Electrical Engineering ⇒ Topic : Coulombs Laws of Electrostatics

Coulomb's Laws of Electrostatics

Charles Coulomb, a French scientist, observed that when two charges are placed near each other, they experience a force. He performed a number of experiments to study the nature and magnitude of the force between the charged bodies. He summed up his conclusions into two laws,known as Coulomb's laws of electrostatics.

First law. This law relates to the nature of force between two charged bodies and may be stated as under :

Like charges repel each other while unlike charges attract each other

In other words, if two charges are of the same nature (i.e. both positive or both negative), the force between them is repulsion. On the other hand, if one charge is positive and the other negative, the force between them is an attraction.

Second law. This law tells about the magnitude of force between two charged bodies and may be stated as under

The force between two *point charges is directly proportional to the product of their magnitudes and inversely proportional to the square of distance between their centres

where k is a constant whose value depends upon the medium in which the charges are placed and the system of units employed. In SI units, force is measured in newtons, charge in coulombs, distance in metres and the value of k is given by ;

                                                  ε₀  = Absolute permittivity of vacuum or air.

                                                   ε_r  = Relative permittivity of the medium in which the charges are placed. For vacuum or air, its value is 1.

                                           FIGURE (A)

The value of  ε₀  = 8.854 x 10^-12 F/m and the value of  ε_r  is different for different media

COULOMB'S LAW

By means of a torsion balance which he had developed, Coulomb in 1785,established the following facts for charges at rest when their diameters are small compared to the distance between their centres.

• Bodies having like charges repel and bodies having unlike charges attract each other.
• The magnitude of the force between the bodies is proportional to the magnitude of the charges upon the bodies.
• The magnitude of the force is inversely proportional to the square of the distance between the charges.
• The force acts along the line joining the centres of the charges.
• The force depends upon the medium in which charges are placed.

In brief, Coulomb's law can be expressed as

                                              ............ (1)

Consider two point charges separated in free space as shown in Figure 2 (a) and  (b), in which

                      F₁₂ = force on charge 1 due to charge 2, and

                      F₂₁ = force on charge 2 due to charge 1.

                      F₁₂ = F₂₁ is magnitude but they are opposite in directions.

In Fig. 2 (b), it is shown that the charges are unlike and therefore the force is attractive in nature.

                                                    figure (2) Charges in free space