Classification of Electrical Engineering Materials

Classification of Electrical Engineering Materials

Electrical engineering materials are the materials used to manufacture electrical machines, equipment, wires, cables, transformers, motors, generators and electronic devices. Different materials have different properties, so each material is selected according to the work it has to perform.

For example, copper is used in electrical wires because it allows electric current to flow easily, while rubber is used to cover wires because it prevents current from flowing into our bodies.

In this note we will first learn the four main groups of materials. Then we will learn about atoms, energy bands and resistivity, which explain why these groups behave differently.

1. What Is the Classification of Electrical Materials?

Classification means arranging materials into different groups according to their properties and behaviour.

For basic electrical engineering, materials are classified into four main groups:

1. Conducting materials
2. Semiconducting materials
3. Insulating materials
4. Magnetic materials

2. Conducting Materials

Definition: Conducting materials are materials that allow electric current to flow through them easily.

Imagine a water pipe. A wide, clear pipe allows water to flow easily. Similarly, a conducting material allows electric current to flow easily. (This is only an analogy to help you picture the idea.)

Conductors generally contain a large number of free electrons. These electrons can move through the material when a voltage is applied.

Examples: Copper, aluminium, silver and gold.

Applications:

• Copper is used in electrical wires and motor windings.
• Aluminium is used in overhead transmission and distribution lines.
• Silver is used in certain electrical contacts because of its excellent electrical conductivity.
• Gold is used in specialised electronic contacts because it resists corrosion.

Important point: Silver is an excellent electrical conductor, but copper and aluminium are more commonly used for many practical wiring applications because of their cost and other useful properties.

3. Semiconducting Materials

Definition: Semiconducting materials have electrical conductivity between that of good conductors and good insulators. Their conductivity can be controlled by temperature, impurities and other conditions.

Think of a semiconductor as a material whose ability to conduct electricity can be controlled. It is neither as conductive as a good metal nor as resistive as a good insulator under ordinary conditions.

Examples: Silicon and germanium.

Applications:

• Diodes
• Transistors
• Integrated circuits (ICs)
• Solar cells
• Electronic control systems

Important point: Silicon is the most widely used semiconductor material in modern electronic devices.

4. Insulating Materials

Definition: Insulating materials are materials that strongly oppose the flow of electric current under normal operating conditions.

Think of insulation as a protective barrier. Just as the plastic covering of a wire separates the conducting metal from the outside, an insulator helps prevent unwanted current flow and electric shock.

Insulating materials generally have very few free electrons available for electrical conduction and a large forbidden energy gap.

Examples: Rubber, glass, porcelain, mica, PVC and dry paper.

Applications:

• PVC and rubber are used to cover electrical wires and cables.
• Porcelain and glass are used in electrical insulators.
• Mica is used in applications requiring good electrical and thermal insulation.
• Insulating oil is used in transformers for insulation and heat removal.

Important point: An insulator does not block current under every possible condition. Excessive voltage can cause electrical breakdown and allow current to flow.

5. Magnetic Materials

Definition: Magnetic materials are materials that respond to a magnetic field and are used where magnetic effects are required.

For example, an electric motor uses magnetic effects to produce rotation, while a transformer uses magnetic flux to transfer electrical energy from one winding to another.

Examples: Iron, silicon steel, nickel and ferrites.

Applications:

• Silicon steel is used in transformer cores.
• Electrical steel is used in the cores of many motors and generators.
• Ferrites are used in high-frequency transformer cores and electronic circuits.
• Hard magnetic materials are used to make permanent magnets.

Important point: Magnetic materials are further classified into soft and hard magnetic materials according to their magnetic behaviour. This topic is studied in detail in the unit on Magnetic Materials.

6. Comparison of the Four Types of Materials

Property	Conductors	Semiconductors	Insulators	Magnetic Materials
Main behaviour	Allow current to flow easily	Conductivity can be controlled	Strongly oppose current flow	Respond to magnetic fields
Free electrons	Many available for conduction	Limited under ordinary conditions	Very few available for conduction	Depends on the material and its electronic structure
Examples	Copper, aluminium	Silicon, germanium	Rubber, glass, PVC	Iron, silicon steel, ferrites
Main uses	Wires and windings	Electronic devices	Electrical protection and insulation	Motors, transformers and magnets

Note: These are four groups based on different properties and functions. Magnetic materials are not necessarily electrically insulating; for example, iron is both electrically conducting and magnetic. Therefore, these categories are not mutually exclusive.

7. Atomic Structure: The Reason Behind Different Behaviour

Why does copper conduct but rubber does not? The answer lies inside the atom.

Atom: An atom is the smallest particle of an element. It has a nucleus at the centre and electrons revolving around the nucleus in fixed paths called shells.

Particle	Charge	Position
Proton	Positive (+)	Inside the nucleus
Neutron	No charge	Inside the nucleus
Electron	Negative (−)	Revolves around the nucleus in shells

• The number of protons is equal to the number of electrons, so a normal atom is electrically neutral.
• The number of protons in an atom is called its atomic number (Z).
• The shells are named K, L, M, N and so on, starting from the nucleus. They can hold at most 2, 8, 18 and 32 electrons respectively.
• The outermost shell can hold at most 8 electrons.

