Electrical Engineering materials MCQ

41. In conductors, conduction of electricity takes place due to the movement of

  1. Electrons only
  2. Positive ions only
  3. Positive and negative ions

Answer.1. Electrons only

Explanation

Conductors are substances through which electric charges can flow easily. They are characterized by the presence of a large number of free electrons. The number density of free electrons in a conductor is the same throughout the conductor. This is because free electrons experience repulsive force between them and the conductor allows the movement of free electrons.

In absence of applied potential difference electrons in the conductor have random motion. The average displacement and average velocity are zero. There is no flow of current due to the thermal motion of free electrons in a conductor. When two ends of conductors are joined to a battery then one end is at higher potential and another at lower potential. This produces an electric field inside the conductor from a point of higher to lower potential i.e. E = V/L.

Current flow conductor

The field exerts an electric force on the free-electron causing acceleration of each electron. In a conductor, the flow of electricity takes place due to the movements of electrons only.

 

42. ________ is the main constituent of glass

  1. Al2O3
  2. B2O3

Answer.2. SiO2

Explanation

SiO2 is the main constituent of glass. Silica is a glass-former, a compound that forms the basic building block of the majority of glasses. It is present in ancient soda-lime glasses at levels of c. 65% to 70%, The elements that form silica (SiO2), silicon and oxygen, are two of the most abundant elements in the earth’s crust.

Silica is a polymorph that has three principal crystalline forms (with the same chemical composition): quartz, tridymite, and cristobalite. Quartz is far more abundant than the others, forming 12% of the earth’s crust. Tridymite and cristobalite are high-temperature forms of silica that occur rarely in certain volcanic rocks and can be produced by heating quartz to high temperatures. They can therefore be found in, for example, pottery and other high-temperature products.

 

43. The conduction of electricity, in semiconductors, takes place due to the movement of

  1. Positive ions only
  2. Negative ions only

Answer.4. Electrons and Holes

Explanation

A pure silicon crystal or germanium crystal is known as an intrinsic semiconductor. For most applications, there are not enough free electrons and holes in an intrinsic semiconductor to produce a usable current.

The electrical action of these can be modified by doping means adding impurity atoms to a crystal to increase either the number of free holes or the number of free electrons. When a crystal has been doped, it is called an extrinsic semiconductor. They are of two types n-type semiconductors having free electrons as majority carriers, p-type semiconductors having free holes as majority carriers.

 

44. In a superconductor, the value of critical density depends upon

  1. Magnetic field strength
  2. Temperature
  3. Both A and B
  4. None of the above

Answer.3. Both A and B

Explanation

Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic flux fields occurring in certain materials, called superconductors when cooled below a characteristic critical temperature.

Properties of Superconductors

Few important properties of superconductors are explained in brief in this section.

(i) Critical temperature TC (Transition Temperature)

The electrical resistance of the superconducting materials becomes zero below the transition temperature (Tc). This property is called the defining property of superconductors. The figure shows the variation of electrical resistance with respect to the temperature for the normal conductor and a superconductor. From the figure, it is noted that the resistance falls to zero at the transition temperature.

Superconducting material

Note:- Good electrical conductors such as silver (Ag), Gold (Au) and copper (Cu) are not good superconductors because the resistivity of these conductors at low temperatures is limited to the low resistivity.

Similarly, good superconducting materials like Zn and Pb are not good electrical conductors.

(ii) Magnetic field effect

If a sufficiently strong magnetic field is applied to a superconductor at any temperature below its critical temperature TC, the superconductor is found to undergo a transition from the superconducting state to the normal state.

This minimum magnetic field required to destroy the superconducting state is called the critical magnetic field HC.

superconductivit

 

45. The coordination number of a simple cubic structure is

  1. 2
  2. 4
  3. 6
  4. 8

Answer.3. 6

Explanation

Coordination Number

The coordination number of any given sphere in a crystalline structure is the number of other spheres by which it is immediately surrounded.

