Introduction
Everybody is familiar with the functions that electricity can perform. It can be used for lighting,
heating, traction and countless other purposes. The question always arises, “What is electricity” ?
Several theories about electricity were developed through experiments and by observation of its
behaviour. The only theory that has survived over the years to explain the nature of electricity is the
Modern Electron theory of matter. This theory has been the result of research work conducted by
scientists like Sir William Crooks, J.J. Thomson, Robert A. Millikan, Sir Earnest Rutherford and
Neils Bohr. In this chapter, we shall deal with some basic concepts concerning electricity.
Nature of Electricity
We know that matter is electrical in nature i.e. it contains particles of electricity viz. protons and
electrons. The positive charge on a proton is equal to the negative charge on an electron. Whether
a given body exhibits electricity (i.e. charge) or not depends upon the relative number of these par-
ticles of electricity.
(i) If the number of protons is equal to the number of electrons in a body, the resultant charge
is zero and the body will be electrically neutral. Thus, the paper of this book is electrically neutral
(i.e. paper exhibits no charge) because it has the same number of protons and electrons.
(ii) If from a neutral body, some *electrons are removed, there occurs a deficit of electrons in
the body. Consequently, the body attains a positive charge.
(iii) If a neutral body is supplied with electrons, there occurs an excess of electrons. Conse-
quently, the body attains a negative charge.
Unit of Charge
The charge on an electron is so small that it is not convenient to select it as the unit of charge. In
practice, coulomb is used as the unit of charge i.e. SI unit of charge is coulomb abbreviated as C. One
coulomb of charge is equal to the charge on 625 × 1016 electrons, i.e.
1 coulomb = Charge on 625 × 1016 electrons
Thus when we say that a body has a positive charge of one coulomb (i.e. +1 C), it means that the body
has a deficit of 625 × 1016 electrons from normal due share. The charge on one electron is given by ;
Charge on electron = 16
1
625 10
− ×
= – 1.6 × 10–19 C
The Electron
Since electrical engineering generally deals with tiny particles called electrons, these small
particles require detailed study. We know that an electron is a negatively charged particle having
negligible mass. Some of the important properties of an electron are :
(i) Charge on an electron, e = 1.602 × 10–19 coulomb
(ii) Mass of an electron, m = 9.0 × 10–31 kg
(iii) Radius of an electron, r = 1.9 × 10–15 metre
The ratio e/m of an electron is 1.77 × 1011 coulombs/kg. This means that mass of an electron is
very small as compared to its charge. It is due to this property of an electron that it is very mobile
and is greatly influenced by electric or magnetic fields.
Energy of an Electron
An electron moving around the nucleus possesses two types of energies viz. kinetic energy due
to its motion and potential energy due to the charge on the nucleus. The total energy of the electron
is the sum of these two energies. The energy of an electron increases as its distance from the nucleus
increases. Thus, an electron in the second orbit possesses more energy than the electron in the first
orbit ; electron in the third orbit has higher energy than in the second orbit. It is clear that electrons
in the last orbit possess very high energy as compared to the electrons in the inner orbits. These last
orbit electrons play an important role in determining the physical, chemical and electrical properties
of a material.
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