Welcome to Physics 212
http://online.physics.uiuc.edu/courses/phys212
This lecture is VERY full. Please sit next to
someone nice. (There won’t be empty seats!).
Festival seating up front !!
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Physics 212 Lecture 1, Slide 1
Physics 212
Lecture 1
Today's Concepts:
a) Coulomb’s Law
b) Superposition
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Physics 212 Lecture 1, Slide 2
Physics 212 - Intro
http://online.physics.uiuc.edu/courses/phys212
Everything can be found online – GO THERE
• You have web based Prelectures and Checkpoints due before every
lecture.
• You have web based homework due every week.
• Be sure to complete Prelecture 2 and Checkpoint 2 before 8 am on
Thursday.
• Bring pen & notebook to lecture – you will be working on problems
• Register your clicker ASAP so you can see your participation grade.
Labs & Discussions
•
•
05
Discussion Sections meet this week
Labs start Jan. 23, must have prelab done before!
Physics 212 Lecture 1, Slide 3
How your grade will be calculated
Prelectures + Checkpoints +
Lectures
100
14 Homework + 10 Quizzes
250
Labs
150
Hour exams (3 x 100 each)
300
Final Exam
200
Prelectures:
50
Checkpoints:
25
Lecture participation: 25
Your top 22 HW/Quiz scores
determine your grade out of
250.
Bonus Points: You can earn up to 1 extra bonus point in every
lecture (for a maximum of 25 bonus points for the semester) by
getting the right answers to all of the clicker questions.
At the end of the semester your lecture bonus points are
added to your HW/Quiz score (250 max).
You can make up for several bad quizzes by
answering correctly in class.
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Physics 212 Lecture 1, Slide 4
Q: What are the benefits of participating ?
A: You learn more
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Viewer
Non-Viewer
25
Data from last year’s first exam
20
15
10
5
0
43
48
52
57
61
65
70
74
78
83
87
91
96
100
Exam Score
Students who…
07
Exam 1 average
Viewed PreLectures
80%
Blew through PreLectures
73%
Physics 212 Lecture 1, Slide 5
Big Brother is Watching
• How Do We Distinguish “Viewers” from “Non-Viewers”?
– We log all keystrokes !!
Results from today’s PreLecture
Physics 212 Lecture 1, Slide 6
Clickers
• Research shows you learn more when you talk!
• 1 point for each lecture
• Maximum of 25 points!
– (No EX’s but there are 28 lectures)
• 0.1 bonus points per correct answer (usually > 10
questions/lecture) maximum 1 point/lecture total
– up to 25 points
– (No EX’s but there are 28 lectures)
09
Physics 212 Lecture 1, Slide 7
Music
Who is the Artist?
A)
B)
C)
D)
E)
Buddy Guy
Ry Cooder
John Lee Hooker
Taj Mahal
Willie Dixon
Example of Krannert Center fare:
Ellnora Guitar Festival (Sept. 2011)
Physics 212 Lecture 1
One Other Key Piece: Homework
1
1
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Physics 212 Lecture 1, Slide 9
Homework: Delayed Feedback
These questions serve as
a test of your
understanding of the
questions posed as
immediate feedback.
Purpose:
Promote
REFLECTION
11
After first deadline
Delayed Feedback
questions turn into
“immediate feedback
questions”. 80% credit
can be obtained by
answering these
questions correctly
before the second
deadline.
Physics 212 Lecture 1, Slide 10
Physics 212 Lecture 1, Slide 11
Coulomb’s Law:
The force on a charge due to another charge is proportional
to the product of the charges and inversely proportional to
the separation squared.
q2
q1
r
q1q2
F 2
r
The force is always parallel to a line connecting the charges,
but the direction depends on the signs of the charges:
11
q2
q1
q2
q1
q2
q1
Opposite signs attract
Like signs repel
Physics 212 Lecture 1, Slide 12
Balloons
Take two balloons and rub them
both with a piece of cloth. After
you rub them they will:
A) Attract each-other
B) Repel each-other
C) Either – it depends on the material of the cloth
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Physics 212 Lecture 1, Slide 13
Balloons
If the same thing is done to both balloons
they will acquire the same sign charge.
They will repel !
+
+ +
+
14
+
+ +
+
Physics 212 Lecture 1, Slide 14
Coulomb’s Law
Our notation
kq1q2
F1,2  2 rˆ1,2
r1,2
F1,2 is the force by 1 on 2 (think “by-on ”)
r̂12 is the unit vector that points from 1 to 2.
Examples
If the charges have the same sign, the force by charge 1
on charge 2 would be in the direction of r12 (to the right)
q1
r1,2
q2
F1,2
If the charges have opposite sign, the force by charge 1
on charge 2 would be opposite the direction of r12 (left)
q1 F1,2
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q2
r1,2
Physics 212 Lecture 1, Slide 15
Example: Coulomb Force
• Two paperclips are separated by 10 meters. Then you remove 1 electron
from each atom on the first paperclip and place it on the second one.
q1 q2
F  k 2 rˆ12
r12
k= 9 x 109 N m2 / C2
electron charge = 1.6 x 10-19 Coulombs
NA = 6.02 x 1023
What will the direction of the force be?
