Classification of Materials MCQ Quiz - Objective Question with Answer for Classification of Materials - Download Free PDF

The correct answer is 4) Electrical insulators for high-voltage transmission lines.

Concept:

• High Mechanical Strength: High-voltage transmission lines require insulators that can support the weight of the conductors and withstand strong winds, ice loads, and other mechanical stresses. Mineral insulators like porcelain and glass possess excellent mechanical strength.
• Electrical Insulating Properties: These materials have high dielectric strength, meaning they can withstand very high voltages without allowing current leakage or breakdown. This is crucial for preventing short circuits and ensuring the safe and efficient transmission of power.

Additional Information

• Batteries and electrodes: While some minerals are used in batteries, they are chosen for their electrochemical properties rather than their bulk mechanical strength and electrical insulation. Insulation within batteries uses different materials.
• Electrical wires for household use: The insulation on household wires is typically made of polymers (plastics or rubber) which offer flexibility and adequate insulation for lower voltages. While some mineral-insulated cables exist for specialized applications (like fire survival circuits), they are not the typical choice for general household wiring.
• Low-voltage electronic devices: Low-voltage electronics often use plastic or ceramic materials for insulation due to their ease of manufacturing and suitability for smaller components. The high mechanical strength of mineral insulators is usually not a primary requirement in these applications.

The Correct Answer is 2) The maximum voltage a dielectric material can withstand without breaking down

Explanation:

Dielectric Strength is a key property of insulating (dielectric) materials and is defined as:

• The maximum electric field (or voltage per unit thickness) that a material can withstand without electrical breakdown.

• It is typically expressed in kV/mm or V/mil.

When the dielectric strength is exceeded:

• The material loses its insulating properties

• It allows current to pass, resulting in dielectric breakdown

Option Analysis

• 1) Electrical conductivity under stress – Conductivity is the opposite of insulation.

• 3) Amount of heat tolerated before melting – Refers to thermal properties, not dielectric.

• 4) Resistance to thermal expansion – Describes mechanical/thermal property, not electrical insulation.

Explanation:

When discussing the primary characteristic of low resistivity materials used in electrical conductors, it is essential to focus on the properties that make these materials suitable for efficient electrical conduction.

Resistivity and Conductivity:

Resistivity (ρ) is a fundamental property of materials that quantifies how strongly a material opposes the flow of electric current. It is usually measured in ohm-meters (Ω·m). A low resistivity indicates that the material allows electric charge to flow through it with minimal resistance, which is desirable for electrical conductors.

Conductivity (σ), on the other hand, is the reciprocal of resistivity and measures a material's ability to conduct electric current. It is measured in siemens per meter (S/m). High conductivity implies that the material has a high capacity for carrying electrical current, making it suitable for use in electrical conductors.

Mathematically, the relationship between resistivity and conductivity is expressed as:

σ = 1 / ρ

Given this relationship, it is clear that materials with low resistivity inherently have high conductivity, making them ideal for electrical conductors.

Characteristics of Low Resistivity Materials:

Materials with low resistivity are chosen for their ability to efficiently conduct electric current with minimal energy loss. Some common examples of low resistivity materials include copper, aluminum, and silver. These materials are frequently used in the manufacturing of electrical conductors such as wires, cables, and busbars.

Key Properties of Low Resistivity Materials:

• High Conductivity: As mentioned, high conductivity is a direct result of low resistivity. This property ensures that the material can carry a large amount of electrical current with minimal voltage drop.
• Thermal Stability: Low resistivity materials often exhibit good thermal stability, meaning they can withstand high temperatures without significant degradation in their electrical properties. This is important for preventing overheating and maintaining the conductor's performance over time.
• Mechanical Strength: While electrical properties are crucial, mechanical strength is also important to ensure that the conductors can withstand physical stresses during installation and operation.
• Corrosion Resistance: Many low resistivity materials, such as copper and aluminum, offer good resistance to corrosion, which helps in maintaining their conductive properties over extended periods.

Calculation:

We are given:

Side of the cube, a = 2 μm = 2 × 10^-6 m

Number of atoms in the domain = 9 × 10¹⁰

Dipole moment of each atom, p = 9 × 10^-24 Am²

The magnetization of the domain is given by the formula:

M = (Number of atoms × Dipole moment) / Volume of the domain

Volume of the domain = a³ = (2 × 10^-6)³ = 8 × 10^-18 m³

Now, calculating the magnetization:

M = (9 × 10¹⁰ × 9 × 10^-24) / 8 × 10^-18

M = (81 × 10^-14) / 8 × 10^-18 = 10 × 10⁴ Am^-1

∴ The magnetization of the domain is 10 × 10⁴ Am^-1.

