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Physics 102: Lecture 02
Coulomb’s Law
and Electric Fields
• Today’s lecture will cover Textbook
Sections 16.3-6
Physics 102: Lecture 2, Slide 1
Recall Coulomb’s Law
• Magnitude of the force between charges q1
and q2 separated a distance r:
F = k q1q2/r2
k = 9x109 Nm2/C2
• Force on nucleus of Hydrogen from eQp=1.6x10-19 C
+
F
Qe = -1.6x10-19 C
-
r = 1x10-10 m
F = (9x109)(1.6x10-19)(1.6x10-19)/(10-10)2 N
= 2.3x10-8 N (to the right)
Physics 102: Lecture 2, Slide 2
Three Charges
• Calculate force on +2mC charge due to other two
charges
Q=+2.0mC
4m
– Calculate force from +7mC charge
– Calculate force from –3.5mC charge
– Add (VECTORS!)
Q=+7.0mC
Physics 102: Lecture 2, Slide 3
6m
Q=-3.5 mC
Electric Field
• Charged particles create electric fields.
– Direction is the same as for the force that a + charge
would feel at that location.
– Magnitude given by:
E  F/q = kq/r2
q=1.6x10-19 C
+
r = 1x10-10 m
E=
Direction?
Physics 102: Lecture 2, Slide 4
Preflight 2.3
What is the direction of the electric field at point B?
1) Left
2) Right
3) Zero
A
y
B
x
Physics 102: Lecture 2, Slide 5
ACT: E Field
What is the direction of the electric field at point C?
1) Left
2) Right
3) Zero
y
C
x
Physics 102: Lecture 2, Slide 6
ACT: E Field
What is the direction of the electric field at point C?
1) Left
Red is negative
2) Right
Blue is positive
3) Zero
Away from positive charge (right)
Towards negative charge (right)
Net E field is to right.
y
C
x
Physics 102: Lecture 2, Slide 7
Comparison:
Electric Force vs. Electric Field
• Electric Force (F) - the actual force felt by a
charge at some location.
• Electric Field (E) - found for a location only
– tells what the electric force would be if a
charge were located there:
F = qE
• Both are vectors, with magnitude and
direction. Add x & y components.
Physics 102: Lecture 2, Slide 8
Preflight 2.2
What is the direction of the electric field at point A?
1) Up
2) Down
3) Left
4) Right
5) Zero
A
y
B
x
Physics 102: Lecture 2, Slide 9
Preflight 2.2
What is the direction of the electric field at point A?
1) Up
Red is negative
2) Down
Blue is positive
3) Left
4) Right
5) Zero
A
y
B
x
Physics 102: Lecture 2, Slide 10
ACT: E Field II
What is the direction of the electric field at point A, if
the two positive charges have equal magnitude?
1) Up
2) Down
3) Right
4) Left
A
y
5) Zero
B
x
Physics 102: Lecture 2, Slide 11
ACT: E Field II
What is the direction of the electric field at point A, if
the two positive charges have equal magnitude?
1) Up
Red is negative
2) Down
Blue is positive
3) Right
4) Left
5) Zero
A
y
B
x
Physics 102: Lecture 2, Slide 12
Preflight 2.5
X
A
Charge A is
Y
B
Field lines start on positive charge, end on negative.
1) positive
Physics 102: Lecture 2, Slide 13
2) negative
3) unknown
Preflight 2.5
X
A
Charge A is
Y
B
Field lines start on positive charge, end on negative.
1) positive
Physics 102: Lecture 2, Slide 14
2) negative
3) unknown
Preflight 2.6
X
A
Y
B
Compare the ratio of charges QA/ QB # lines proportional to |Q|
1) QA= 0.5QB
Physics 102: Lecture 2, Slide 15
2) QA= QB
3) QA= 2 QB
Preflight 2.6
X
A
Y
B
Compare the ratio of charges QA/ QB # lines proportional to Q
1) |QA|= |0.5QB|
Physics 102: Lecture 2, Slide 16
2) |QA| = |QB|
3) |QA| = |2 QB|
Preflight 2.8
X
A
Y
B
The magnitude of the electric field at point X is greater than at point Y
1) True
Physics 102: Lecture 2, Slide 17
2) False
Preflight 2.8
X
A
Y
B
The electric field is stronger when the
lines are located closer to one another.
The magnitude of the electric field at point X is greater than at point Y
1) True
Physics 102: Lecture 2, Slide 18
2) False
Density of field lines gives E
ACT: E Field Lines
B
A
Compare the magnitude of the electric field at
point A and B
1) EA>EB
Physics 102: Lecture 2, Slide 19
2) EA=EB
3) EA<EB
ACT: E Field Lines
B
A
Compare the magnitude of the electric field at
point A and B
1) EA>EB
Physics 102: Lecture 2, Slide 20
2) EA=EB
3) EA<EB
E inside of conductor
• Conductor  electrons free to move
– Electrons feels electric force - will move until
they feel no more force (F=0)
– F=qE: if F=0 then E=0
• E=0 inside a conductor (Always!)
Physics 102: Lecture 2, Slide 21
E inside of conductor
• Conductor  electrons free to move
– Electrons feels electric force - will move until
they feel no more force (F=0)
– F=qE: if F=0 then E=0
• E=0 inside a conductor (Always!)
Physics 102: Lecture 2, Slide 22
Preflight 2.10
X
A
Y
B
"Charge A" is actually a small, charged metal ball (a conductor). The
magnitude of the electric field inside the ball is:
(1) Negative
Physics 102: Lecture 2, Slide 23
(2) Zero
(3) Positive
Preflight 2.10
X
A
Y
B
"Charge A" is actually a small, charged metal ball (a conductor). The
magnitude of the electric field inside the ball is:
(1) Negative
Physics 102: Lecture 2, Slide 24
(2) Zero
(3) Positive
Recap
• E Field has magnitude and direction:
– EF/q
– Calculate just like Coulomb’s law
– Careful when adding vectors
• Electric Field Lines
– Density gives strength (# proportional to charge.)
– Arrow gives direction (Start + end on -)
• Conductors
– Electrons free to move  E=0
Physics 102: Lecture 2, Slide 25
To Do
• Read Sections 17.1-17.3
• Extra problems from book Ch 16:
– Concepts 12-18
– Problems 11, 13, 25, 31
• Do your preflight.
See you next time!
Physics 102: Lecture 2, Slide 26