A) Attractive
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B) Repulsive
Physics 212 Lecture 1, Slide 16
Example: Coulomb Force
• Two paperclips are separated by 10 meters. Then you remove 1 electron
from each atom on the first paperclip and place it on the second one.
q1 q2
F  k 2 rˆ12
r12
k= 9 x 109 N m2 / C2
electron charge = 1.6 x 10-19 Coulombs
NA = 6.02 x 1023
Which weight is closest to the approximate force between those
paperclips (recall that weight = mg, g = 9.8 m/s2)?
A) Paperclip (1g x g)
B) Text book (1kg x g)
C) Truck (104 kg x g)
D) Aircraft carrier (108 kg x g)
E) Mt. Everest (1014 kg x g)
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Physics 212 Lecture 1, Slide 17
Two charges, q with +1 mC and Q with +10 mC are placed near each other as
shown in the figure. Which of the following diagrams best depicts the forces
acting on the charges?
Checkpoint 1
The forces are in opposite directions because they have like
charges. The arrow on the +10 is longer because it has a
greater magnitude than the +1
80
60
Determining the force of one charge on the other requires
that the charges be multiplied. The order of multiplication is
irrelevant. Since all other factors are constant between the
two, the charges must be equal. Also, Newton's Third Law.
40
20
0
24
Because the right hand sphere has a charge of +10C, it
would impact the smaller sphere by -10C and visa versa.
Physics 212 Lecture 1, Slide 18
Superposition:
If there are more than two charges present, the total force
on any given charge is just the vector sum of the forces due
to each of the other charges:
q2
F4,1
F1
q1
F3,1
F2,1
F1
F3,1
F2,1
F4,1
q4
q3
F1  F2,1  F3,1  F4,1  ...
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Physics 212 Lecture 1, Slide 19
Superposition ACT
What happens to Force on q1 if its sign is changed
A) |F1| increases
B)
|F1| remains the same
C)
|F1| decreases
q2
D) Need more information to determine
q1
q4
q3
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Physics 212 Lecture 1, Slide 20
The direction of all forces changes by 180o – the magnitudes stay the same:
q2
q2
F4,1
F4,1
F1
q1
F2,1
F1
F3,1
q3
F2,1
q3
F3,1
F4,1
F1  F2,1  F3,1  F4,1  ...
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F2,1
F3,1
q4
F1
q1
q4
F2,1
F1
F3,1
F4,1
 F1   F2,1  F3,1  F4,1  ...
Physics 212 Lecture 1, Slide 21
Try using our Coulomb Force simulation
(“simulations” link on the homepage)
Physics 212 Lecture 1, Slide 22
Checkpoint
Compare the magnitude of the net force
on q in the two cases.
A)
|F1 | > |F2|
B)
|F1 | = |F2|
C)
|F1 |< |F2|
D)
Depends on sign of q
+Q
+Q
q
q
F2 = 0
F1
-Q
+Q
Case 1
Case 2
70
60
50
40
30
20
10
0
The vector sum of the coulomb forces in case 1 results in an
effectively doubled force, while the case 2 coulomb forces would
cancel each other out.
Because in Case 2, both charges have the same sign, the charge in
the middle will have a net force pointing downwards, whereas in Case
1 the charge in the middle does not have a net force exerted on it.
they are all the same because magnitude of
force is kqq/r^2
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Physics 212 Lecture 1, Slide 23
Checkpoint
Four charged particles are placed on a
circular ring with radius 3 m as shown
below. A particle with charge Q is
placed in the center of the ring
y
What is the direction of
horizontal force on Q?
A) Fx > 0
B) Fx = 0
3q
x
C) Fx < 0
Q
100
80
60
q
q
40
q
20
0
Great Job !
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Physics 212 Lecture 1, Slide 24
Checkpoint
3q
Four charged particles are placed on a
circular ring with radius 3 m as shown
below. A particle with charge Q is
placed in the center of the ring
y
Q
What is vertical force on Q?
A) Fy > 0
B) Fy = 0
C) Fy < 0
x
q
q
60
50
q
40
30
20
10
0
The magnitude and y component of the particle above the origin is greater than the magnitudes and
y components of the particles below the origin.
They are equal magnitudes opposite directions which cancel out
The outer two charges exert only a fraction of their force in the y direction, therefore, the sum of the
three lower charges must be less than the upper charge who's force is three times as large.
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Physics 212 Lecture 1, Slide 25
See you Thursday!
• Discussion Sections meet this week!
• Be sure to complete Prelecture 2 and Checkpoint2
• Labs begin Jan. 23
Physics 212 Lecture 1, Slide 26