
Rabbit Fur
Glass
Mic a
Acryli c
Human Hair
Nylon
Wool
Fur
Lead
Silk
Alumi num
Paper
Cotton
Steel
Wood
Amber
Seali ng Wax
Hard Rubbe r
Nickel, Copper
Brass, Silver
Gold, Platinum
Sulfur
Acetate, Rayon
Polyes ter
Styrene
Orlon
Saran
Balloon
Polyur ethane
Polypropy lene
Vinyl (PVC)
Silicon
Teflon
Negative (Gains electrons easily)
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Electrostatics is based on 4 four empirical
facts
• Conservation of charge
• Quantization of charge
• Coulombs Law
• The principle of superposition
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Conservation of charge
• Rubbing does not create charge, it is transferred from object to
another
• Teflon negative - silk positive
• Acrylic positive - silk negative
Demo: Show electronic electroscope with cage: gives magnitude
and sign of charge. Use teflon and silk to show + and -.
• Nuclear reactions
0 = e+ + e-
• Radioactive decay
238U
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= 234Th90 + 4He2
• High energy particle reactions e- + p+ = e- + p+ + n0
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What is meant by quantization of charge?
• Discovered in 1911 by Robert A. Millikan in the oil drop experiment
• The unit of charge is so tiny that we will never notice it comes in
indivisible lumps.
• Example: Suppose in a typical experiment we charge an object up
with a nanoCoulomb of charge (Q = 10-9 C). How many
elementary units, N, of charge is this?
Elementary unit of charge
e  +1.6  1019 C
Q
109 C
10
9
N 

0.625

10

6.25

10
e +1.6  1019 C
Six billion units of charge
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Coulombs Law
Lab Experiment
In 1785 Charles Augustin Coulomb reported in the Royal Academy Memoires using a
torsion balance two charged mulberry pithballs repelled each other with a force that is
inversely proportional to the distance.
m1m2
F=G 2
r
q1q 2
F=k 2 where k = 8.99  10 9 Nm 2 /C2
r
Also F=
q1
r
1
q1q 2
-12
2
2
where

=
8.85

10
C
/
N.
m
0
4p 0 r 2
Electrical Permittivity of vacuum
+
q2
Point charges
Spheres same
as points
+
Repulsion
+
-
-
Attraction
Repulsion
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Coulombs Law
Two Positive Charges
• What is the magnitude of the force between two positive charges,
each 1 nanoCoulomb, and 1cm apart in a typical demo?
q1
F=
F
q2
r

1 nC
1 cm
1 nC
kq1q 2
r2
10 c
10 m 
8.99  10 9
9
Nm 2
C2
2
2
2
 9  10 5 N
(Equivalent to the weight
of a long strand of hair)
1 nC
• What is the direction of the force?
1 cm
1 nC
Repulsion
What is the direction of the force in unit vector notation?
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• What is the direction of the force in unit vector notation?
1 nC
1
1 cm
1 nC
2
y
ĵ
iˆ
x
F1  9  10 5 N î
F
F2  +9  10 5 N î

8.99  10 9
Nm 2
C2

10 9 c
10 m 
2
2

2
 9  10 5 N
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Coulombs Law in vector notation.
v
q1 q2
F12  k
r
2
r̂12
F12
q1
.
.q
r̂12
2
r
where:
F12 is the force exerted by particle 1 on particle 2,
rφ12 is a unit vector in the direction from 1 to 2Σ, and
k, q1, and q2 are defined as before (the Coulomb constant, the charge on particle 1, and the
charge on particle 2 respectively).
F12


8.99  10 9
Nm 2
C2
10 c10 cr̂
9
10 m
2
2
9
12
 +9  10 5 N r̂12
The positive sign means the force on q2 is in the same direction as the unit vector
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How do you write the Force on q1
v
q1 q2
F21  k 2 r̂21
r
F21
.q
r̂21
1
.q
2
r
where:
F21 is the force exerted by particle 2 on particle 1,
r̂21 is a unit vector in the direction from 2 to 1Σ, and
k, q1, and q2 are defined as before (the Coulomb constant, the charge on particle 1, and the
charge on particle 2 respectively).
F21