Valence Electrons and Free Electrons

Valence electrons are the electrons present in the outermost shell of an atom. The electrical behaviour of a material depends mainly on these electrons.

Free electrons are electrons that are free to move from atom to atom through the material. They are the electrons that carry electric current.

Point	Valence Electrons	Free Electrons
Meaning	Electrons in the outermost shell of an atom	Electrons that are free to move from atom to atom
Position	In the outermost shell, bound to the atom	Move through the material, not bound to one atom
Role in conduction	Do not carry current unless they become free	Carry the electric current
Number	Fixed by the type of atom	Depends on the material and its temperature

Important point: Free electrons come from valence electrons, but all valence electrons are not free electrons. In a conductor many valence electrons become free. In an insulator they are tightly held and do not become free under normal conditions.

Fig. 1: Atoms of copper (1 valence electron) and silicon (4 valence electrons)

Diagram in words: The figure shows two atoms. The copper atom has four shells holding 2, 8, 18 and 1 electrons, so it has only one valence electron. The silicon atom has three shells holding 2, 8 and 4 electrons, so it has four valence electrons. The valence electrons are shown as open circles.

8. Classification Based on Atomic Structure

The number of valence electrons and how firmly they are held help us to group materials as conductors, semiconductors and insulators.

Material	Valence Electrons (Generally)	Behaviour of Electrons
Conductor	1 to 3	Loosely held. They easily become free, so current flows easily.
Semiconductor	4	Shared with neighbouring atoms by covalent bonds. At very low temperature there are almost no free electrons. At room temperature some bonds break and a few free electrons become available.
Insulator	5 to 8	Outer shell is nearly full and the electrons are tightly held. Almost no free electrons are available.

Important point: This is only a general guide, not a strict law. For example, lead has 4 valence electrons but is a conductor, and diamond has 4 valence electrons but is an insulator. The energy-band model given below explains the behaviour more correctly.

9. Energy Band Theory

In an isolated atom, electrons can have only certain fixed energies. These are called energy levels, and they correspond to the shells.

Think of the floors of a building. A person can stand on the first floor or the second floor, but not in the empty space between them. In the same way, an electron can have only allowed energies and not the energies in between. (Again, this is only an analogy.)

Formation of bands: In a solid, a very large number of atoms are very close to each other. Their energy levels affect each other, so each energy level spreads out into a band of many closely spaced levels.

Fig. 2: Formation of energy bands in a solid

Diagram in words: On the left, an isolated atom has three separate energy levels shown as short horizontal lines. On the right, in a solid, each level has spread into a wider rectangle called a band. The valence band is in the middle, the conduction band is at the top, and an empty space between them is the forbidden energy gap.

Important Terms

• Valence band: The band of energy levels occupied by the valence electrons.
• Conduction band: The band of energy levels above the valence band. Electrons in this band are free to move and contribute to electrical conduction.
• Forbidden energy gap (E_g): The energy range between the top of the valence band and the bottom of the conduction band in which allowed electron states are absent in the ideal band model. It is measured in electron volt (eV).

How is current conducted? For current to flow, electrons must be in the conduction band or in a partly filled band. A completely filled band cannot carry current. An electron of the valence band can move to the conduction band only if it gets energy equal to or more than E_g. This energy can be supplied by heat, light or an electric field.

Energy Bands of Conductors, Semiconductors and Insulators

1. Conductor: The valence band and conduction band overlap, or a band is only partly filled. There is no forbidden gap, so electrons can move easily even at ordinary temperature.

Fig. 3: Energy band diagram of a conductor

Diagram in words: Two bands are drawn one above the other. The conduction band and the valence band overlap in a shaded region, so E_g = 0.

2. Semiconductor: A small forbidden gap (about 1 eV) separates the two bands. At very low temperature the valence band is almost full and the conduction band is almost empty. At room temperature some electrons gain enough energy to cross the gap, so the material conducts a little.

Fig. 4: Energy band diagram of a semiconductor

Diagram in words: The conduction band is at the top and the valence band is at the bottom. They are separated by a small gap, marked E_g of about 1 eV.

3. Insulator: A very large forbidden gap (generally more than about 5 eV) separates the two bands. Under normal conditions, very few electrons can cross the gap, so almost no current flows.

Fig. 5: Energy band diagram of an insulator

Diagram in words: The conduction band is at the top and the valence band is at the bottom, with a very wide gap between them.

Material	Forbidden Energy Gap (Eg)	Electrical Behaviour
Conductor	Absent or effectively zero (bands overlap)	Electrons can move easily
Semiconductor	Small. Silicon about 1.1 eV, germanium about 0.67 eV	Some electrons can enter the conduction band under suitable conditions
Insulator	Large, generally more than about 5 eV	Very few electrons enter the conduction band under normal conditions

Exam tip: The forbidden energy gap is generally largest in insulators, smaller in semiconductors, and absent or effectively zero in the simple band model of conductors. The eV values above are approximate. Real materials can have more complex band structures.

