Page 30 - Basic Electrical Engineering

The outermost shell of an atom may have less than eight electrons. As for
example, copper has an atomic number of 29. This means, copper atom has
29 protons and 29 electrons. The protons are concentrated in the nucleus

while the electrons are distributed in the K, L, M, and N shells as 2, 8, 18,
and 1 electrons, respectively. The outermost shell of a copper atom has one

electron only whereas this shell could have 8 electrons.
The position occupied by an electron in an orbit signifies its energy. There

exists a force of attraction between the orbiting electron and the nucleus due
to the opposite charge the of electron and the proton. The electrons in the

inner orbits are closely bound to the nucleus than the electrons of the outer or
outermost orbit. If the electron is far away from the nucleus, the force of
attraction is weak, and hence the electrons of outermost orbit are often called

free electrons. For example, a copper atom has only one atom in the last orbit
which otherwise could have eight electrons.

In a copper wire consisting of large number of copper atoms, the atoms are
held close together. The outermost electrons of atoms in the copper wire are

not sure about which atom they belong to. They can move easily from one
atom to the other in a random fashion. Such electrons which can move easily

from one atom to the other in a random fashion are called free electrons. It is
the movement of free electrons in a material like copper that constitutes flow
of current. Here, of course, the net current flow will be zero as the movement

of the free electrons is in random directions. When we apply a potential,
which is nothing but a force, it will direct the flow of electrons in a particular

direction, i.e., from a point of higher potential towards a point of lower
potential. Thus, current flow is established between two points when there
exists a potential difference between the points.

When in a material the electrons can move freely from one atom to another
atom, the material is called a conductor. Silver, copper, gold, and aluminium

are good conductors of electricity. In general, all metals are good conductors
of electricity. Although silver is the best conductor of electricity, the second

best conductor, i.e., copper, is mostly used as conductor because of the cost
factor. In electrical and electronic engineering fields, the purpose of using a