Electricity
What is electricity?
The collection or flow of
electrons in the form of
an electric charge
What is the difference between static
electricity and current electricity?
Static electricity is stationary or collects on
the surface of an object, whereas current
electricity is flowing very rapidly through a
conductor.
What Is Static Electricity?
A stationary electrical
charge that is built up on
the surface of a material
Two kinds of charges
After being rubbed, a
plastic ruler can attract
paper scraps.
Ruler carries electric charge.
It exerts electric force on paper.
This charging method is called charging by friction.
The interaction between static electric charges is called
electrostatics.
Where do charges come from?
Matter is made up of atoms.
+ Proton (positive charge)
–
neutron (neutral)
+
+ +
– electron (negative charge)
– –
atom nucleus
Where do charges come from?
If electrons = protons neutral
If electrons > protons gaining electrons, negative
charge
If electrons < protons losing electrons, positive charge
Relative Electro-negativity
electro-negativity
ranking for some Glass
+++++
common materials ++++ Human Hair
from electron donating Nylon
+++
Silk
materials (+, glass) to ++
Fur
electron accepting + Aluminum
materials (-, teflon) Paper
- Cotton
-- Copper
--- Rubber
---- PVC
----- Teflon
Where do charges come from?
Rubbing materials does NOT create
electric charges. It just transfers
electrons from one material to the
other.
Where do charges come from?
When a balloon rubs a piece of wool...
– + electrons are pulled from the
wool to the balloon.
– +
–
– + The balloon has more electrons than
– usual.
– +
wool + The balloon: – charged,
The wool: +charged
Insulators and conductors
Insulators: materials that do NOT
allow electrons to flow through them
easily.
Insulators can be easily charged by friction as the extra
electrons gained CANNOT easily escape.
4 Insulators and conductors
Conductors: materials that allow electrons to flow
through them easily.
Conductors CANNOT be easily charged by friction as
the extra electrons gained can easily escape.
Induction: The production of a charge
in an uncharged body by bringing a
charged object close to it
When negatively charged rod is put near a
metal can...
electrons of the can are pushed
- - - - - - - away from the rod.
induced
attraction top of the can: positive
charges
++ + buttom of the can: negative
+ +
& attraction > repulsion
- -
- -
metal can -
repulsion
Attraction of uncharged objects
Similarly, when charged rod is close to paper
scrap...
molecules of paper align.
- - - - - - -
attraction between the rod
attraction
++ + + and + charge > repulsion
–––– between the rod and - charge.
paper ++ + +
––––
repulsion
Static Discharge
Human body can not feel less than
2,000 volts of static discharge
Static charge built up by scuffing
shoes on a carpet can exceed 20,000
volts?
A balloon has a negative charge
when rubbed by a woollen cloth.
1 If the balloon can attract some paper scraps, which of the
following cannot be the charge of paper scraps?
A Neutral B Positive
C Negative
How does a positively charged rod attract a
neutral object?
When a + charged rod is put near neutral object,
______________
negative charge is induced on the side of the object
near the rod and _____________
positive charge is induced on the side
away from the rod. The rod can attract the netural object
attraction between rod and – induced charge >
because _________
the ________
repulsion between rod and + induced charge.
Grounding
What is grounding?
An object is grounded when it is
connected to the earth through a
connecting wire.
If a charged conductor is grounded, it
will become neutral.
Grounding
b How does grounding occur?
When we touch a metal ball of
positive charge...
+ +
+ electrons flow from the
+ +
earth to the metal ball to
neutralize the metal ball.
Metal ball becomes neutral.
Why do gasoline tankers usually have metal chains at
the back?
When cars run, their tires and bodies are usually charged
by friction
_______. For gasoline tankers, if the accumulated
sparks
charge is large enough, _______can be produced and
explosion
_________ will occur if gasoline vapor is ignited. Those
metal chains conduct the charge on the bodies of tankers to
ground and avoid the danger.
the _______
Lightning
kills more than 60 one mile every fiv
people and seconds
injures more than about 20,000 C
400 people a year Voltage of up to
in the US 1.2x108 volts
Electrostatic Force
Law of Charges
• Like charges repel and opposite charges attract
+ +
- -
+ -
Note that electricity is different from gravitation, in
which the force is always attractive
m1 m2
February 27, 2020 23
The Unit of Charge
The unit of charge is the coulomb, abbreviated C
The coulomb is defined in terms of the SI unit for
electric current, the ampere, abbreviated A
The ampere is a basic SI unit like the meter, the
second, and the kilogram.
The unit of charge is defined as
1 C=1As
February 27, 2020 24
Charge of an Electron
We can define the unit of charge in terms of the
charge of one electron
• An electron is an elementary particle with charge
q = -e where
• e = 1.602 · 10-19 C
• A proton is a particle with q = +e
e = 1.602 · 10-19 C
February 27, 2020 25
Coulomb of Charge
A full coulomb is a very large amount of charge!