10. Characteristics of Materials

Resistivity

Resistivity (ρ) is the resistance of a material of unit length and unit area of cross-section. For a conductor of length l and area of cross-section A, the resistance is:

R = ρl / A   so   ρ = RA / l

Resistivity depends on the material and its temperature. It does not depend on the size of the conductor. Its unit is ohm-metre (Ω·m).

Conductivity

Conductivity (σ) is the reciprocal of resistivity, that is σ = 1 / ρ. It shows how easily a material allows current to flow. Its unit is siemens per metre (S/m).

Effect of Temperature on Resistance

Material	Effect of Temperature on Resistance	Reason
Conductor	Increases (positive temperature coefficient)	At higher temperature the atoms vibrate more and collide more with the free electrons, so the flow of current is opposed more.
Semiconductor	Decreases (negative temperature coefficient)	More bonds break and more electrons enter the conduction band, so more charge carriers are available.
Insulator	Generally decreases	At high temperature some electrons gain enough energy to move, so the insulator becomes a poorer insulator.

Important point: Carbon also has a negative temperature coefficient of resistance. Alloys such as manganin have a temperature coefficient of resistance almost equal to zero, so their resistance remains almost constant when the temperature changes. These materials are studied in detail in the unit on Conducting Materials.

Summary of Characteristics

Point	Conductor	Semiconductor	Insulator
Valence electrons (generally)	1 to 3	4	5 to 8
Free electrons	Very many	Few	Almost none
Forbidden energy gap	Nil (bands overlap)	Small, about 1 eV	Large, above about 5 eV
Resistivity	Very low	Medium	Very high
Conductivity	High	Medium	Very low
Resistance with rising temperature	Increases	Decreases	Generally decreases
Examples	Copper, aluminium	Silicon, germanium	Rubber, glass

11. Everyday Examples to Remember

• Electric wire: Copper carries current, while its PVC covering provides insulation.
• Mobile phone: Silicon-based semiconductors are used in electronic chips.
• Transformer: Copper windings carry current, insulating materials separate the windings, and a magnetic core guides magnetic flux.
• Electric motor: Conductors carry current, insulating materials prevent short circuits, and magnetic materials help produce torque.

12. MCQ and True/False Facts

These facts match the type of questions asked in the previous question papers.

• The forbidden energy gap is largest in insulators.
• In an insulator, the valence band and conduction band are separated by a large gap.
• A semiconductor has an almost empty conduction band and an almost filled valence band (at low temperature).
• The forbidden gap of a conductor is not 1 eV. It is absent or effectively zero. (The statement "the gap is 1 eV in conductors" is False.)
• The electrons of the outermost shell are valence electrons. They are not all free electrons. (The statement "outermost electrons are known as free electrons" is False.)
• Air is an insulating material (True).
• Carbon has a negative temperature coefficient of resistance. Manganin has an almost zero temperature coefficient.

13. Important Questions for Examinations

Short-answer questions:

1. Define conducting materials and give two examples.
2. Define semiconductor and mention two applications.
3. What are insulating materials? Give two examples.
4. What are magnetic materials? Give two applications.
5. What are valence electrons?
6. Differentiate between valence electrons and free electrons.
7. Describe the structure of an atom.
8. Define valence band, conduction band and forbidden energy gap.
9. Define resistivity and conductivity. Write their units.
10. Explain the effect of temperature on the resistance of conductors, semiconductors and insulators.

Long-answer questions:

1. Classify electrical engineering materials into conducting, semiconducting, insulating and magnetic materials, giving examples and applications.
2. Explain the energy band theory of solids.
3. Differentiate between conductors, semiconductors and insulators on the basis of atomic structure and energy bands, with diagrams.

14. Quick Revision

• Conductors: Allow current to flow easily — copper and aluminium.
• Semiconductors: Have controllable conductivity — silicon and germanium.
• Insulators: Strongly oppose current flow — rubber, glass and PVC.
• Magnetic materials: Respond to magnetic fields — iron, silicon steel and ferrites.
• Valence electrons: Electrons in the outermost shell of an atom.
• Free electrons: Electrons free to move through the material and carry current.
• Atomic number: Number of protons in an atom.
• Forbidden energy gap: Energy range between the valence and conduction bands where allowed states are absent in the ideal band model.
• Largest forbidden energy gap: Generally found in insulators.
• Approximate gaps: Silicon about 1.1 eV, germanium about 0.67 eV, insulators more than about 5 eV, conductors about zero.
• Resistivity: Unit is ohm-metre (Ω·m). Conductivity: Unit is siemens per metre (S/m).
• Temperature effect: Resistance of conductors increases with temperature; resistance of semiconductors decreases.

Related topics to study next: Semiconductor Fundamentals, Conducting and Semiconducting Materials, Insulating Materials, Magnetic Materials.

Conclusion: Electrical engineering materials are selected according to their electrical, magnetic and other physical properties. Conductors carry current, semiconductors control current, insulators prevent unwanted current flow, and magnetic materials support useful magnetic effects in electrical equipment. The number of valence electrons and the energy-band structure explain why these materials behave so differently.