1. Electrostatics
Teacher : Piyanuch Plaon
Subject : Physics 4

2. Discovery of charge
Benjamin Franklin arbitrarily called the
two kinds of charge positive and negative.
In most cases, only the negative charge is
mobile.

3. Properties of charge
Like charges repel, and
unlike charges attract.
Charge is conserved, meaning it
cannot be created or destroyed, only
transferred from one location to
another.
In all atoms, electrons have negative
charge and protons have positive

4. Insulators
In insulators, electrons
are bound in “orbit” to
the nucleus in each
atom.
When charge is placed
on an insulator, it stays
in one region and does
not distribute.
Wood, plastic, glass,
air, and cloth are good
insulators.

5. Conductors
In conductors
electrons can move
from atom to atom,
thus electricity can
“flow”.
When charge is
placed on a
conductor, it
redistributes to the
outer surface.
Metals (copper, gold,

6. Charging by Friction
When insulators are rubbed
together, one gives up electrons and
becomes positively charged, while
the other gains electrons and
becomes negatively charged.

7. Charging by
ConductionWhen a charged conductor makes contact
with a neutral conductor there is a transfer of
charge.
Electrons are transferred
from the rod to the ball,
leaving them both
negatively charged.
Electrons are transferred
from the ball to the rod,
leaving them both
positively charged.
CHARGING
NEGATIVELY
CHARGING
POSITIVELY

8. Step 1. A charged rod is
brought near an isolated
conductor. The influence
of the charge object
polarizes the conductor
but does not yet charge
Step 2. The conductor
is grounded to the
Earth, allowing charge
to flow out between it
and the Earth.
Charging by Induction

9. Charging by Induction
(cont.)
Step 3. The ground is
removed while the
charge rod is still
nearby the conductor.
Step 4. The rod is
removed and the
conductor is now
charge (opposite of
rod).

10. Electric Forces and Electric Fields
CHARLES COULOMB
(1736-1806)
MICHAEL FARADAY
(1791-1867)

11. Electrostatic Charges
The charge of an electron (qe) is -1.6 x
10-19 C
Electrostatic charge is a fundamental
quantity like length, mass, and time.
The symbol for charge is q. The SI unit for
charge is called the coulomb (C).
ATTRACTION
AND
REPULSION

12. The Electrostatic Force
The constant of proportionality, k, is equal to
9.0 x 109 Nm2/C2.
COULOMB’S LAW
OF ELECTROSTATIC
FORCE
Fe 
kq1q2
r2
consta
nt
distan
ce
charg
es
electrost
atic force
The electrostatic force depends
directly on the magnitude of the
charges.
The force depends inversely on the
square of distance between charges
(another “inverse square law”)!

13. Electric Field Strength
g 
Fg
m
E 
Fe
q0
DEFINITION
OF
GRAVITATIO
NAL FIELD
DEFINITION
OF
ELECTRIC
FIELD
g field 
force
mass
E field 
force
charge
SI unit of electric
field
newton
coulomb

N
C
Electric field is a vector
quantity
E field points toward negative charges
E field points away from positive charges
q0 is a small, positive test
charge

14. Electric Field Lines
Density of
field lines
indicates
electric
field
strength
Definition of E Field for single
point charge
POSITIVE
CHARGE
NEGATIVE
CHARGE
E 
Fe
q0

kq0q / r2
q0
E 
kq
r2
consta
nt
distan
ce
charg
e
electri
c field
Single Point
Charges

15. Electric Field Lines
Electric fields for multiple
point charges
POSITIVE AND NEGATIVE
POINT CHARGES
TWO POSITIVE
POINT CHARGES

16. E 
kq
r2
EXAMPL
E 1
EXAMPL
E 2
E 
9 109
Nm2
/C2
 5 103
C 
2 m 2
Electric Fields
Find the force on an proton placed 2 meters from
the 5 millicoulomb charge in the problem above.
E 
Fe
q
Fe  qE  1.6 10-19
C 1.13107
N/C  1.8110-12
N, to the right
Fe 
9 109
Nm2
/C2
 5 103
C 1.6 10-19
C 
2 m 2  1.8 10-12
N, to the right
OR
Find the electric field strength
at 2 meters from the 5
millicoulomb charge.
E=1.13107
N/C, to the right
E

