1
PHYS 1022: Chap. 26, Pg 2
2
PHYS 1022: Chap. 26, Pg 3
and Conductors • Coulomb’s Law The Field Model
Topics: • Developing a Charge Model • Charge • Insulators
PHYS 1022: Chap. 26, Pg 4 New Topic
3
PHYS 1022: Chap. 26, Pg 5
Charging by contact direct transfer
Charging by induction no contact
Can happen to both conductors and insulators
separation of charge is called polarization
There are two kinds of charge: positive (+): glass rubbed by silk negative (–): wax or plastic rubbed by wool like charges repel unlike charges attract
PHYS 1022: Chap. 26, Pg 6
1. positive (+) 2. negative (–)
3. neutral
4
PHYS 1022: Chap. 26, Pg 7
1. positive (+) 2. negative (–)
3. neutral
PHYS 1022: Chap. 26, Pg 8
1. positive (+) 2. negative (–)
3. neutral
5
PHYS 1022: Chap. 26, Pg 9
PHYS 1022: Chap. 26, Pg 10
6
PHYS 1022: Chap. 26, Pg 11 New Topic
PHYS 1022: Chap. 26, Pg 12
Electric force is a non-contact force (acts at a distance)
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!
k = 9 x 109 N m2/C2
7
PHYS 1022: Chap. 26, Pg 13
-q
+q1
+q2
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.
Superposition principle
PHYS 1022: Chap. 26, Pg 14
8
PHYS 1022: Chap. 26, Pg 15
1. left 2. right
3. zero
PHYS 1022: Chap. 26, Pg 16
9
PHYS 1022: Chap. 26, Pg 17
PHYS 1022: Chap. 26, Pg 18
10
PHYS 1022: Chap. 26, Pg 19
PHYS 1022: Chap. 26, Pg 20
11
PHYS 1022: Chap. 26, Pg 21
1 cm
1 nC +
+ +
1 cm
1 cm 2 nC 2 nC
PHYS 1022: Chap. 26, Pg 22 New Topic
12
PHYS 1022: Chap. 26, Pg 23
Recall our discussion of the two kinds of forces: Contact force:
» You push on the desk » The ground pushes on the chair » Tension in a rope
Hmm...
Faraday and Newton had some trouble with this one
PHYS 1022: Chap. 26, Pg 24
13
PHYS 1022: Chap. 26, Pg 25
These isolated Temperatures make up a Scalar Field
(you learn only the temperature at a place you choose)
PHYS 1022: Chap. 26, Pg 26
That would require a VECTOR field. (you learn how fast the wind is blowing AND in what direction)
You may prefer to know which way the wind is blowing …
14
PHYS 1022: Chap. 26, Pg 27
Definition of the Electric Field:
E ≡ F / qtest
PHYS 1022: Chap. 26, Pg 28
€
Fg =G Mr2m = mg
M m
Q q
15
PHYS 1022: Chap. 26, Pg 29
+Q
-Q +Q
+Q +4Q
-Q +Q
PHYS 1022: Chap. 26, Pg 30
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.
Superposition principle The field at a given point is a vector sum of the electric field caused by each
charge individually.