Energy Band Diagaram ( Conductor, Semiconductor and Insulator )

Valance electrons or band

• The electrons in the outer most orbit of an atom are called as valance electrons.
• The valance electrons have highest energy. 

In this theory, Neil Bohr’s atomic structure, definitions of matter, molecules, atoms, nucleus, atomic weight, extra nucleus, electrons in orbit, atomic number and energy of an electron are explained.

Atomic structure

• The scientist Neil Bohr explains the atomic structure. 
• According to Neil an atom consists of positive charged nucleus and negative charged electrons revolve in circular orbit. 
• The electrons can revolve around the nucleus in certain orbits. 
• Each orbit consists of certain energy level. 
• As the distance between nucleus and orbit increases, energy of electrons increases. 
• If the energy is given to an electron, the electron lifts to higher energy level. This is called as excitation.

Matter

• The matter is defined as anything which occupies space and weight such as wood, copper, water, air etc.

Molecule

• The matter is made up of very small particles known as molecules.

Atoms

• It is smallest into which matter can be divided without release of charged particles. It is smallest unit of matter.

Nucleus

• The nucleus of an atom is mainly consists of two particles protons and neutrons. 
• The proton has positive charge whereas the neutron has no charge. The mass of neutron is same as that of proton. 
• Therefore the nucleus has positive charge.

Atomic weight

• It is sum of weight of protons and weight of neutrons.
• Atomic weight = weight of protons + weight of neutrons
• A = Z + N
• According to electron theory, the atoms of all elements composed mainly two parts namely nucleus at the center and electrons surrounding it. 
• The nuclear symbol consists of three parts, symbol of element, atomic number and atomic mass. 
• Let us try to understand it.

²⁶Al₁₃ = ^A Al _Z = ^{Number of protons + Number of neutrons} Al _{Number of protons}

Mass of aluminum (A) is 26

Atomic number of aluminum (Z) is 13

Therefore the number of neutron in the aluminum = A – Z = 26 – 13

= 13

Extra nucleus (Electrons)

• It is outer part of an atom which contains electrons only. 
• The charge of an electron is equal and opposite to that of charge of proton. The number of electron is equal to number of proton. 
• Therefore we can say that an atom is neutral.
• Charge of an electron = 1.602 × 10 ^{– 19} Coulomb
• Mass of an electron = 9.0 × 10 ^{– 19} Kg
• Radius of an electron = 1.9 × 10 ^{– 15} Meter
• The ratio of charge of an electron to the mass of an electron is 1.77 × 10¹¹ Coulomb per kg. 
• It means that mass of an electron is very small as compared to its charge. The electron is greatly affected by electric or magnetic field.

Electrons in orbit

• The electrons of an atom around nucleus revolve in different orbits. 
• The number of electrons in any orbit is equal to 2n².

Where n is orbit number

Number of electrons in first orbit = 2(1)² = 2

                                Second orbit = 2(2)² = 8

                                   Third orbit = 2(3)² = 18

• The number of electron in the last orbit should not greater than 8
• Any orbit should not contain more than 18 electrons. 
• The atomic number of silicon is 14. It means that 2 electrons lies in first orbit, 8 electrons in second orbit and remaining 4 electrons in the third or last orbit.

Atomic number

• It is defined as the number of proton is equal to number of electrons.

        Atomic number = Number of proton

                                  = Number of electrons

• The atomic number of sodium is eleven (11), it means that it has 11 number of protons and 11 number of electrons. 
• It has 2 electronics in first orbit, 8 electrons in the second orbit and only one electron in the last or third orbit. List of number is given here.

https://education.jlab.org/itselemental/index_num.html

Energy of an electron

• Each electron consists of kinetic energy due to its motion and potential energy due to charge of electron. 
• As the distance between nucleus and electron increases, the energy of an electron also increases. Therefore
• Energy of 1^st orbit electrons < Energy of 2^nd orbit electrons
• Energy of 2^nd orbit electrons < Energy of 3^rd orbit electrons
• Energy of 3^rd orbit electrons < Energy of 4^th orbit electrons
• It means that electrons of first orbit consists lowest energy whereas the electrons of last orbit have highest energy. 
• The electrons of last orbit play important role to determine electrical, chemical and physical properties of a material.

 You may also like :

 Function of Freewheeling diode

Working Principle of DC Motor

Structure, Working and Characteristic of IGBT

Practical DC Generator

Voltage Source and Current Source

Ideal Voltage Source

• An ideal voltage source is one in which terminal voltage remains constant regardless current flow through its terminals. 
• The ideal voltage source has zero internal resistance. 

Speed Control of the DC Shunt Motor

Introduction

The speed of DC motor is related to following equation

N a E_b / F

N = K ( V – I_aR_a ) / F

Characteristics of DC Shunt Motor, DC Series Motor, DC Compound Motor

The importance characteristic of DC Motor is as follows

Torque – Armature current ( T – I_a )

• It represents relation between torque and armature current in the DC motor. 

Neutral Shift in the Transformer

Describe the effect on transformer when unbalanced load is connected to Star / Star connected transformer? What is neutral shift? Describe its causes?

• The star – star connected transformer works on satisfactory if the load is balanced. 

Per unit Resistance and Reactance

Per unit resistance & Per unit reactance

Per unit resistance

• It is defined as the resistance drop at rated current and frequency expressed at rated voltage.

Per unit resistance = I₁R₀₁ / V₁

Voltage Regulation of the Transformer

• When the secondary of the transformer is not loaded, the secondary terminal voltage ₀V₂ is equal to secondary induced emf E₂.

        E₂ = V₂ + I₂ ( R₂ + jX₂ )

Transformer Efficiency and Condition for Maximum Efficiency

EFFICIENCY OF TRANSFORMER

• The efficiency of the transformer at any load power factor is defined as the ratio of the output power and input power.

Efficiency = ( Output power / Input power ) × 100%

Working of Auto transformer, Saving in Copper Material

• It consists of only single winding which is common to both primary and secondary winding. 
• It has two end terminals ( PR) acts as primary and one end terminal ( either P or R ) and inter mediate terminal Q acts as a secondary winding ( Figure A ). 

Parallel Operation of the Single Phase Transformer

Parallel Operation

• When the primaries of two or more transformers are connected to common supply bus bars and secondary are connected to common load bus bars, the transformers are said to connect in parallel.

Losses in the Transformer

The following losses are occurs in the transformer.

Core Loss

• The core loss depends upon the supply voltage.