q k q q where r is the unit vector from q to q along the line joining the two charges and r is the distance between the charges q and q . The electrostatic force between two charges is not affected by the presence of other charges in the neighbourhood. The force on q exerted by the charge q is k q q By continuing this, the total force acting on the charge q due to all other charges is given by tot n + .......... k q q q q q q tot ...
.... + q q n n n ( . ) The gravitational force between the proton and the electron is attractive. The magnitude of the gravitational force between these particles is Gm m G e p = .
× – N The ratio of the two forces e G = . × Note that F e G » The electrostatic force between a proton and an electron is enormously greater than the gravitational force between them. Thus the gravitational force is negligible when compared with the electrostatic force in many situations such as for small size objects and in the atomic domain. This is the reason why a charged comb attracts an uncharged piece of paper with greater force even though the piece of paper is attracted downward by the Earth.
This is shown in Figure . F e F G Figure . Electrostatic attraction between a comb and pieces of papers 12th - 12th - - - - - Unit Electrostatics r r F sin F sin F