the line process, the net change in charge will always be zero. (iii) Quantisation of charges What is the smallest amount of charge that can be found in nature? Experiments show that the charge on an electron is − e and the charge on the proton is + e . Here, e denotes the fundamental unit of charge.
The charge q on any object is equal to an integral multiple of this fundamental unit of charge e . q = ne ( . ) Here n is any integer ( , ± , ± , ± , ± ………..). This is called quantisation of electric charge.
Robert Millikan in his famous experiment found that the value of e = . × – C. The charge of an electron is − . × – C and the charge of the proton is + .
× – C. When a glass rod is rubbed with silk cloth, the number of charges transferred is usually very large, typically of the order of . So the charge quantisation is not appreciable at the macroscopic level. Hence the charges are treated to be continuous (not discrete).
But at the microscopic level, quantisation of charge plays a vital role. EXAMPLE . Calculate the number of electrons in one coulomb of negative charge. Solution According to the quantisation of charge, q = ne Here q = 1C.
So the number of electrons in coulomb of charge is n e electrons 12th - 12th - - - - - Unit Electrostatics (vi) Coulomb’s law has same structure as Newton’s law of gravitation. Both are inversely proportional to the square of the distance between the particles. The electrostatic force is directly proportional to the product of the magnitude of two point charges and gravitational force is directly proportional to the product of two masses. But there are some important differences between these two laws.
• The gravitational force between two masses is always attractive but Coulomb force between two charges can be attractive or repulsive, depending