1.
Introduction
2.
Charges of matter
3.
Electric Charges
4.
Statement
5.
Mathematical Derivation
Discovered by a French scientist
Charles Augustin De Coulomb.
 The year of discovery was 1785.
 Created a device that helped him
develop his theories on charge,
electric force and field.
 The device was called Torsion
Balance.
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A matter is said to be charged when there is an imbalance
of protons and electrons.
The greater is the accumulation of elecrons ,the greater
is the charge.
Charge is measured in coulombs (C).
An electron has 𝟏. 𝟔 × 𝟏𝟎−𝟔 C.
Electron is derived from Greek
word Elektron meaning amber.
 Protons have positive charge
and electrons have negative
charge.
 Like Charges always repel each
other and unlike charges always
attract each other.

“The Force of attraction or repulsion between any two
point charges is directly proportional to the magnitude of
the charges and inversely proportional to the square of
distance between their centers.”
Limitations:
The Charges must be point charges ,i.e., small in magnitude and
positive.
o The charges must be symmetrical and distinct.
o
Consider a system of two point charges, q1 and q2 ,
separated by a distance r in vacuum. The force exerted by q1
on q2 is given by Coulomb's law:
Then, according to Coulomb`s Law;
𝑓 ∝ 𝑞1 ……..(1)
𝑓 ∝ 𝑞2 ……..(2)
𝑓 ∝ 𝑟 2 ……..(3)
Combining (1) , (2) and (3) :
𝑞1 𝑞2
𝑓∝ 2
𝑟
It can also be written as:
𝑞1 𝑞2
𝑓=𝑘 2
𝑟
Where “k” is constant of proportionality called
Coulomb`s Contant.
1
𝑘=
4𝜋𝜀°
Where 𝜖° is constant of proportionality called
permitivity constant.
And;
𝜀° = 8.85 × 10−12 𝐶 2 𝑁 −1 𝑚−2
Then;
k= 9 × 109 𝑁𝑚2 𝐶 −2
The equation can also be written as:
1 𝑞1 𝑞2
𝐹=
4𝜋𝜀° 𝑟 2
In vector form:
𝐹=
1 𝑞1 𝑞2 ^
(
)𝑟
4𝜋𝜀° 𝑟 2
End