**The French engineer Charles Coulomb investigated
the quantitative relation of forces between charged objects during the 1780's.
Using a torsion balance device, created by Coulomb himself, he could determine
how an electric force varies as a function of the magnitude of the charges
and the distance between them.**

**In that way Coulomb demonstrated that the
electric force between two stationary charged particles is :
- Inversely proportional to the square of the distance r between the particles
and is directed along the line that joins them.
- Proportional to the product of the charges q**

**Coulomb 's Law can be expressed in the form
of an equation :**

**The validity of Coulomb's Law has been verified
with modern devices that have detected that the exponent 2 has an exactitude
of one part in 10 ^{16}.
K_{e} is a constant known as the Coulomb 's constant, which in the
International System units has the value K_{e} = 8.987x10^{9}
Nm^{2}/C^{2}.
The International System unit for charge is the Coulomb.
The smallest known charge in nature is the charge of an electron or proton,
which has an absolute value of e = 1.60219x10^{-19} C.
Thus, a 1 Coulomb charge is approximately equal to a charge of 6.24x10^{18}
(= 1C/e) electrons or protons. **

**We should notice that the force is a vectorial
quantity, that is, has magnitude and direction. Coulomb 's Law expressed in
vectorial form for the electric force F _{12} exerted by a charge q_{1}
over a second charge q_{2} is (bold type is used to denote vectorial
quantities):**

**As every force obeys Newton's third Law,
the electric force exerted by q _{2} over q_{1} is equal in
magnitude to the force exerted by q_{1} over q_{2} and in
the opposite direction, that is F_{21}= - F_{12}.**

**If q _{1} and q_{2} have
the same sign, F_{12} takes the direction of r. If q_{1} and
q_{2} have opposite sign, the product q_{1}q_{2} is
negative and F_{12 }points opposite to r.**

**When two or more charges are present, the
force between any pair of them is given by the above equation. Hence, the
resultant force on any of them is equal to the vectorial sum of the forces
exerted by the different individual charges. For example, with three charges,
the resultant force exerted by particles 2 and 3 over 1 is**

**F _{1} = F_{21} + F_{31}**

**Most Related Sites
· Coulomb's
Law Exercises
· Energy, Work
and Power: Concepts
· Power
· Kinetic
Energy
· Potential
Energy**

**Related Sites:**

** · Physics, Main Page
· Physics, Mathematics
· Physics, Detailed Homework Scope Help
· Energy, Work and Power: Concepts
· Kinetic Energy
· Potential Energy
· Power
· Ohm's Law, Principle
· Ohm's Law Exercises
· Gauss' Law
· Gauss' Law Exercises
· Second Newton's Law
· Second Newton's Law Examples, Part One
· Second Newton's Law Examples, Part Two
· Physics Problems, Example
· Physics Homework - Mechanical Energy Conservation Problems
· Physics Homework - Mechanical Power Problems
· Electric Field Charges
· Electric Field Exercises
· Electric Potential Energy
· Exercises, Electric Potential Energy
· Sound Waves
· Sound Waves: Standing, Interference, Doppler Effect - Examples
· Vectors, Scalars
· Vectors, Scalars - Analytic Method
· Addition Vector Tools, Problems
· **