8.99  10 9
Nm 2
C2
10 c10 cr̂
9
10 m
2
2
9
12
 +9  10 5 N r̂21
The positive sign means the force on q1 is in the same direction as the unit vector.
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Principle of Superposition
Three charges In a line
Example
Three charges lie on the x axis: q1=+25 nC at the origin, q2= -12 nC at
x = 2m, q3=+18 nC at x=3 m. What is the net force on q1 and the direction of
the force? We simply add the two forces keeping track of their directions.
r̂31
r̂21
2
1
x
3
r
r
Fnet ,1  F21 + F31
r
 q2
q1 q2
q1 q3
q3 
Fnet ,1  k
r̂ + k 2 r̂31  kq1  2 r̂21 + 2 r̂31 
2 21
r21
r31
r31
 r21


 8.99  10

 8.99  109
9 Nm 2
C2
Nm2
C2
 12  10 9 C
18  10 9 C 
(25  10 C) 
r̂21 +
r̂31 
2
2
(2m)
(3m)



(25  10
 2.25  107 N r̂21
9
9

C) 3  109 C r̂21 + 2  109 C r̂31

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(Directed in the positive x direction)
Uniformly charged metal spheres of Radius R
kq 2
F= 2
(r)
kq 2
F(r+2R)2
Demo: Show uniformity of charge around sphere using
electrometer.
Demo: Show charging spheres by induction using
electrometer
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Coulombs Law
Two Pennies without electrons
What is the force between two 3 gm pennies one meter apart if we remove
all the electrons from the copper atoms?
(Use Elmo)
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Force on one charge due to many charges lying in a
plane -I want to show you another notation.
Fnet  F12 + F13
F12 
N
Fnet   F1i
-q3
i2
kq1q2
r̂
2 12
(r12 )
r13
kq1q3
F13 
r̂13
(r13 )2
r̂13
r̂12
+q1
r12
+q2
-q3
F13
Fnet
Fnet
F13
F12
F12 +q1
+q2
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Example Three point charges in a plane
Use Elmo)
q3= - 2 nC
+y
5 cm
2 cm
1 cm
q1= + 1 nC
+x
q2= + 1 nC
Question: What is the net force on q1 and in what direction?
Hint : Find x and y components of force on q1 due to q2 and q3 and add
them up.
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In an atom can we neglect the gravitational force between the
electrons and protons? What is the ratio of Coulomb’s electric
force to Newton’s gravity force for 2 electrons separated by a
distance r ?
q
q
r
Fc =
m
kee
r2
m
r
Fg =
Gmm
r2
Fc ke 2
=
Fg Gm 2


19


8.99  10
1.6  10 C
Fc

2
31
Fg
6.67  10 11 Nm
9.1

10
kg
2
kg

9 Nm 2
C2

2
2
 4.2  10 42
Huge number - pure number - no units
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Why are neutral objects always attracted to positive
or negative charged objects.
For example:
•Rubbed balloon is attracted to wall
•Comb is attracted to small bits of paper
•Clothes in the dryer stick together.
Demo: Put wood on the spinner and place charged teflon
and plastic rods near it.
What is the explanation of all of these phenomena?
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Explanation: The neutral objects atoms and molecules orient
themselves in the following way so that the Coulomb forces due to attraction are greater
than those due to repulsion because the latter are further away. (Inverse square Law)
Acrylic Rod
Wooden block
F
Acrylic Rod
kq1q2
r2
Wooden block
Repulsion
Attraction
+
+
+
+
+
+
-
+
+
+
+
+
+
+
+
+
+
+
+
Attractive forces >> Repulsive Forces
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• Show induction: Click on
http://people.virginia.edu/~ral5q/classes/phys632/summer
08/lecture_1.html
-- Demo: Using conducting spheres and electrometer
– Demo: Electrophorus
– Demo: Electroscope
– Demo: hanging charged/conducting pith ball- first
attraction by induction, then contact, then conduction of
charge, then repulsion
Problem: Two equally charged hanging pith balls (Use Elmo)
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