# Electrostatics by 93WVi17

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```									 Ch 21 Electrostatics

The study of the properties of
stationary charges
Properties of Charge
• Fundamental Property of Matter
• Two types of charge
   positive
   negative
• Like charges repel
• Unlike charges attract
• Charge is quantized (distinct fundamental units)
   e = 1.602  10-19 C
   Coulomb (C) is the SI unit of charge
• Law of Conservation of Charge: The total charge
of any isolated system is a constant
Charge Conservation
238
92
U  Th  He
234
90
4
2
There are 92 protons on either side of this
nuclear reaction equation.
Charge Conservation Particle
Physics

 e e            

The decay of a neutral photon
into and electron and a
positron (positively charge
electron) in GREEN !!!
Method of generating static charge
• Process of
inducing a
positive charge
on a conducting
sphere.
• Contact or
induction on
isolated
(insulated)
conductors
Material Behaviors
• Insulator - large effort required to move
charges through material
– Examples: wood, plastic, rubber, glass, dry air,
vacuum
• Conductor - very little effort required to move
charges through material
– Examples: most metals, ionic solutions, plasmas
• Semi-conductor - intermediate effort required
to move charges through material
– Examples: germanium, silicon
Charge and Force

Insulators        Conductor and Inuslator
Coulomb’s Law (Force Law
between charges)
• Charles Auguste Coulomb - 1785
• Force between two point charges is
proportional to each charge
• Force between two point charges is
inversely proportional to the distance
separating them
• Force between charges obey the Law of
Coulomb’s Law (point Charges)
q1 q2
F k     2
ˆ
r
r
9        2    2
k  8.99  10 N  m / C
like charges
1
k
4  0
 0  8.85  10 12 C2 / N  m 2    unlike charges
Example 1
A positive 1.0 mC charge is located at the origin and a
-0.3 mC charge is located at x = 2.0 cm. What is the
force on the negative charge?

q1 q2
F k     2                    1.0 mC        -0.3 mC
r                      0.0 cm        2.0 cm
6               6
N  m 1.0 10 C 0.3 10 C
2
F  8.99 10   9

 0.02m 
2                   2
C

F  6.74N                            
F  6.74Ni
Example 2
A positive 0.1 mC charge is located at the origin,
a +0.2 mC charge is located at (0.0 cm, 1.5 cm),
and a -0.2 mC charge is located at (1.0 cm, 0.0
cm). What is the force on the negative charge?
3   0.2 mC
(0.0 cm, 1.5 cm)
q1 q2
F k     2
ˆ
r
1     a       2
r
0.1 mC        -0.2 mC
(0 cm. 0cm)   (1.0 cm, 0.0 cm)
Determine the force between the negative charge
and each positive charge.
3   0.2 mC
(0.0 cm, 1.5 cm)
6          6
N  m 2 0.110 C 0.2 10 C ˆ
F21  8.99 109                              i
 0.01m 
2                    2
C

1             2                       

F21  180Ni
.
0.1 mC        -0.2 mC
(0 cm. 0cm)   (1.0 cm, 0.0 cm)

The force between charges 1 and 2.
q1 q2
F21  k             ˆ
i
2
r21
3   0.2 mC
(0.0 cm, 1.5 cm)

1     a           2
0.1 mC            -0.2 mC
(0 cm. 0cm)       (1.0 cm, 0.0 cm)

The force between charges 3 and 2.
9 N m
0.2 106 C 0.2 10 6 C
2
F23    8.99 10
 0.01m    0.015m 
2             2
C2

F23  111N
.
3   0.2 mC
(0.0 cm, 1.5 cm)

1       a     2
0.1 mC        -0.2 mC
(0 cm. 0cm)   (1.0 cm, 0.0 cm)
Determine the components of each force.


F23   F23 cos ai  F23 sin a
j
                        0.01m                        0.015m
F23  111N
.                                .
i  111N               
j
0.01m2  0.015m2   0.01m2  0.015m2


F23  0.62Ni  0.92Nj
3    0.2 mC
(0.0 cm, 1.5 cm)

1     a       2
0.1 mC        -0.2 mC
(0 cm. 0cm)   (1.0 cm, 0.0 cm)
Determine the sum of the components of the forces.

                
F  1.80Ni  0.62Ni  0.92Nj
Determine the resultant force
F   2.42 N i  0.92 N
         j

F  2.59 N @ 159 o from +x axis
Sample prob 21-3

a) What is force between
spheres

b) Ground sphere A, then
what is the force ?
Field
Region of space influenced by some physical
phenomena; e.g., temperature, gravity,
electric, magnetic behaviors
Electric Field
• Region of space where another charge would
be influenced by a charge or distribution of
charges
• Units of electric field is Newtons/Coulomb
(N/C)
• Electric Field points away from positive charge
and toward negative charge

         F
E  lim
q0 0 q
0
Field of a point charge

         F
E  lim
q0 0 q
0
1    qq0
ˆ
r
          4 0 r    2
E  lim
q0 0        q0
            1 q
E  lim                ˆ
r
q0  0 4      2
0 r
      1 q
E              rˆ
4 0 r   2
Effect of a finite test
charge on a conductor

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