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Transcript
```PHYS 1022: Chap. 26, Pg 2
1
and Conductors
•  Coulomb’s Law
The Field Model
Topics:
•  Developing a Charge Model
•  Charge
•  Insulators
PHYS 1022: Chap. 26, Pg 3
New Topic
PHYS 1022: Chap. 26, Pg 4
2

There are two kinds of charge:
  positive (+): glass rubbed by silk
  negative (–): wax or plastic rubbed by wool
  like charges repel
  unlike charges attract

Charging by contact
  direct transfer
Charging by induction
  no contact
  Can happen to both conductors and
insulators
  separation of charge is called
polarization

PHYS 1022: Chap. 26, Pg 5
1.
positive (+)
2.
negative (–)
3.
neutral
PHYS 1022: Chap. 26, Pg 6
3
1.
positive (+)
2.
negative (–)
3.
neutral
PHYS 1022: Chap. 26, Pg 7
1.
positive (+)
2.
negative (–)
3.
neutral
PHYS 1022: Chap. 26, Pg 8
4
PHYS 1022: Chap. 26, Pg 9
PHYS 1022: Chap. 26, Pg 10
5
New Topic

PHYS 1022: Chap. 26, Pg 11
Electric force is a non-contact force (acts at a
distance)
k = 9 x 109 N m2/C2
The unit for charge is Coulomb (C)
The smallest charge: e = 1.6 x 10-19 C.
Two 1-C charges separated by 1 m has Coulomb force of 9x109 N!
PHYS 1022: Chap. 26, Pg 12
6
+q1

If there are several charges, the net force on any one of them is
the vector sum of the individual forces due to all the others. -q
  Superposition principle
+q2
PHYS 1022: Chap. 26, Pg 13
PHYS 1022: Chap. 26, Pg 14
7
1.
left
2.
right
3.
zero
PHYS 1022: Chap. 26, Pg 15
PHYS 1022: Chap. 26, Pg 16
8
PHYS 1022: Chap. 26, Pg 17
PHYS 1022: Chap. 26, Pg 18
9
PHYS 1022: Chap. 26, Pg 19
PHYS 1022: Chap. 26, Pg 20
10
1 nC
+
2 nC
m
1c
1c
m
+
1 cm
+
2 nC
PHYS 1022: Chap. 26, Pg 21
New Topic
PHYS 1022: Chap. 26, Pg 22
11
  Recall our discussion of the two kinds of forces:
  Contact force:
»  You push on the desk
Hmm...
»  The ground pushes on the chair
»  Tension in a rope
some trouble
with this one
PHYS 1022: Chap. 26, Pg 23
PHYS 1022: Chap. 26, Pg 24
12
These isolated Temperatures make up a Scalar Field
(you learn only the temperature at a place you choose)
PHYS 1022: Chap. 26, Pg 25
You may prefer to know which way the wind is blowing …
That would require a VECTOR field.
(you learn how fast the wind is blowing AND in what direction)
PHYS 1022: Chap. 26, Pg 26
13
  Definition of the Electric Field:
E ≡ F / qtest
PHYS 1022: Chap. 26, Pg 27
M
Fg = G 2 m = mg
r
€
M
m
Q
q
PHYS 1022: Chap. 26, Pg 28
14
+Q
+4Q
+Q
-Q
-Q
+Q
+Q
PHYS 1022: Chap. 26, Pg 29
  Superposition principle
  The field at a given point is a vector sum of the electric field caused by each
charge individually.

Example: electric dipole
 Point charges q1=+12nC and
q2=-12nC are placed 10 cm
apart. This combination of
two charges with equal
magnitude and opposite sign
is called an electric dipole.
 Compute the electric field at
points a, b, and c.
PHYS 1022: Chap. 26, Pg 30
15
```