The correct answer is 4) Copper and aluminium. Here's why:

• Copper and Aluminium:
  • These are the most commonly used materials for electrical transmission lines due to their excellent conductivity (low resistivity) and relatively low cost.
  • Aluminium is lighter than copper, making it advantageous for long-distance overhead lines.
  • Copper has a better conductivity than aluminium, so it is used in situations where the best possible conductivity is required.
• Gold and Silver:
  • While gold and silver have even lower resistivity than copper and aluminium, they are far too expensive for widespread use in transmission lines.
• Lead and Tin:
  • Lead and tin have relatively high resistivity, making them unsuitable for efficient electrical transmission.
• Tungsten and Iron:
  • Iron has a higher resistivity than copper and aluminium, and tungsten, while strong, is also higher resistivity, and very expensive. Tungsten is primarily used for filaments in incandescent light bulbs due to its high melting point.

The correct answer is Graphite.

Key Points

Graphite is not used as a coolant in nuclear reactors.

• A coolant in a nuclear reactor is used to remove heat from the machine core and transfer it to the environment. 
• Almost all currently operating nuclear power plants are light water reactors (LWRs) using ordinary water under high pressure as coolant.
• Heavy water reactors use deuterium (isotope of Hydrogen) oxide which has identical properties to ordinary water but much lower neutron capture.

Additional Information

Parameters for a good coolant:

• Temperature coefficient of resistance explains the variation in the resistance by the change in temperature. It is expressed in ohms/ohms°C
• If it is positive, resistance increases with an increase in temperature.
• If it is negative, resistance decreases with an increase in temperature.

Among the given options, carbon has a negative temperature coefficient.

Important Notes:

Semiconductors have a negative temperature coefficient as well.

Concept:

There are 7 class of insulating materials, each suitable for different application:

The permissible temperature limit at which the insulators may be worked safely without deterioration depends upon the type and class of the insulation as detailed below.

Explanation:

Thermoplastics:

• Thermoplastics are defined as polymers that can be melted and recast almost indefinitely.
• They are molten when heated and harden upon cooling.
• When frozen, however, thermoplastic becomes glass-like and subject to fracture.
•  These characteristics, which lend the material its name, are reversible, so the material can be reheated, reshaped, and frozen repeatedly. As a result, thermoplastics are mechanically recyclable.
• Some of the most common types of thermoplastic are polypropylene, polyethylene, polyvinylchloride, polystyrene, Polyethylenetheraphthalate (PET), Teflon, Nylon, and polycarbonate.

Additional Information

Thermosetting plastic.

• The plastic which keeps its shape and does not soften i.e the plastic which when molded once, can not be softened by heating is called Thermosetting plastic.
• Thermosetting plastics are synthesized by condensation polymerization and have primary bonds between molecular chains and held together by strong cross-links.
• Thermosetting plastics have a high melting point and tensile strength as compared to thermoplastics.
• one of the most common types of thermoplastic are Bakelite, Melamine, Polyesters etc.

Transition temperature (T_c): The temperature at which the material changes its state from normal conductor to superconductor.

For Mercury (Hg), T_c = 4.12°K

Note:

• Transition temperature varies for different materials but generally is below 20 K (− 253 °C).
• For superconductors, transition temperatures usually lie between 1°K and 10°K.
• Tungsten has the lowest transition temperature, which is 0.015°K.
• Niobium the highest transition temperature, which is 9.2°K.

Concept:

Martensite:

• It is the hardest constituent of steel. The primary reasons accounting for this could be, the internal strains within BCC iron due to the excess carbon presence and due to the plastic deformation of parent FCC iron (Austenite) surrounding the martensitic plate.
• The rate of cooling and the amount of carbon percentage in steel are directly proportional to the amount of hardness achieved in martensitic transformation.

Important Points

Bainite:

• It is a plate-like microstructure that forms in steels at a temperature of 125-550 (depending on alloy content).
• It forms by the decomposition of austenite at a temperature which is above MS but below that at which fine pearlite forms.

Austenite:

• It is also known as Gamma-phase iron is a metallic, non-magnetic allotrope of iron or solid solution of iron, with an alloying element.
• In plain carbon steel, austenite exists above the critical eutectoid temperature of 1000 K.
• Austenite is of FCC crystal structure.

Ledeburite:

• In iron and steel metallurgy, ledeburite is a mixture of 4.3 % Carbon in iron and is a eutectic mixture of austenite and cementite.
• Ledeburite is not a type of steel as the carbon level is too high although it may occur as a separate constituent in some high carbon steel. 

