4. Coulomb's Law

Statement of Coulomb’s Law

Coulomb’s First Law

electrically charged bodies

Coulomb’s first law states that like-charged objects (bodies or particles) repel each other and unlike charged objects (bodies or particles) attract each other.

Coulomb’s Second Law

Coulomb’s second law states that the force of attraction or repulsion between two electrically charged objects is directly proportional to the magnitude of their charge and inversely proportional to the square of the distance between them. Hence, according to the Coulomb’s second law,


  1. ‘F’ is the repulsion or attraction force between two charged objects.
  2. ‘Q1’ and ‘Q2’ are the electrical charged of the objects.
  3. ‘d’ is distance between center of the two charged objects.
  4. ‘k’ is a constant that depends on the medium in which charged objects are placed. In S.I. system, as well as in M.K.S. system k=1/4πεoεr. Hence, the above equation becomes.

The value of εo = 8.854 × 10-12 C2/Nm2.

Hence, Coulomb’s law can be written for medium as,

Then, in air or vacuum εr = 1. Hence, Coulomb’s law can be written for air medium as,

The value of εr would change depends on the medium. The expression for relative permittivity εr is as follows;

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