Page 37 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Neutron: It is the same mass as the proton but no charge. There for the nucleus of an atom is positively charged.
The sum of protons and neutrons constitutes the entire weight of an atom and is called atomic weight.
Atomic weight=Number of proton + Number of neutron.
Electrons

It is the small particle revolving round the nucleus. It has negative electric charge, having negligible mass. The
charge on an electron is equal but opposite to that on a proton. Number of electron is equal to the number of
proton in an atom under ordinary condition. Therefore, an atom is neutral as a whole. The number of proton or
electron in an atom is called atomic number.
Charge on an electron, e =1.602 X 10 coulomb.
-19
Mass of electron, m = 9X10 kg.
-31
Radius of an electron, r = 1.9X 10 meter.
-15
The ratio of e/m of an electron is small. this means that mass of an electron is very small as compared to its
charge. It is due to the property of an electron that its very mobile and is gently influenced by electric or magnetic
field. (Fig 2)

Fig 2

Energy shell: Electron in an atom revolves around the nucleus in different orbits or paths called shell. The
number of electron in any orbits given by 2n 2
Where ‘n’ is the number of orbit.

For e.g. First orbit contains - 2X1 = 2electron.
2
Second orbit contains - 2X2 = 8 electrons….and so on.
2
The energy of an electron is increased as its distance from the nucleus increases. Thus, an electron in the second
orbit possesses more energy than the electron in the first orbit; the electron in the third orbit has higher energy
than second orbit. It is clear that in the last orbit possess very high energy as compared to the electrons in the
inner orbit.

Valence electron: The electron in the outermost orbit of an atom are known as valence electron. The max.number
of valence electron can be 8. These valence electrons play an important role in determining physical, chemical
and electrical properties of material. These electrons determine whether or not the material is chemically active;
metal or non-metal or, a gas or solid.
Free electron: The greater the energy of valence electron, the lesser it is bound to the nucleus. The valence
electron possesses so much energy, they are loosely attached the nucleus. These loosely attached valence
electrons move within the material. The valence electron which are loosely attached to the nucleus are known as
free electrons. (Fig 3)

Fig 3

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CITS : Power - Electrician & Wireman - Lesson 5 - 12