1. Properties of Electric Charges
2. Coulomb’s law
3. The Electric Fields
4. Electrics Field of a Continuous Charge
Distribution
5. Electric Field Lines
Why each
individual hair
stand-up??
Each individual hair on their heads becomes charged and exerts a
repulsive force on the other hairs, resulting in the “stand-up’’
hairdos that you see here
1. Properties of Electric Charges
(a) A negatively charged rubber rod suspended by a thread is
attracted to a positively charged glass rod.
(b) A negatively charged rubber rod is repelled by another
negatively charged rubber rod.
When a glass rod is rubbed with silk, electrons are transferred
from the glass to the silk. Because of conservation of charge, each
electron adds negative charge to the silk, and an equal positive
charge is left behind on the rod. Also, because the charges are
transferred in discrete bundles, the charges on the two objects are
$e, or $2e, or $3e, and so on.
We conclude that electric charge has the
following important properties:
 There are two kinds of charge in nature; charges of
the same sign repel one another and charges of
opposite sign attract one another
 Total charge in an isolated system is conserved
 Charges is quantized
A charged comb attracts bits of paper because charges in
molecules in the paper are realigned.
Quick Quiz
If you rub an inflated balloon
against your hair, the two
materials attract each other, as
shown in Figure. Is the amount
of charge present in the system
of the balloon and your hair after
rubbing (a) less than, (b) the
same as, or (c) more than the
amount of charge present before
rubbing?
Coulomb’s Law
Coulomb’s torsion balance
Coulomb’s experiments
F
Garis Fr-2
r
The electric force:
• is inversely proportional to the
square of the separation r
between the particles and
directed along the line joining
them or (|F12|  1/ r122)
• is proportional to the product of
the charges q1 and q2 on the two
particles or (|F12|  |Q1| |Q2|);
• is attractive if the charges are of
opposite sign and repulsive if the
charges have the same sign;
• is a conservative force.
The Coulomb’s law expressed in vector form:
F12
r12
+
-
Q2
F12
rˆ12
+
F21
F21
Q1
+
Quick Quiz
Object A has a charge of +2 C, and object B
has a charge of +6 C. Which statement is true
about the electric forces on the objects?
FBA?
A
+2 C
FAB?
B
+6 C
A: FAB=-3FBA
B: FAB=-FBA
C: 3FAB=-FBA
D: FAB=12FBA
Example
Consider three point
charges located at the
corners of a right
triangle as shown in
Figure, where q1 = q3 =
5.0 C, q2 = 2.0 C,
and a = 0.10 m. Find
the resultant force
exerted on q3.
Superposition principle
Q2
F41
Q1
-
F21
+
F31
-
+
Q4
Q3
Gaya pada muatan
adalah jumlah vektor
gaya dari semua muatan
F1  F21  F31  F41
Gaya dari muatan kontinu
:
Q
R-r
dq
r
R
FQ 
1
4

Q dq
0
R  r 
Rr
3
Gaya dari muatan kontinu :
Simetri Bola
Q
dq    dV    s ds sin   d   d 
2
z
R-r
R  L kˆ
R

r  s rˆ  s sin  cos   iˆ  sin  sin   ˆj  cos  kˆ
r dq
y
x
FQ 
1
4

Q dq
0
R  r 
Rr
3

Gaya dari muatan kontinu :
muatan batang (4 SKS)
dq   ( x )dL   ( x )dx
y
R  h  ˆj
R-r
R
r  x iˆ
r
dq
x
FQ 
1
4

Q dq
0
R  r 
Rr
3
Gaya dari muatan kontinu :
muatan cakram (4 SKS)
R  h  ˆj
dq    dA   ( s ,  ) s  ds  d 

r  s rˆ  s cos   iˆ  sin   ˆj
Q
R-r
R
r
dq
x
y
FQ 
1
4

Q dq
0
R  r 
Rr
3

Hukum Coulomb
vs Hukum Gravitasi Newton
F12 
1
Q 1Q 2
4 0 | r12 |
2
rˆ12
F12  G
m1m2
| r12 |
2
rˆ12
 Tarik menarik atau
 Selalu tarik menarik
tolak menolak
 1/r2
 Sangat kuat
 Skala lokal
 1/r2
e
2
 Sangat lemah 4
0
 Skala besar, planet,
alam semesta
  Gm
2
Ringkasan
 Hukum Coulomb
 Gaya elektrostatik diantara muatan
 Bentuk vektor hukum Coulomb
 Gaya Coulomb vs Gravitasi
 Gaya Elektrostatik lebih kuat
 Superposisi
F1  F21  F31  F41