material objects made up of atoms are neutral in general. When an object is rubbed with another object (for example rubber with silk cloth), some amount of charge is transferred from one object to another due to the friction between them and the object is then said to be electrically charged. Charging the objects through rubbing is called triboelectric charging. .
. Basic properties of charges (i) Electric charge Most objects in the universe are made up of atoms, which in turn are made up of protons, neutrons and electrons. These particles have mass, an inherent property of particles. Similarly, the electric charge Rubber __ __ _ ++ Glass (a) Rubber Rubber __ __ _ __ __ _ (b) A negatively charged rubber rod is attracted to a positively charged glass rod A negatively charged rubber rod is repelled by another negatively charged rubber rod Figure .
(a) Unlike charges attract each other (b) Like charges repel each other 12th - 12th - - - - - Unit Electrostatics . COULOMB’S LAW In the year , Coulomb deduced the expression for the force between two stationary point charges in vacuum or free space. Consider two point charges q and q at rest in vacuum, and separated by a distance of r as shown in Figure . .
According to Coulomb, the force on the point charge q exerted by another point charge q is k q q ( . ) where r is the unit vector directed from charge q to charge q and k is the proportionality constant. F q q r Figure . Coulomb force between two positive point charges Important aspects of Coulomb’s law (i) Coulomb's law states that the electrostatic force is directly proportional to the product of the magnitude of the two point charges and is inversely proportional to the square of the distance between the two point charges.
(ii) The force on the charge q exerted by the charge q always lies along