SEM-3^RD�SUB-EEM�PREPARED BY-SUMAN SAHU,ASST. H.O.D. ELECTRICAL ENGINEERING�CHAPTER 1:CONDUCTING MATERIAL �AY: 2021 - 22�

CONTENTS

• Introduction
• Classification
• Resistivity
• Temperature Coefficient of Resistance.
• Factors effecting Resistivity.
• Properties of conducting Materials.
• Classification of conducting material as low resistivity and high resistivity materials.
• Superconductors and their applications.

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��������INTRODUCTION��An electrical engineer should posses the knowledge of the properties of materials used in electrical engineering. This knowledge helps to choose the correct materials for a given application. Hence, the materials available can be employed effectively and economically for a specific purpose.

Classification of Electrical Materials

•  Materials used in the electrical engineering field are classified basing on their properties and applications.
• a. Conductor materials.
• b. Resistor materials.
• c. Insulating materials.
• d. Semiconductor materials
• e. Magnetic materials
• f. Refractory materials
• g. Structural materials

RESISTIVITY

Resistivity-. Resistivity or specific resistance of a material may be defined as the resistance offered between the opposite faces of a metre cube of that material. T

• he unit of resistivity is ohm metre (Ω-m).
• We have according to law of resistance:
• The resistance of a material ( R ) depends directly to its length (L) -inversely to the sectional area ( A )

• So, R α L / A
• Or R= ρ L/A
• ( where ρ is known as resistivity of material) Therefore ρ = A R/L When R= Resistance in Ohms (Ω )
• L= Length in m
• A= Area of cross section in m2
• ρ = resistivity or Specific resistance in Ω-m

Temperature Coefficient of Resistance.

The resistance-change factor per degree Celsius of temperature change is called the temperature coefficient of resistance. This factor is represented by the Greek lower-case letter “alpha” (α). A positive coefficient for a material means that its resistance increases with an increase in temperature.

FACTORS EFFECTING RESISTIVITY

• Factors effecting the resistivity of electrical materials are listed below –

1.Temperature.

2.Alloying.

3.Mechanical stressing.

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EFFECT OF TEMPERATURE ON RESISTIVITY

• The resistivity of materials changes with temperature. Resistivity of most of the metals increase with temperature. The change in the resistivity of material with change in temperature is given by formula given below-

�Where ρ_t1 is the resistivity of material at temperature of t₁^o C.

ρ_t2 is the resistivity of material at temperature of t₂^oC.

α₁ is temperature coefficient of resistance of material at temperature of t₁^o C.�

EFFECT OF ALLOYING ON RESISTIVITY

• Alloying is a solid solution of two or more metals. Alloying of metals is used to achieve some mechanical and electrical properties. The atomic structure of a solid solution is irregular as compared to pure metals. Due to which the electrical resistivity of the solid solution increases more rapidly with increase of alloy content. A small content of impurity may increase the resistivity metal considerably. Even the impurity of low resistivity increases the resistivity of base metal considerably. For example the impurity of silver (having lowest resistivity among all metals) in copper increase the resistivity of copper.

EFFECT OF MECHANICAL STRESS

• Mechanical stressing of the crystal structure of material develops the localized strains in the material crystal structure. These localized stains disturb the movement of free electrons through the material. Which results in an increase in resistivity of the material. Subsequently, annealing, of metal reduces the resistivity of metal. Annealing of metal, relieve the mechanical stressing of material due to which the localized stains got removed from the crystal structure of the metal. Due to which the resistivity of metal decrease. For example, the resistivity of hard drawn copper is more as compared to annealed copper.

Properties of conducting Materials

• Resistance of conductor is very low or negligible.
• The electric filed inside the conductor is zero
• The charge density inside the conductor is zero
• High conductivity
• Casted, forged, rolled, drawn and machined
• Malleable and Ductile
• Positive temperature coefficient of resistance.�

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�Classification of conducting material as low resistivity and high resistivity materials �

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• Low resistivity materials Copper, Aluminium, steel, Silver, Gold, Platinum , copper alloys Brass, Bronze.

• High resistivity materials manganin, constantan, Nichrome, mercury, platinum, carbon and tungsten

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Copper and its properties

• A good electrical conductor
• A good thermal conductor
• Corrosion resistant
• Antibacterial
• Easily joined
• Ductile
• Tough
• Non magnetic
• Easy to alloy
• Recyclable
• Catalytic

Properties of Aluminium

• It is silver- white in colour
• It posses high ductility
• It has high thermal and electrical conductivity
• It has high contact resistance
• High reflectivity for light rays
• It can be rolled into thin sheets and drawn into wires

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Copper alloys Brass, Bronze

• An alloy is a combination of metals
• Brass and bronze are both alloys of copper that is, they are solid solutions," or mixtures of copper with another metal.
• The main difference between brasses and bronzes is that brass is mostly made of copper and zinc while bronze is mostly made of copper and tin.

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High resistivity material properties

• High resistivity.
• High melting point.
• High mechanical strength.
• High ductility, so that can be drawn in the form of wire easily.
• High corrosion resistance mean free from oxidation.
• Low cost.
• Long life or durable.
• High flexibility.

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• Tungsten
• 1. It is grayish in colour when in metallic form.
• 2. It has a very high melting point (33000C)
• 3. It is a very hard metal and does not become brittle at high temperature.
• 4. It can be drawn into very thin wires for making filaments.
• 5. Its resistivity is about twice that of aluminium.
• 6. In its thinnest form, it has very high tensile strength.
• 7. It oxidizes very quickly in the presence of oxygen even at a temperature of a few hundred degrees centigrade.
• 8. In the atmosphere of an inert gas like nitrogen or argon, or in vacuum, it will reliably work up to 20000C.

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• Uses : It is used as filaments of electric lamps and as a heater in electron tubes. It is also used in thermionic valves, radars. Grids of electronic valves, sparking and contact points.

�Carbon �

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• Carbon is mostly available as graphite which contains about 90% of carbon. Amorphous carbon is found in the form of coal, coke, charcoal, petroleum, etc.
• Electrical carbon is obtained by grinding the raw carbon materials, mixing with binding agents, moulding and baking it.
• Properties :
• 1. Carbon has very high resistivity (about 4600 micro ohm cm).
• 2. It has negative temperature coefficient of resistance.
• 3. It has a pressure-sensitive reistance material and has low surface friction.
• 4. The current density is 55 to 65 A/cm2.
• 5. This oxidizes at about 3000C and is very weak.
• 6. It has very good abrasive resistance.
• 7. It withstands arcing and maintains its properties at high temperature.

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Superconductors and their applications

• Superconductivity is a phenomenon of exactly zero electrical resistance when cooled below a characteristic critical temperature
• Superconducting magnets are used in MRI (Magnetic Resonance Imaging) which is a way of looking at the soft
• Low and high field magnets for Nuclear Magnetic Resonance (NMR) parts of the body.
• Industrial magnets for materials magnetic separation

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