Lecture 1-1
Physics 241: Electricity and Optics
All Lectures take place in Rm. PHYS112
Lecture 0101:
9:30 – 10:20 (T,Th)
Prof. Wei Xie (wxie@purdue.edu)
Office: PHYS Room 246
10:30 – 11:20 (T,Th)
11:30 – 12:20 (T,Th)
Prof. Rolf Scharenberg (schrnbrg@purdue.edu)
Office: PHYS Room 245
Course Web Page:: http://www.physics.purdue.edu/phys241/
CHIP website: http://chip.physics.purdue.edu/public/241/spring2010/
Lecture 0201:
0301:
Textbook: Physics for Scientists and Engineers (6th edition ),
volume 2 or newer by Paul A. Tipler and Gene Mosca
Exams: 2 evening one-hour exams and a two-hour final exam
Quizzes: during lectures using iClickers.
Homework: CHIP (26 assignments)
Lecture 1-2
An Old (and Fundamental!) Question:
What is the Structure of Matter in the Universe?
Lecture 1-3
Structure of matter in the Universe
Wood
Huge scale
Iron
Universe
Gravitational
Current building block
scale ~ 10-10 m
Electromagnetic
Leptons:
electron,
muon,
etc
scale ~ 10-15 m Strong
Lecture 1-4
Quantization of Charge
• Fundamental unit: elementary charge e
q  ne, n  1,  2,  3, 
• An electron carries a charge of –e ; a proton carries
a charge of +e
It is typically the electrons that move
between objects.
• Coulomb (C): one coulomb is the amount of charge
that is transferred through the cross section of a wire
in 1 second when there is a current of 1 ampere in the
wire.
e  1.602177  1019 C  1.60 1019 C
Lecture 1-5
Conductor vs. Insulator
• Conductors: material in which electric charges can
move around “freely”
Cu 1029 / m3
– Metals, tap water, human body, …
• Insulators: material in which electric charges are
“frozen” in place
– Air, glass, plastic, …
• Semi-conductor: material in which electric charges
can move around but not as freely as in conductors
– Silicon, germanium, …
Ge 1019 / m3
Lecture 1-6
Conservation of Charge
The net electric charge is conserved in any physical process.
But …
• Charge can be transferred from one
object to another.
Zn( s)  2 H  (aq)  SO42 (aq)
 Zn 2  SO42 (aq)  H 2 ( g )
• Individual charges can also, in fact, be
“destroyed” or “created”, but not net charges


e e  
( annihilation)
( pair production)
Lecture 1-7
Coulomb’s Law
• Charges with the same sign repel each other, and charges
with opposite signs attract each other.
• The electrostatic force between two particles is
proportional to the amount of electric charge that each
possesses and is inversely proportional to the distance
between the two squared.
q1q2
F1,2 k 2 rˆ1,2
r1,2
 r1,2
by 1
on 2
q1
q2
r
• Coulomb constant:
k
1
4 0
 8.99  109 N  m 2 / C 2
where 0 is called the permittivity constant.
Lecture 1-8
Warm-up Quiz
• One known charge Q1 = Q > 0 and the other
unknown positive charge Q2 > 0 are held fixed at a
separation d = R as shown.
• Another (non-zero) charge Q3 is introduced
somewhere along the line connecting Q1 and Q2.
– Which of the following statements is true?
1. The force on Q3 can be zero only if Q3 is placed to the left of Q1.
2. The force on Q3 can be zero only if Q3 is placed between Q1 and Q2.
3. The answer to above depends on the sign of Q3.
4. The answer to above depends on the magnitudes of Q1 and Q2 .
5. The force on Q3 can never be zero, no matter what the (non-zero!)
charge Q3 is.
Lecture 1-9
friction can cause electrons to move
from one object to another.
Lecture 1-10
Charging by rubbing
Lecture 1-11
Charging by induction
polarization
by induction
grounding
Lecture 1-12
How strong are Coulomb forces?
• Electron and proton in a hydrogen atom
e2 (8.99  109 Nm2 / C 2 )(1.60  1019 C )2
7
F k 2 

O
(10
)N
11
2
r
(5.3  10 m)
O(107 ) N
23
2
a  F / me 

O
(10
)
m
/
s
O(1030 )kg
• Compare electric and gravitational forces
electron and
proton
2
e2
Fe  k 2
r
Fe
ke

 2.27 1039
Fg Gm p me
Fg  G
m p me
r2
me = 9.11x10-31 kg,
mp =1.67x10-27 kg
Lecture 1-13
•
Question
A Human weight 120 lb, which of the
following is correct?
a) A large fraction of the weight come from the
attraction force between the charges on
human body and earth.
b) All the weight comes the attraction force
between the charges on human body and
earth.
c) All the weight come from the gravitational
forces. The electric forces are negligible.
Lecture 1-14
Principle of Superposition
 


F1  F12  F13  F14  
q1
F13
• Add
F12
q2
F1
q3
by components or
• Magnitude and direction
separately by using
trigonometry
Lecture 1-15
Physics 241 – Sample Quiz A – Jan. 8, 2008
Two point charges are separated by distance d
as shown. Where can you put a third charge of
+1 C so that there is no net electric force acting
on it? (Take Q > 0.)
a)
b)
c)
d)
e)
to the right of charge -Q
to the left of charge 2Q
between the two charges
some other place
nowhere
e = 1.610-19 C
k = 8.99109 Nm2/C2
2Q
d
-Q
Lecture 1-16
Physics 241 – Sample Quiz B – Jan. 8, 2008
Two point charges are separated by distance d
as shown. Where can you put a third charge of
+1 C so that there is no net electric force acting
on it? (Take Q > 0.)
a)
b)
c)
d)
e)
to the right of charge -2Q
to the left of charge Q
between the two charges
some other place
nowhere
e = 1.610-19 C
k = 8.99109 Nm2/C2
Q
d
- 2Q
Lecture 1-17
Physics 241 – Sample Quiz C – Jan. 8, 2008
Two point charges are separated by distance d
as shown. Where can you put a third charge of
1 C so that there is no net electric force acting
on it? (Take Q > 0.)
a)
b)
c)
d)
e)
to the right of charge Q
to the left of charge 2Q
between the two charges
some other place
nowhere
e = 1.610-19 C
k = 8.99109 Nm2/C2
2Q
d
Q