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EE3321 ELECTROMAGNETIC FIELD THEORY
Lecture 3
Highlights
1. Coulomb’s Law
As reported by the Ancient Greek philosopher Thales of Miletus around 600 BC, charge could be
accumulated by rubbing fur on various substances, such as amber (or “electron” in Greek which
is spelled ήλεκτρον). The Greeks noted that the charged amber buttons could attract light objects
such as hair. They also noted that if they rubbed the amber for long enough, they could even get
a spark to jump.
The law of electrostatic attraction and repulsion, developed in the 1780s by French physicist
Charles Augustine de Coulomb, may be stated in scalar form as follows:
The magnitude of the electrostatic force between two point electric charges is directly
proportional to the product of the magnitudes of each charge and inversely proportional
to the square of the distance between the charges.
Thus, the magnitude of the force exerted by two charges over each other is given by
F = k Q
1
Q
2
Newtons (N= kg m s
2
)
R
2
k =
__1___
=
9 x 10
9
N m
2
/ (As)
2
4πε
o
ε
o
= 8.85 x 10
12
F/m
permittivity of free space
Coulomb’s Law falls into the category of inversesquare laws found in physics.
The diagram
below shows how the law works.
Lines represent the “flux” emanating from the source. The total number of flux lines depends on
the strength of a charge and is constant with increasing distance. A greater density of flux lines
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(lines per unit area) means a stronger force field.
The density of flux lines is inversely
proportional to the square of the distance from the source because the surface area of a sphere
increases with the square of the radius. Thus the strength of the field is inversely proportional to
the square of the distance from the source.
2. Electric Charge Convention
Electric charge
is a fundamental conserved property of some subatomic particles, which
determines their electromagnetic interaction.
Electrically charged matter is influenced by, and
produces, electromagnetic fields.
The interaction between a moving charge and an
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 Fall '08
 FLORES
 Charge, Electromagnet, Electric charge, Charge Convention Electric

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