Download PHYS 1022: Chap. 26, Pg 2

PHYS 1022: Chap. 26, Pg 2
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and Conductors
•  Coulomb’s Law
The Field Model
Topics:
•  Developing a Charge Model
•  Charge
•  Insulators
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New Topic
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 
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
 
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1. 
positive (+)
2. 
negative (–)
3. 
neutral
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1. 
positive (+)
2. 
negative (–)
3. 
neutral
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1. 
positive (+)
2. 
negative (–)
3. 
neutral
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New Topic
 
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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!
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+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
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1. 
left
2. 
right
3. 
zero
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1 nC
+
2 nC
m
1c
1c
m
+
1 cm
+
2 nC
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New Topic
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  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
Faraday and
Newton had
some trouble
with this one
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These isolated Temperatures make up a Scalar Field
(you learn only the temperature at a place you choose)
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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)
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  Definition of the Electric Field:
E ≡ F / qtest
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M
Fg = G 2 m = mg
r
€
M
m
Q
q
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+Q
+4Q
+Q
-Q
-Q
+Q
+Q
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  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.
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