17. PE for Two Point Charges
PE 
kq1q2
r
Potential energy is zero at infinite
distance
Potential energy is positive for like
chargesPotential energy is negative for opposite
charges
Potential Energy is force
times distance
PE  Fed 
kq1q2
r2
 r
charg
es
distanc
e
electric
potential
energy
consta
nt
Exampl
eHow much electrostatic potential energy in a hydrogen atom,
which consists of one electron at a distance of 5.3 x 10-11
meters from the nucleus (proton).
PE 
kq1q2
r

(9 109
)(1.6 1019
)(–1.6 1019
)
5.31011
 4.35 1018
J

18. Potential Difference (Voltage)
Potential 
Energy
Charge
V 
PE
q
A volt (v) is the unit for voltage
named in honor of Alessandro Volta,
inventor of the first battery.
1volt 
1 joule
1 coulomb
SI Units
source voltage (V)
common dry cell 1.5
car battery 12
household (US) 120
comb through hair 500
utility pole 4,400
transmission line 120,000
Van de Graaff 400,000
lightning 1,000,000,000
V 
J
C
A good analogy: potential is to
temperature, as potential energy is to
heat.
Electric potential is average energy per
charge.
Potential difference is often called
voltage.
Energy is a relative quantity (absolute
energy doesn’t exist), so the change in
electric potential, called potential
difference, is meaningful.
Voltage is only dangerous
when a lot of energy is
transferred.
Voltage, like energy, is a scalar.

19. Potential Difference for Constant
Electric Field
V  EdV 
PE
q

qEd
q
voltag
e
E
field
distanc
e
Potential energy is often stored in a
capacitor.
Most capacitors have constant electric
fields.
Capacitors are made by putting an
insulator in between two conductors.
Exampl
e
Calculate the magnitude of the
electric field set up in a 2-
millimeter wide capacitor connected
to a 9-volt battery.V  Ed  9  E(0.002)  E  4500 N/C

20. Consider a test
charge to measure
potential
Potential Difference for Point
Charge
V 
kq
r
charg
e
distanc
e
potential
difference
consta
nt
V 
PE
q0

kqq0 / r
q0
Exampl
e
V1 
kq1
r

(9 109
)(6 109
)
0.3
 180 V
V2 
kq2
r

(9 109
)(4 109
)
0.4
 90 V
V3 
kq3
r

(9 109
)(10 109
)
0.5
 180 V
V  V1  V2  V3  180  90 180  270 V
-4 nC
10 nC 6 nC
0.3 m
0.4 m
find
∆V
here

21. CAPACITORS
 A basic capacitor has two parallel
plates separated by an insulating
material
 A capacitor stores an electrical
charge between the two plates
 The unit of capacitance is Farads (F)
 Capacitance values are normally
smaller, such as µF, nF or pF

22.  Basic capacitor construction
Dielectri
c
material
Plate
1
Plate
2
The dielectric
material is an
insulator therefore
no current flows
through the
capacitor
CAPACITORS

23. Storing a charge between
the plates
 Electrons on the left
plate are attracted
toward the positive
terminal of the voltage
source
 This leaves an excess of
positively charged holes
 The electrons are
pushed toward the right
+ -
+ _
+ _
CAPACITORS

24. Types of capacitors
 The dielectric material
determines the type of
capacitor
 Common types of
capacitors are:
Mica
Ceramic
CAPACITORS

25.  Variable capacitors
are used in
communication
equipment, radios,
televisions and
VCRs
 They can be
adjusted by
consumers by
tuning controls
CAPACITORS

26.  Fringing – At the edge of the
capacitor plates the flux lines extend
outside the common surface area of
the plates.
CAPACITANCE

27. THE CURRENT : IC
Current ic associated with the
capacitance C is related to the voltage
across the capacitor by

Where dvc/dt is a measure of the
change in vc in a vanishingly small
period of time.
The function dvc/dt is called the

28. CAPACITORS IN SERIES AND
PARALLEL
- Capacitors, like
resistors, can be
placed in series and
in parallel.
- When placed in
series, the charge is
the same on each
capacitor.

29. CAPACITORS IN SERIES AND
PARALLEL
 Placing capacitors in
parallel the voltage
across each capacitor
is the same.
The total charge is
the sum of that on
each capacitor.