Lecture 1
Chapter 25
Physics II
01.20.2015
Electric Charges and Forces
95.144
Course website:
http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII
Lecture Capture:
http://echo360.uml.edu/danylov201415/physics2spring.html
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Course Website
Everything you need to know about the course can be
found on the course website:
http://faculty.uml.edu/Andriy_Danylov/Teaching/PhysicsII.aspx
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Course Organization
Physics II meets 4 times a week
•
•
Lectures : 8:00-8:50 TuFr, Olney 150
Recitation Sections
Various times (M/W)
and locations
Lecture slides will be posted on-line
Lecture recording (video and audio) will be posted:
http://echo360.uml.edu/danylov201415/physics2spring.html
The textbook: “Physics for Scientists and Engineers, a Strategic Approach,”
Third Edition by Randall Knight (Pearson, 2012)
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Weekly Homework
• Online HW is on www.masteringphysics.com
 Course title:
95144Spring2014
 Course ID:
DANYLOVSPRING2015
• The online HW is typically due midnight on Sunday
(You are penalized 25% for each day late)
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Course Grading
Item
HW
Quizzes
Exam 1
Exam 2
Final Exam
Lecture
Attendance
Points
100
100
100
100
200
50
Total
650
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Range
Grade
>80%
A
75-80%
A-
70-75%
B+
65-70%
B
60-65%
B-
55-60%
C+
50-55%
C
45-50%
C-
40-45%
D+
35-40%
D
<35%
F
Clicker registration
http://student.turningtechnologies.com/
Channel Number
of a clicker:
61
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
 Class name:
DANYLOVSPRING2015
Physics II Resources
 Get help early and often.
 The first person to contact with problems in Physics I is your
Recitation Instructor!!
 Recitation Instructors will have office hours each week during
which they can be met.
 Physics Department Tutoring Center:
 Tutors are available free of charge in the Physics Department
tutoring room (9 am-5 pm), located adjacent to the Department
office on the 1st floor of Olney Hall.
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Electricity
• Electricity and magnetism is all around us.
microphones, calculators, televisions, radio, computers.
• The colors of the rainbow in the blue sky are there
because of electricity
• Your nerve system is driven by electricity.
• You could not see without electricity.
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Electrostatics
Charges are at rest
Electrical forces arise from particles in atoms
Electrons – negatively charged
Protons – positively charged (by convention)
Like charges repel
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Opposite charges attract
Coulomb’s law
F1on2
r
F2on1
q2
q1
When two charged particles are a distance, r, apart, they each
experience a force.
In SI units K = 8.99  109 N m2/C2.
Enormous!!!
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Coulomb’s law
 We can make many future equations easier to use if we
rewrite Coulomb’s law in a somewhat more complicated
way.
 Let’s define a new constant, called
the permittivity constant 0:

95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics

 Rewriting Coulomb’s law in terms of
0
gives us:
ConcepTest 1
A.
The force between two charges separated by a
distance d is F. If the charges are pulled apart to
B.
a distance 3d, what is the force on each charge?
C.
F
F
Q
Q
D.
E.
d
?
Q
9F
3F
F
(1/3)F
(1/9)F
?
Q
93%
3d
Originally we had:
Fbefore = k(Q)(Q)/d2 = F
)F
(1
/9
)F
1%
(1
/3
2%
F
3%
3F
Fafter = k(Q)(Q)/(3d)2 = 1/9F
1%
9F
Now we have:
Principle of superposition
If multiple charges are present,
the net electric force on a charge q due to all other charges is
ଵ+ ଶ+ ଷ+ ସ
F1
௡
q1
q
F2
F3
F4
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
q4
q2
q3
ConcepTest 2 Forces in 2D
1
A.
2
Which of the arrows best represents
3
B.
the direction of the net force on
4
charge +Q due to the other two
charges?
C.
d
+2Q
+Q
The charge +2Q repels +Q
5 D.
E.
d
toward the right. The
+4Q
charge +4Q repels +Q
upward, but with a stronger
88%
force. Therefore, the net
force is up and to the right,
+2Q
but mostly up.
0%
5
3
2
1
2%
4
8%
2%
+4Q
1
2
3
4
5
Charged ions
Usually an atom is neutral
If an atom loses one or more electrons,
it becomes positively charged
called a positive ion
If an atom gains one or more electrons,
it becomes negatively charged
called a negative ion
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
electrons
Insulators and conductors
 The electrons in an insulator are
all tightly bound to the positive
nuclei and not free to move
around.
 In metals, the outer atomic
electrons are only weakly
bound to the nuclei.
 These outer electrons
become detached from
their parent nuclei and
are free to wander about
through the entire solid.
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Charging
When a plastic rod is rubbed with a towel, the plastic acquires a negative charge
and the towel acquires an equal amount of positive charge.
(The charges are separated but the sum is zero)
Rubbed glass
Rubbed amber/plastic
positively charged (lack of electrons)
negatively charged (excess electrons)
It leads to
CONSERVATION OF CHARGE
The net amount of electric charge produced in any process is zero
Demo:rod/paper
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Charging metal
The figure shows how a conductor is
charged by contact with a charged
plastic rod.
 Electrons in a conductor
are free to move.
 Once charge is transferred
to the metal, repulsive forces between
the electrons cause them to move apart
from each other.
Demo:rod/electroscope
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Induced charge in conductor
 Although the metal
as a whole is still electrically
neutral,
we say that the object has been
polarized.
 Charge polarization
is a slight separation
of the positive and negative
charges in
a neutral object.
Demo:rod/electroscope/induction and rod/baloon
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Metal
ConcepTest Conductors
A neutral metal ball hangs from the ceiling by an
insulating thread. The ball is attracted to a
Positive
Negative
Neutral
A.
B.
C.
100%
positive-charged rod held near the ball. The
charge of the ball must be:
the ball can be separated by
l
Ne
ut
ra
ive
ga
t
Ne
Po
sit
iv
If the ball is neutral, the charges in
0%
e
0%
induction (polarization), leading to a
net attraction.
Remember
the ball is a
conductor!
Charge induction in an insulator
The figure shows how a neutral atom is
polarized by an external charge, forming
an electric dipole.
Center of
negative charge
electric dipole
F
negatively charged
surface
positively charged
surface
Demo:rod/paper
95.144, Spring 2015, Lecture 1
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Demonstrations
A Van de Graff generator (named after its inventor) is a high voltage
generator. It basically loads a spherical hollow conductor with static
charges which brings the conductor to a very high (or very low)
potential.
Electroscope
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics
Thank you
See you on Friday
95.144, Spring 2015, Lecture 1
Department of Physics and Applied Physics