Coulomb's inverse square law related to electrostatic

• Coulomb’s Law: This law states that the mutual electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of the distance separating them. Direction of  these  force  along  the  line  joining  two  point charges .
•                                           “Explanation”: let there be two points charges having mangnitude  q and q' are separated at a distance  r  in air or vacuum and the magnitude  of force acting b/w them be F. 

               

       

Thus  according  to  coulomb's  law: 

Fα q.q'

And.  Fα 1/r²

Adding above relation 

Fα q.q'/r²

              F = a.q.q'/r².........(1)                  

Here ‘a’ is a proportionality constant the value of which   depends  upon the system of measurment. 

 (1)  In c.g.s system for air/vacuum

                      a = 1

       putting this value in equal (1)

     

                F = 1× q.q'/r²   dyne

(2) In S.I system for air/ vacuum 

      

      a = 1/4πEo = 9×10^9  Nm²/C

Here Eo= permittivity of free space 

            Eo = 8.86×10–¹²  C²/Nm²

This value of a is substitute in equ(1)

    

               F = 1×q×q'/4πEo×r²   N

     

     

Note: Next topic is comparsion between electrostatic force and gravitational force