The number of electrons required to produce 1 coulomb
of charge is 1C
Ne 6.24 1018
1.602 10 -19 C
Because a coulomb is a large amount of charge, everyday
examples of static electricity typically involve
• 1 micro coulomb = 1 C = 10-6 C
• 1 nano coulomb = 1 nC = 10-9 C
• 1 pico coulomb = 1 pC = 10-12 C
February 27, 2020 26
Insulators and Conductors
The electronic structure of materials determines their
ability to conduct electricity
• “Conducting electricity” means the transport of electrons
Materials that conduct electricity well are called
conductors
• Electrons can move freely (i.e., some of the electrons)
• Metals
• Water with dissolved materials
Materials that conduct electricity poorly are called
insulators
• Electrons cannot move freely
• Glass
• Pure water
February 27, 2020 27
Semiconductors
Semiconductors are materials that can be switched
between being an insulator and being a conductor.
Semiconductors are the backbone of modern
electronics and computers.
Replica of first transitor Modern computer chip with
in 1947 millions of transitors
February 27, 2020 28
Electrostatic Charging
There are two ways to charge an object
• Conduction
• Induction
Charging by conduction
• We can charge an object by connecting a source of
charge directly to the object and then disconnecting the
source of charge
• The object will remain charged
– Conservation of charge
February 27, 2020 29
Charging by Conduction
++++++++++++++++++
Electroscope
We brought charge onto
the electroscope through
contact with the electrode.
February 27, 2020 30
Induction
++++++++++++++++++
---------
The presence of the
positively charged rod
leads to a redistribution
of charge (a kind of
polarization).
It pulls electrons up to
the electrode.
February 27, 2020 31
Charging by Induction
++++++++++++++++++
---------
The presence of the
positively charged rod
leads to a redistribution
of charge.
Grounding pushes
positive charge to Earth
(or rather pulls electrons
from Earth!) leaving the
electroscope negative.
ground
February 27, 2020 32
Electric Force - Coulomb’s Law
Consider two electric charges: q1 and q2
The electric force F between these two
charges separated by a distance r is
given by Coulomb’s Law kq1q2
F 2
r
The constant k is called Coulomb’s
constant and is given by
k 8.99 10 Nm /C
9 2 2
February 27, 2020 33
Coulomb’s Law (2)
The coulomb constant is also written as
1 C2
k where 0 8.85 10 12
4 0 Nm 2
0 is the
“electric permittivity of vacuum”
• A fundamental constant of nature
1q1q 2
F
4 0 r 2
February 27, 2020 34
Example: Force between Two Charges
What is the force between two 1 C charges 1 meter
apart?
1
q1q 2
F
4 0 r 2
9 N m 1 C 1 C
2
F 8.99 10
1 m
2 2
C
8.99 109 N
which is the weight of 450 Space Shuttles at launch
February 27, 2020 35
Electric Force
The electric force is given by
q1q2
The electric force, unlike the
gravitational force, can be positive
Fk 2
or negative r
• If the charges have opposite
signs, the force is negative + -
• Attractive
• If the charges have the same
sign, the force is positive + +
• Repulsive - -
February 27, 2020 36
Electric Force Vector
Electric force in vector form
q1
y
r r r2 r1
r1
q2 r r2 r1
r2 rˆ
x r r
q1q2 q1q2
F2 k 2 r̂ F1 k 2 r̂
r r
February 27, 2020 37
Superposition Principle
The net force acting on any charge is the
vector sum of the forces due to the remaining
charges in the distribution.
F1,net F1,2 F1,3 F1,n
F1x F1,2, x F1,3, x ... F1,n , x
F1 y F1,2, y F1,3, y ... F1,n , y
F1z F1,2, z F1,3, z ... F1, n , z
February 27, 2020 University Physics, Chapter 21 38
Example - The Helium Nucleus
The nucleus of a helium atom has two protons and two
neutrons. What is the magnitude of the electric force
between the two protons in the helium nucleus?
The distance between the two protons is approximately 2.0 10-15 m
Each proton has charge q 1.602 1019 C
The force is given by
2
19
q1q2 9 Nm
2
1.602 10 C
F k 2 8.99 10 58 N
C 2.0 1015 m
2 2
r
February 27, 2020 University Physics, Chapter 21 39
2مثال
]Determine magnitude of the force between He
nuclei (2+P) and Ne nuclei (+10P)
Distance between them r=3x10-9m
charge of one proton P=1.6x+19C
Two equally charges (q2&q1( )0.8x10-9C ) one +ve and
one –ve separated by r= 0.1m المؤثرة علىcalculate electric
force on third charge
qo= 0.4x10-9C )if it puted at a&b. as shown
q2 aq q1
At point a - +3 0.04m
+
0.06m
r=0.1m
r=0.1m r=0.1m b
At point b - + +
q2 F2 q1 F1 q3
calculate electric force affected on q2 due to
؟q1 and q3 as shown in figure.
- q =1.2ϻc
3
F2
5cm F1
+ q =2.5ϻc
2
- q =3.6ϻc
1
5cm
find electric force on q3 due to q1 and 3مثال
q2 as shown below
