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Electricity
Chapter 19
Introduction
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One of the oldest problem in physics
phenomena related to electric charge
deals with
-interactions between charges
-movement of charges -->
(electric currents)
-production of charges
-detection of charges
-measurement of charges
-properties of electric currents
-electric apparatus
Electric charges: a reality
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Experiments showing the existence of electric charges
- combing the hair
- rubbing a balloon with a piece of wool
- glass rod rubbed with silk
- amber rubbed with wool
- rubber rod rubbed with fur
– Two different kind of charges: positive and negative
(one kind on the wool and the other type of the balloon)
– Like charges repel; unlike charges attract
- Materials that are able to transfer
charges are known as conductors
- Materials that cannot transfer charges are
known as insulators
- Grounding --> giving the charges to the
Earth
Conservation of charges
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When creating a charge with a given sign we have to
create the same amount of opposite charge
“The amount of electric fluid should remain the same” -->
Benjamin Franklin
 Objects are electrically neutral not because they
contain no charges! --> they contain the same
amount of positive and negative charges
– The law of charge conservation:
In an isolated system the total charge
is conserved
total charge: algebraic sum of the
charges
Induced Attractions
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Attraction is more common than repulsion
Charged objects can attract uncharged ones
A charged rod attracts a neutral metal ball
 Explaining attraction between charged and
uncharged objects --> induced charges
separation of charges in the uncharged object
(it can be conductor or isolator)
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Measuring the amount of charge: the electroscope
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The electroscope can be used to detect charges
- Working principle: same type of charges repel
each other
- Construction and working principle
- The electroscope does not indicate
the sign of the charges!
Experiments with the electroscope:
The electric force
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Calculating the magnitude of the attractive or
repulsive forces between the charges
Variation with distance and amount of
charges
Q1Q2
F k 2
r
Coulomb’s law
-Charles Coulomb 1785
Q1 and Q2 : the two charges
(measured in coulomb: C)
r: distance between the charges
k: 9 x 109 N m2/C2
Coulomb’s constant
Difference and Similarities between
Electricity and Gravity
Mathematical form of the Coulomb law and law of
gravitation very similar
2
- gravitation is always attractive (no
9 Nm
k  9 10
2
C
negative mass)
Q1Q2 - electrical force can be both
Fe  k 2
attractive or repulsive
r
- Electric force is dominant in the
m1m2 atomic world
Fg  G 2
r
- Gravitational forces dominates on the
2
macroscopic scale: people, planets, galaxies
11 Nm
G  6.7 10
- Electric forces are more stronger !!!
kg 2
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Lightning
- During the storm clouds develop a separation of
electric charge
- Tops of the clouds positively, bottoms
negatively charged
- The ground positively charged due to the
repulsion of the electrons in the ground
-downward moving electrons
- upward flowing positive charges
The Electric Field
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Electric charges modify the space around them--> concept of electric
field
This electric field is manifested by the force acting on charges
Concept of electric field is analogous with the concept of gravitational
field
Definition: The value of the electric field (E) at any point in space is
equal to the force experienced by one unit of positive charge at that
 
point
 F
Q
Units: N/C
E k 2
electric field is a vector: E 
q
- electric field can be represented by a set
of arrows around the object
(size of the arrows representing the
magnitude of the field)
- electric field can be represented by a set
of continuous lines starting from positive
charges and ending at negative ones
(density of the lines ~ intensity of the
field)
r
Electric potential
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Characterizes energetically the electric field
Similar quantity to the gravitational potential
Definition of electric potential energy:
For each point, the electric potential energy is equal to the work done
against the field in bringing the charged object from some zero
reference location to that point (if we do the work EPE> 0, if the field
does the work EPE <0)
- It’s value does not depend on the path but on where we choose the
zero reference location
Qq at a distance r from Q
EPE  k
- Measured in : Joules (J)
zero level at infinite
r
Definition of the electric potential: electric potential energy of unit
charge (EPE/charge)
at a distance r from Q
Q
V  k
- Units: J/C= Volt (V)
r
By knowing the electric potential difference between two points we
can calculate the work done by the field when moving a given charge
between the two points
Summary
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Objects become electrically charged or uncharged by transferring charges
Two different charges exist: positive and negative
In Isolated systems the total charge is conserved
Like charges repel, unlike charges attract
The electric force between two charges is given by the Coulomb law
Electric charges are surrounded by an electric field equal to the force
experienced by a unit charge
electric potential characterizes energetically the electric field
The electric potential energy gives the work made against the electric
field, when we bring the given charge from a zero reference location to the
given point
The electric potential equals the electric potential energy divided by the
objects charge
Home-work Assignment:
Part I: 492/3-8,11,15-27; 493/28-31; 495/2-12
Part II: 493/33-35, 37-42; 494/43-48,50-58; 495/59,60,13-18,23,24