on the nature of charges. • The value of the gravitational constant G = . × – N m kg – . The value of the constant k in Coulomb law is k = × N m C – .
Since k is much more greater than G , the electrostatic force is always greater in magnitude than gravitational force for smaller size objects. • The gravitational force between two masses is independent of the medium. For example, if kg of two masses are kept in air or inside water, the gravitational force between two masses remains the same. But the electrostatic force between the two charges depends on nature of the medium in which the two charges are kept at rest.
(vii) The force on a charge q exerted by a point charge q is given by q q π e Here r is the unit vector from charge q to q . But r =− , joining the two charges. r is the unit vector pointing from charge q to q .It is shown in the Figure . .
Likewise, the force on the charge q exerted by q is along - r (i.e., in the direction opposite to r ). (iii) In SI units, k o π e and its value is × N m C – . Here e o is the permittivity of free space or vacuum and its value is e o k C N m (iv) The magnitude of the electrostatic force between two charges each of one coulomb and separated by a distance of m is calculated as follows: F = × × × = × N . This is a huge quantity, almost equivalent to the weight of one million ton.
We never come across coulomb of charge in practice. Most of the electrical phenomena in