Basic properties of charges
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 is another intrinsic and fundamental property of
particles. The nature of charges is understood through various experiments
performed in the 19th and 20th century. The SI unit of charge is coulomb.
Benjamin Franklin argued
that when one object is rubbed with another object, charges get transferred
from one to the other. Before rubbing, both objects are electrically neutral
and rubbing simply transfers the charges from one object to the other. (For
example, when a glass rod is rubbed against silk cloth, some negative charge
are transferred from glass to silk. As a result, the glass rod is positively
charged and silk cloth becomes negatively charged). From these observations, he
concluded that charges are neither created or nor destroyed but can only be
transferred from one object to other. This is called conservation of total
charges and is one of the fundamental conservation laws in physics. It is stated
more generally in the following way.
‘The total electric
charge in the universe is constant and charge can neither be created nor
be destroyed. In any physical process, the net change in charge will always be
zero.
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 (1.1)
Here n is any
integer (0,±1,±2, ±3,
±4………..). This is called quantisation of electric charge. Robert Millikan in
his famous experiment found that the value of e = 1.6 × 10-19C. The charge
of an electron is −1.6 ×
10-19 C and the charge of the proton is +1.6 × 10-19C.
When a glass rod is
rubbed with silk cloth, the number of charges transferred is usually very
large, typically of the order of 1010. 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.
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