Explanation:

Classification of steels:

1) Based on the chemical composition:

i) Low carbon steels: 

• Composition: 0%C to 0.25 % C.
• Microstructure: Predominantly α - ferrite and small quantities of pearlite.
• Properties: Outstanding ductility and toughness. good machinability and weldability, high formability, toughness, high ductility, etc.
• Applications: Automobile body components, structural shapes, pipes, sheets, etc.

ii) Medium carbon steels: 

• Composition: From 0.25% to 0.55% C.
• Microstructure: α - ferrite and pearlite.
• Properties: Stronger than low–carbon steel but less tough than it.
• Applications: Railway wheels & tracks, gears, etc.

iii) High carbon steels: 

• Composition: From 0.55% up to 2.1 % C
• Microstructure: Fe3C, Pearlite(C >0.8%), - ferrite and pearlite (C < 0.8%).
• Properties: Hardness, strongest, and least ductile compared to Low carbon steels.
• Applications: Knives, hack saw blades, chisels, hammers, drills, dies, machine tool cutters, punches, etc.

2) Based on Notch toughness:

Notch toughness: Notch toughness is an indication of the capacity of the steel to absorb energy when a stress concentrator or notch is present.

i) Class H steels: These are usually used for primary structure members, piling, jacket braces and legs, and deck beams. Because this class of steel has a good record of application in welded structures at service temperatures above freezing.

ii) Class N steels: These are used where the service temperature is 10°C to 0°C.

iii) Class C steels: These are used in subfreezing service temperatures (lower than 0°C), as in the North Sea or another cold climate. Because in the colder regions the temperature can reach –40°C

3) Based on manufacturing method:

i) Bessemer steel method:

• The principle of Bessemer Converter is the removal of impurities from the iron by oxidation and the air is being blown through the molten iron.
• The furnace is made of steel with fire clay bricks to resist heat.
• The impurities manganese(mn) and Silicon(Si) are converted into their respective oxides and that can be expelled out.

​ii) Electric Arc Furnace Method:

• it is an extremely hot enclosed region, where heat is produced employing electrodes for melting certain materials such as steel (scrap) without changing the electrochemical properties of the material(metal).
• The electric arc produced between the electrodes and the metal is used for melting the metal(scrap).

​
Heat treatment of steels: Heat treatment is the secondary process applied to steel to improve mechanical properties without changing the chemical properties but only change in grain structure.

Steps involved in heat treatment:

i) Heating: The specimen is heated up to a certain high temperature and during heating initially, residual stresses are relieved and at high temperature enlargement of grains takes place.

ii) Soaking/Holding: After heating the specimen is hold for some time to get uniform grain formation, the holding time or soaking time depends on the size of the specimen.

iii) Cooling: After holding the specimen is cooled in different ways based on the requirement like slow cooling or fast cooling.

The heat treatment processes are:

• Hardening
• Annealing
• Normalizing
• Tempering

Diamagnetic materials

• Weak, negative susceptibility to magnetic fields
• Diamagnetic materials are slightly repelled by a magnetic field
• All the electrons are paired so there is no permanent net magnetic moment per atom
• Most elements in the periodic table, including copper, silver, and gold, are diamagnetic

Paramagnetic materials

• Small, positive susceptibility to magnetic fields
• These materials are slightly attracted by a magnetic field
• Paramagnetic properties are due to the presence of some unpaired electrons, and from the realignment of the electron paths caused by the external magnetic field
• Paramagnetic materials include magnesium, molybdenum, lithium, and tantalum

Ferromagnetic materials

• Have a large, positive susceptibility to an external magnetic field
• They exhibit a strong attraction to magnetic fields and can retain their magnetic properties after the external field has been removed
• Ferromagnetic materials have some unpaired electrons, so their atoms have a net magnetic moment
• Iron, nickel, and cobalt are examples of ferromagnetic materials.

Concept:

Ferromagnetism:

• Ferromagnetism is a unique magnetic behavior that is exhibited by certain materials such as Nickel, Iron, Cobalt, Alloys, etc.
• It is a phenomenon where these materials attain permanent magnetism or they acquire attractive powers.
• It is also described as a process where some of the electrically charged materials attract each other strongly.
• It is a property that considers not only the chemical make-up of material but it also takes into account the microstructure and the crystalline structure.

Ferrimagnetism:

• In a ferrimagnet, the magnetic moment of one type ion on one type of lattice site in the crystal are aligned anti-parallel to those of ion on another site. Because the magnetic moment is not of the same magnitude they only partially cancel each other and the material has net magnetic moment.
• Ferrimagnetism has several similarities to ferromagnetism in that the cooperative alignment between magnet dipoles leads to a net magnetic moment even in the absence of applied field.
• Ferrimagnetism has lost above the critical temperature.