Electric field
Physics 114
7/24/2016
Lecture II
1
Introduction
•
•
•
•
•
Instructor
Prof. Regina Demina
Office
B&L 367
Phone
275-7357
Email
regina@pas.rochester.edu
Office hour Tue 4:30-5:30 pm
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Lecture II
2
Workshops
• Workshops start this
week.
• The first homework
assignment is due next
week
• Have questions – use
office hours
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Lecture II
3
Lab sign up
• Information about the labs is available at
http://www.pas.rochester.edu/~physlabs/
• Questions about the labs will get answered most quickly by email
to physlabs@pas.rochester.edu
• Labs begin Jan 28th for "A" week students and February 4th for
"B" week students. You will receive your "A" or "B" week
assignment by email during the week of January 21st. If you do
not receive your assignment or have concerns about your
"A"/"B" week assignment, please email
physlabs@pas.rochester.edu
• A pre-lab homework assignment is to be handed in at the
beginning of each lab. The manuals, which include the pre-lab
homework, are available on the lab web site. Bring a print-out of
each lab manual to lab. There will be some changes in grading
policy this term. You will receive a handout detailing the new
procedures at your first lab.
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Lecture II
4
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Lecture II
5
Concepts
• Primary concepts:
– Electric field
• Secondary concepts:
– Field lines
– Shields
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Lecture II
6
Laws
• Superposition principle
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Lecture II
7
Skills
• Calculate electric field of a system of charges
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Lecture II
8
Coulomb’s law
F12
F21
+
+
1
2
F12
F21
-
+
1
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Q1Q2
F k 2
r
2
Lecture II
9
Test problem #1
• Two point charges are separated by 10meters. If
the distance between them is reduced to
5.0meters, the magnitude of the electrostatic
force exerted on each charge
–
–
–
–
A
B
C
D
7/24/2016
decreases to half its original value.
increases to twice its original value.
decreases to one quarter of its original value.
increases to four times its original value.
Lecture II
10
Test problem #2
• If the magnitude of the charge on two identical
charged bodies is doubled, the electrostatic force
between the bodies will be
–
–
–
–
A
B
C
D
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doubled.
halved.
quadrupled.
unchanged.
Lecture II
11
Test problem #3
• If the charge of one point charge is doubled and
that of the other charge stays the same, the force
will be
–
–
–
–
A
B
C
D
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doubled.
halved.
quadrupled.
unchanged.
Lecture II
12
System of charges
Calculate the net
electrostatic force on
particle 3.
Input:
Q1Q2
F k 2
r
9
2
2
k  9.0 10 Nm / C
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–
–
–
–
–
Q1= - 86mC= - 86.10-6 C
Q2= + 50mC= + 50.10-6 C
Q3= + 65mC= + 65.10-6 C
r13=60cm=0.60m
r23=30cm=0.30m
• What if the value of Q3
changes?
Lecture II
13
Electric field
Q1Q2
F k 2
r


F  Q1 E

E
+
+
1
2
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• F – force between two charges(N)
• Q – electric charge (C= Coulomb)


F  Q1  E
Q2
Ek 2
r
• E – electric field created at point 1
by charge 2
• Charge 2 has changed the property
of space at point 1
• Charge 1 is experiencing this
change
Lecture II
14
Electric field
F
E
q
• E – electric field is the force F exerted on a small
positive test charge, divided by the magnitude of this
charge q.
• Electric field is a vector.
• Electric field is measured in N/C.
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Lecture II
15
Electric field
• Electric field is like the
ski slope
• Charge – a skier
• The slope is there
whether you ski down
or not
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Lecture II
16
Field lines
+
+
+
+
+
+
+
+
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+
Field lines at each point are
parallel to the electric
field at this point.
Let’s take a sample (+)
charge and examine
electric field pattern
around a point positive
charge.
• Like signs repel.
• Field lines are going
radially away from the
positive charge
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17
Field lines
+
+
+
-
+
+
+
+
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+
Field lines at each point are
parallel to the electric
field at this point.
Let’s take a sample (+)
charge and examine
electric field pattern
around a point negative
charge.
• Unlike signs attract.
• Field lines are going
radially toward the
negative charge
Lecture II
18
Superposition of fields
  
E  E1  E2

E1
+
Positive test charge

E2
+
-
1
2
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Principle of superposition:
Net field created by a
system of charges is a
vector sum of fields
created by individual
charges:
   
E  E1  E2  E3  ....
Lecture II
19
System of charges
Calculate the net
electrostatic force on
particle 3.
Input:
Q1Q2
F k 2
r
9
2
2
k  9.0 10 Nm / C
7/24/2016
–
–
–
–
–
Q1= - 86mC= - 86.10-6 C
Q2= + 50mC= + 50.10-6 C
Q3= + 65mC= + 65.10-6 C
r13=60cm=0.60m
r23=30cm=0.30m
• What if the value of Q3
changes?
Lecture II
20
Rules of field lines
Field lines
• Are parallel to the electric field
• Point away from positive
charges
• Point toward negative charges
• Never cross
• Can be open ( go to infinity)
• Their density represents the
intensity of the electric field.
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Lecture II
21
Constant field
+Q
-Q
• Electric field is constant
between two very large
parallel plates,
• one with a positive charge
+Q,
• the other one with a negative
charge - Q
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Lecture II
22
Charges in electric fields

F

E
+
Positive charges experience
force along the direction
of the field.
Negative charges – against
the direction of the field
-
F
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F=qE
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23
Electric field in conductors

Eexternal
+
+
+
+
+
-
-
-
-
-
 

E  Eexernal  Einternal  0
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Conductor – a material with
abundant free (to move)
charge
E=0 in good conductors in the
static situation.
E is perpendicular to the surface
of conductor.
Metal hollow boxes are used to
shield electric fields.
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24
Test problem #4
• Points A, B and C are at various distances from a
given point charge.Which statement is most
accurate? The electric field strength is
–
–
–
–
A
B
C
D
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Greatest at point A.
Greatest at point B.
Greatest at point C.
The same at all three points.
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25
Test problem #5
• Which diagram best represents the electric field
lines around two oppositely charged particles?
–A
-C
–B
-D
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Lecture II
26
Motion in electric field
• Remember F=ma ?
• This tells about the effect
the force will have on an
object with mass m
• In electric field F=qE
• This tells you about the
amount of force electric
field exerts on the object
• Answer the question: what
is the origin of forces?
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Lecture II
E
v
d
E=2.0x104N/C
e=1.6x10-19C
m=9.1x10-31kg
d=1.5cm=0.015m
v=?
27
Motion in electric fields
• a is always || to net F
• Remember v is not always
|| to a (projectile, circular
motion)
-
-
-
-
vx
+
+
+
vy
+
vx=const
vy=at=tF/m=tEq/m
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Lecture II
28