Electrical Engineering materials MCQ

Electrical Engineering material MCQ | Electrical Engineering material MCQ with Explanation

1. Which of the following variety of copper has the best conductivity?

  1. Pure annealed copper
  2. Hard drawn copper
  3. Induction hardened copper
  4. Copper containing traces of silicon

Answer.1. Pure annealed copper

Explanation:-

The conductor is defined as materials that allow electricity. Conductors are usually applied to materials, generally, metals, used to conduct electric current, though heat conductors and sound conductors have important uses. Good conductors of electricity tend to be good conductors of heat, too.

Silver is the best conductor of electricity, but copper is the most commonly used. The conductivity of pure copper is 97.6% that of silver. The electrical conductivity of metals is often expressed as a percentage of the electrical conductivity of copper, which is arbitrarily set at 100%. Tough-pitch copper is the standard conductivity metal, and it is designated as the International Annealed Copper Standard (IACS).

 

2. The tiny block formed by the arrangement of a small group of atoms is called the

  1. Unit cell
  2. Space lattice
  3. Either A or B
  4. None of the above

Answer.1. Unit cell

Explanation

Unit Cell: A unit cell is the smallest assemblage of atoms, ions, or molecules in a solid, beyond which the structure repeats to form the 3-dimensional crystal lattice.

 

3. On which of the following factors does the resistivity of a material depend?

  1. Temperature
  2. Both 1 and 2
  3. None of the above

Answer.3. Both 1 and 2

Explanation

Material can be characterized by its resistivity, denoted by p, which is independent of size and shape, The resistance of a homogeneous sample of material having resistivity ρ, length l, and uniform cross-sectional area A is given by

R = \left[ {\frac{m}{{n{e^2}\tau }}} \right]\frac{L}{A}

Where

m = mass of the electron

n = number of free electrons per unit volume

τ = Average relaxation time

or

R = \rho \frac{L}{A}

 

Resistivity is defined as the resistance of a conductor of unit length and unit area of cross-section. Its SI unit is Ohm-m (Ω-m).

Comparing

\rho = \left[ {\frac{m}{{n{e^2}\tau }}} \right]

 

It follows from here that the resistivity of the material of a conductor depends upon the following factors :

(i) Resistivity is inversely proportional to n i.e., the number of free electrons per unit volume. But the value of n depends upon the nature of the material. So, the resistivity of a conductor depends upon the nature of the material of the conductor.

(ii) Resistivity is inversely proportional to i.e.» the average relaxation time of the free electrons in the conductor. t decreases with rise of temperature. So, resistivity increases with the increase in the temperature of the conductor.

The free electron density and the relaxation time are different for different materials. So, the resistivity for different materials is different.

 

4. Power cables capable of supplying large currents (30 A) have solid copper cores of cross-sectional area equal to 1.5 mm2. What is the resistance of a 10m length of such a cable?

  1. 1.1 Ω
  2. 0.11 Ω
  3. 111 Ω
  4. 11.1 Ω

Answer.2. 0.11 Ω

Explanation

Given

Current (I) = 30 A

Area (A) = 1.5 mm2 = 1.5 × 10−6m

Length (L) = 10 m

Resistivity of copper = 1.7 × 10−8 Ωm

Resitance of the conductor is given by

R = ρL/A

= (1.7 × 10−8  × 10)/1.5 × 10−6

R = 0.11Ω

 

5. The resistivity of pure copper at 20° C is 1.72 × 10−8 Ωm. At 20° C the resistance of a pure copper cylindrical wire having diameter d= 0.2 cm and length L = 5km is

  1. 27.4 Ω
  2. 274 Ω
  3. 2.74 Ω
  4. 0.274 Ω

Answer.1. 27.4 Ω

Explanation:-

Given

Diameter d = 0.2 cm

Radius (r) = 0.2/2 = 0.1 cm = 0.001 m

Area of cylinder (A) = πr2 = π × 0.001−6m

Length (L) = 5 km = 5000 m

Resistivity of copper = 1.72 × 10−8 Ωm

Resitance of the conductor is given by

R = ρL/A

= (1.72 × 10−8  × 5000)/(π × 0.001−6)

R = 27.4 Ω

 

6. The resistivity of pure copper at 20° C is 1.72 × 10−8 Ωm. At 20° C, the resistance of a pure copper cylindrical wire is 27.4Ω  having a diameter d= 0.2 cm and length L = 5km. What will be the voltage drop from end to end if the current in the wire is 15 A