Simple cubic lattice:- In the simple cubic lattice, one atom is present at each of the eight corners of the unit cell. The geometry of the structure suggests that if we consider an atom at one corner as the center, it will be surrounded by six equidistant neighbours and hence the coordination number of the simple cubic lattice is six. If “a” represents the side of the unit cell, the distance between the nearest neighbours will be “a”

 

46. Which of the following effect greatly the resistivity of electrical conductors?

  1. Composition
  2. Pressure
  3. Size

Answer.1. Composition

Explanation

Resistivity is an important property in any material that conducts electricity. It is essential for conducting materials to have a low resistivity and non-conductive or insulating materials to have high resistivity.

The resistivity of conductors depends on their composition, physical condition (e.g. dampness in the case of non-metals), alloying, manufacturing and heat treatment, chemical purity, mechanical working, and aging.

The resistivity of a material depends upon the chemical composition of the material together with the physical treatment applied to the material. For example, impurities and the addition of an alloying metal alter the atomic lattice of the material, while heat treatment and cold working modify the grain structure.

 

47. Is an element used in semiconductors whose atoms have three valence electrons?

  1. An acceptor
  2. A donor
  3. Silicon

Answer.1. An acceptor

Explanation

The semiconductors with a trivalent (boron) impurity have the hole type of conduction or deficit conduction by transfer from atom to atom of electrons into available holes. A semiconductor in which the conduction is due to holes referred to as a p-type semiconductor. Here, p stands for positive because of the carriers acting like positive charges, for the hole travels in a direction opposite to that of the electrons filling it. A trivalent atom, the one that has three valence electrons is called an acceptor or recipient. Each acceptor produces one hole in a silicon crystal. In a p-type semiconductor, the holes are the majority carriers, while the free electrons are the minority carriers because the holes outnumber the free electrons.

 

48. Due to which of the following factors the brush wear rate is altered?

  1. Speed of the machine
  2. Contact pressure
  3. Surface conditions of brush and commutator
  4. All of the above

Answer.4. All of the above

Explanation

Brushes are needed to complete the electrical path for current flow through the armature, which puts some very heavy demands on them. Brushes must be held securely to the surface of the commutator, which usually requires spring action. There are four popular types of brush materials: carbon graphite brush, electro-graphite brush, natural graphite brush, and metal-graphite brush. The latter one is made of copper and graphite mixed according to the demand for certain characteristics. The metal graphite brush can carry extremely high currents.

Brush wear is affected by various factors including temperature, material properties, sliding speed, contact force, and interfacial and environmental conditions. The mechanical wear of brushes is proportional to the brush spring pressure and sliding speed, while the electrical wear of brushes is associated with current and contact voltage drop.

 

49. The coils of the D.C. motor starter are wound with wire of

  1. Copper
  2. Kanthal
  3. Nichrome

Answer.3. Manganin

Explanation

Starters are used in DC motors to limit the starting current within about 2 to 3 times the rated current By reducing the resistance in series with the armature circuit.

The DC motor starter is wound from manganin wire since manganin is an alloy with a low-temperature coefficient of resistance and a low thermoelectric e.m.f. to copper. Manganin has been the traditional material for high-grade standard resistors.

 

50. Carbon resistors are used extensively because they are

  1. Easy to make
  2. Compact
  3. Inexpensive

Answer.4.  All of the above reasons

Explanation

A fixed resistor is one that has a single value of resistance that remains constant under normal conditions. The two main types of fixed resistors are carbon-composition and wire-wound resistors.

The advantages of the carbon-composition resistor were its low price, the availability of many popular resistance values, and relatively constant resistance value over a wide frequency range. The disadvantages were the tendency for the resistor’s value to vary over time, and with changes in temperature and humidity. They are not usually manufactured with power dissipation capabilities beyond 5 watts.

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