Quiz: Electrical Engineering 07 Oct 2020

Quiz: Electrical Engineering
Exam: SSC JE
Topic: Magnetic circuit

Each question carries 1 mark.
Negative marking: 1/4 mark
Time: 10 Minute

Q1. Magnetic field intensity at Centre of circular coil, of diameter 2 m and carrying a current of 2 A is:
(a) 1 A/m
(b) 2 A/m
(c) 4 A/m
(d) 8 A/m

Q2. A coil of 100 turns is rotated at 1200 rpm in a magnetic field having uniform density of 0.04 T. therefore, the value of frequency is……………….
(a) 20 Hz
(b) 25 Hz
(c) 30 Hz
(d) 50 Hz

Q3. Which of the following statements is true about magnetic lines of force?
(a) Magnetic lines of force are always closed.
(b) Magnetic lines of force always intersect each other.
(c) Magnetic lines of force tend to crowd far away from the poles of the magnet
(d) Magnetic lines of force do not pass through the vacuum.

Q4. The qualities aspired to obtain a good permanent magnet is/are ____________
(a) high residual flux
(b) low coercivity
(c) high coercivity
(d) high residual flux and high coercivity

Q5. For which of the following is magnetic susceptibility negative?
(a) Paramagnetic and Ferromagnetic materials
(b) Paramagnetic Materials only
(c) Ferromagnetic Materials only
(d) Diamagnetic Materials

Q6. Which of the following statements are true about the magnetic susceptibility xm of paramagnetic substance?
(a) Value of xm is inversely proportional to the absolute temperature of the sample
(b) xm is negative at all temperature
(c) xm is does not depend on the temperature of the sample.
(d) All of the above

Q7. …………materials having relative permeability much greater than unity
(a) Ferromagnetic
(b) Paramagnetic
(c) Diamagnetic
(d) None of the above

Q8. The materials having high retentivity and high coercivity are suitable for making
(a) Weak magnets
(b) Temporary magnets
(c) Permanent magnets
(d) None of the above

Q9. Both the number of turns and the core length of an inductive coil are doubled. Its self-inductance will be:
(a) Doubled
(b) Halved
(c) Tripled
(d) Quadrupled

Q10. Curie temperature is the temperature above which a ferro-magnetic material becomes……
(a) Paramagnetic
(b) Diamagnetic
(c) Either of the above
(d) None of the above

SOLUTIONS
S1. Ans.(a)
Sol. magnetic field intensity at Centre of circular coil is given by:H=I/2R=2/(2×1)=1 A/m

S2. Ans.(a)
Sol. speed of rotation =1200 rotations per minute=1200/60=20 rotations per second
Frequency is defined as rotations per second=20 Hz

S3. Ans.(a)
Sol. Magnetic lines of force are always closed.

S4. Ans.(d)
Sol. It’s both high residual flux as well as coercivity is desired for a good permanent magnet.

S5. Ans.(d)
Sol. Diamagnetic material: Magnetic materials which align against the magnetic field are known as diamagnetic materials. Magnetic susceptibility is χ<0 which means it is always a negative value for diamagnetic material.
These materials are independent of temperature. As these materials magnetize in the opposite direction, they do have a small amount of magnetization intensity. Gold, tin, mercury, water, etc are examples of diamagnetic materials.
For a diamagnetic material, the relative permeability lies between 0 ≤ μr < 1 and its susceptibility lies between −1< χ< 0. 𝑥=𝑢𝑟−1
Where 𝑥 is a dimensionless quantity defines as the magnetic susceptibility of the medium.

S6. Ans.(a)
Sol. The mathematical definition of magnetic susceptibility is the ratio of magnetization to applied magnetizing field intensity. This is a dimensionless quantity.
x=M/H
Since it is the ratio of two magnetic fields, susceptibility is a dimensionless number. paramagnetic, superparamagnetic, and ferromagnetic substances have positive susceptibilities (χ>0).
For paramagnetic substance value of χm is inversely proportional to the absolute temperature of the sample
These materials (Paramagnetic)are temperature dependent and are weekly attracted by magnets with relative permeability 1.00001 to 1.003.

S7. Ans.(a)
Sol. for ferromagnetic materials, relative permeability is much greater than unity.
For a paramagnetic material, its susceptibility is positive. This is because its relative permeability is slightly greater than unity. For a diamagnetic material, the relative permeability lies between 0≤μr<1 and its susceptibility lie between −1<x <0.

S8. Ans.(c)
Sol. The permanent magnets are made from hard ferromagnetic materials (steel, carbon steel, cobalt steel etc). since, these materials have high retentivity, the magnet is quite strong. Due to their high coercivity, they are unlikely to be demagnetised by stray magnetic fields.

S9. Ans.(a)
Sol. self- inductance L=(µ0 µr AN^2)/L
Where, A=Area
N= number of turns
L=length of coil
If, both number of turns and core length of an inductive coil are doubled. Its self-inductance will be doubled.

S10. Ans.(a)
Sol. Curie temperature (TC) is the temperature at which certain materials lose their permanent magnetic properties, to be replaced by induced magnetism.
Below the Curie temperature, the atoms are aligned and parallel, causing spontaneous magnetism; the material is ferromagnetic.
Above the Curie temperature the material is paramagnetic, as the atoms lose their ordered magnetic moments when the material undergoes a phase transition.

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