  1. 4.11 V
  2. 41.1 V
  3. 0.411 V
  4. 411 V

Answer.4. 411 V

Explanation:-

Given

Current (I) = 15 A

Diameter d = 0.2 cm

Radius (r) = 0.2/2 = 0.1 cm = 0.001 m

Area of cylinder (A) = πr2 = π × 0.001−6m

Length (L) = 5 km = 5000 m

Resistivity of copper = 1.72 × 10−8 Ωm

Resitance of the conductor is given by

R = ρL/A

= (1.72 × 10−8  × 5000)/(π × 0.001−6)

R = 27.4 Ω

Voltage drop

V = R.I

= 27.4 × 15

V ≅ 411 V

 

7. The number of atoms per unit cell in B.C.C. structure is

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

Answer.3. 2

Explanation:-

If the corners of the cube and the center of the cube are occupied by identical atoms then the crystalline structure is called a body-centered cubic structure (B.C.C). This structure is found in many materials like Na, Li, K etc.

The bcc unit cell is a cube (all sides are of the same length and all faces sharing a corner are perpendicular to each other) with an atom at each corner of the unit cell and an atom in the center of the unit cell. Each of the corner atoms is the corner of another cube, thus the corner atoms are shared among eight unit cells. It is said that bcc has a coordination number of 8 and a bcc unit cell consists of a net total of two atoms; one in the center and eight eighths from corner atoms. Some of the materials that have a bcc structure include lithium, sodium, potassium, chromium, barium, vanadium, alpha-iron, and tungsten. Metals that have a bcc structure are usually harder and less malleable than close-packed (e.g., face-centered cubic structured) metals such as gold.

8 corners × 1/8 per corner atom = 8 × 1/8 = 1 atom

1 body centre atom = 1 + 1 = 2 atom

 

8. Microwaves serve as a source of ____

  1. Resonant waves
  2. Musical waves
  3. U
  4. Microwaves

Answer.3. Ultrasonic Waves

Explanation:- 

Piezoelectric effects are defined as “the phenomena of separation of charge in a crystal by mechanical stresses and the converse.”

Applications of  Piezoelectric material

Piezoelectric materials find uses in applications where mechanical and electric energy must be interconverted. The mechanical displacements and movements of electrical charge tend to be of limited magnitudes in these applications.

Piezoelectrics are used in microphones and ultrasound detectors, where they convert the oscillations of a diaphragm, caused by incident pressure waves, into a varying electrical signal. When an electric current is applied to a piezoelectric crystal, it starts to vibrate and these vibrations generate sound waves with frequencies between 1.5 and 8 MHz (i.e ultrasound). A related use is in record-player pickups, where the force from the stylus varies between 0.01 and 0.05 N and the resulting output signal varies from 0.1 to 0.5 volts.

 

9. The number of atoms per unit cell in the F.C.C. structure is

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

Answer.2. 4

Explanation:-

One of the close-packed structures is the face-centered cubic structure. The face-centered cubic structure has atoms located at the eight corners and the centers of all the cubic faces as shown in Fig.

  • Each of the corner atoms is the corner of eight cubes so the corner atoms are shared among eight unit cells. Additionally, each of its six face-centered atoms is shared with an adjacent unit cell as shown in Fig.
  • It has a coordination number of 12.
  • The FCC unit cell consists of a net total of four atoms; one eight of eight corner atoms and six halves of the face-centered atoms.

Number of atoms per unit cell

8 corners × 18 per corner atom = 8 × 18 = 1 atom

6 face-centered atoms × 12 atoms per unit cell = 3 atoms

Hence, the total number of atoms in a unit cell =  3+ 1 = 4 atoms

 

10. The resistance of 1.5 km of wire of cross-sectional area 0.17 mm2 is 150 Ω. Determine the resistivity of the wire

  1. 17 µ Ωm
  2. 0.17 µ Ωm
  3. 0.017µ Ωm
  4. 0.171µ Ωm

Answer.3. 0.017µ Ωm

Explanation

Given

Area of cylinder (A) = 0.17mm2 = 0.17 × 10−6m

Length (L) = 1.5 km = 1500 m

Resistance = 150 Ω

Resistivity of the conductor is given by

ρ = RA/L

= (0.17 × 10−6  × 150)/(1500)

ρ = 0.017µ Ωm

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top