Answer.4. All of the above

Explanation

Low resistivity materials possess the lower value of resistivity i.e. higher conductivity, Hence, they are suitable for those applications where the voltage drop and power loss are to be kept to a minimum value. The application of low resistivity materials are used in

⇒ Windings of alternators, motors, transformers

⇒ Domestic wiring

⇒ Electricity transmission from power stations to other places

Gold, silver, copper, and aluminum are examples of low resistivity materials. Amongst these materials, copper is commercially the most acceptable.

Answer.1. Cubic

Explanation

Solid metals and most minerals have a crystalline structure. Here the atoms are arranged in a three-dimensional geometric array known as a lattice. Lattices are describable through a unit building block, or unit cell, that is essentially repeated throughout space.

The most common metals are face-centered cubic (FCC), body-centered cubic (BCC), or hexagonal in structure

The three most common crystal lattice structures are

1. Face Centered Cubic (FCC)
2. Body-Centered Cubic (BCC)
3. Hexagonal Close-Packed (HCP)

In FCC structure, the unit cell is a cube with one atom at each corner of the cube and one each face. Examples are — Ca, Ni, Cu, Ag, Pt, Au, Pb, Al, and alloys of these. Generally, soft metals have a face-centered cubic structure. They have good ductility and good electrical conductivity.

In the BCC structure, atoms are present at all eight corners of the cube and another one at the body center. Examples are — Fe, Na, K, V, Mo, Ta, W. Mild steels and less ductile metals have body-centered cubic structures. They have more tendency to deform plastically.

In HCP structure, atoms are at each corner of a hexagonal prism and one atom each at the center of top and bottom faces, And three atoms in the midplane. Examples are — Be, Mg, Zn, Cd, Co, Ti. They have good ductility and can be easily deformed.

Answer.4. All of the above

Explanation

Low resistivity materials possess the lower value of resistivity i.e. higher conductivity, Hence, they are suitable for those applications where the voltage drop and power loss are to be kept to a minimum value. Consequently, they find use in the windings of alternators, motors, transformers; in domestic wiring, and in electricity transmission from power stations to other places, Gold, silver, copper and aluminum are examples of low resistivity materials. Amongst these materials, copper is commercially the most acceptable.

Characteristics of Low Resistivity

Materials Besides having a lower value of resistivity, the low resistivity materials are also required to possess the following characteristics.

Low coefficient of temperature resistance α:- The value of α should be low to avoid (or minimize) the variation in voltage drop and power loss, with a change in temperature. If the value of α is high, the voltage drop and power loss will increase. A low value of a helps in reducing the heat in windings of electrical machines and in increasing the resistance of transmission lines during the hot season.

High resistance to corrosion:- This is a desired requirement to prevent corrosion due to environmental effects. The conducting material should not easily corrode, particularly when used in atmospheric exposure without insulation.

Good solderability:- Conductors are often required to be jointed. The joints should offer a minimum contact resistance, Joining is normally done by soldering because it offers minimum contact resistance. All materials do not possess good solderability. Hence, the solderability of conducting materials should be such as to develop minimum resistance.

High mechanical strength:- Overhead transmission and distribution lines are subjected to stresses and strains under their self-weight and also due to winds. The conductors used in the winding of electrical machines and transformers develop mechanical and thermal stresses also when loaded. Therefore, the low resistivity materials should be mechanically strong enough to resist these stresses.

Good ductility:- The conducting materials should be ductile enough to enable themselves to be drawn into different shapes and sizes.

Answer.2. Copper and Nickel

Explanation

Constantan is an alloy of copper and nickel that has moderate resistivities and low-temperature coefficients of resistance. Nominally, constantans are 55 Cu-45Ni alloys, but specific compositions vary from approximately 50 Cu-50Ni to approximately 65Cu- 35Ni.

Answer.1. Vacuum tube

Thermionic emission occurs in a vacuum tube when the additional energy is supplied to the electrons thermally, i.e. by heating the metal.

Thermionic emission is the process of electron emission from a metal surface by heating the metal. The electrons so liberated are called thermions.

Vacuum tubes and sprytrons are some of the electronic switchings and amplifying devices based on vacuum conductivity.

Answer.3. Silicon

Explanation

Semiconductors are those materials that have electrical conductivities lying between those of good conductors and insulators. The commonly used semiconductor elements are silicon, germanium, and gallium arsenide. Silicon is the most widely used semiconductor material.

Answer.4. All of the above

Explanation

Due to its high conductivity and reasonable cost, copper is the most widely used metal for electrical purposes. It is a crystalline, non-ferrous, nonmagnetic (diamagnetic), reddish-colored metal. It possesses several advantageous properties such as given below.

High ductility:- It is a ductile metal having a ductility of more than 15%. By virtue of this property, it can be easily drawn into thin bars and wires. Hence, it is very useful for making cables, strands, and conductors.

High tensile strength:- Its ultimate tensile strength is high enough (300—350 MPa) which makes it substantially strong to sustain mechanical loads.

High melting point:- Its melting point is sufficiently high (1083°C) that makes it suitable for use at high temperatures also.

Corrosion Resistance:- When exposed to an atmospheric environment, it forms a protective layer of copper oxide (CuO). Thus, the copper is highly resistant to corrosion which is a desired property for bare/open overhead conductors.

Winding and soldering:- It can be easily brazed (a kind of welding) which is a necessary requirement in electrical wiring and other connections.

Low contact resistance:-  Coating or plating the copper alloy with silver, gold, palladium, and similarly noble metal or alloys provides excellent stability and low contact resistance with little film growth.

Less bulky:-  Copper requires less area of conductor than Aluminum for the same current rating. Hence it is less bulky and machines become compact

Annealed copper is more ductile than hard-drawn copper. It can withstand severe bending and forging stresses without failure, It is used as power cables, winding wires for electrical machines and transformers, and in making coils.

Hard-drawn copper possesses high mechanical strength. It is suitable for overhead transmission wires and bus bars etc.

Answer.4. Manganin

Explanation

Materials with Zero Temperature Coefficients 

Manganin is an alloy of copper manganese and nickel. It can be observed that manganin has almost zero temperature coefficient at 20°C and likewise, constantan has almost zero value, Hence, resistances made using wires of these materials do not change in value with the variation of temperature. They are used as swamping resistances. In circuits that should maintain constant resistance values at all temperatures: one typical example is in electrical meters.

Answer.4. Gallium

Explanation

Zero Resistivity Materials:- The material which conducts electricity at zero resistance is known as zero resistivity material. The superconducting materials like alloys of aluminum, zinc, gallium, nichrome, niobium, etc., conduct electricity almost at zero resistance below the transition temperature. The above superconducting materials are perfect diamagnets. The applications of these materials are energy saving in power systems, superconducting magnets, memory storage, etc.

Answer.1. Schottky Defect

Explanation

Schottky defect:- When a pair of positive and negative ions are missing from ionic crystal, the defect is known as the Schottky defect. Due to this defect, there is no change in the neutrality of the crystal.