Ques 91. A circuit has inductance of 2 H. If the circuit current changes at the rate of 10A/sec, then self induced emf is (SSC-2014)
5V
0.2V
20V
10V
Answer.3. 20V
Explanation:-
The self induced EMF is given as
E = Ldi/dt
= 2 x 10
= 20 V.
Ques 92. The B-H curve for _______ will be the straight line passing through the origin. (SSC-2014)
Air
Soft Iron
Hardened Steel
Silicon Steel
Answer.1. Air
Explanation:-
The amount of magnetic field we apply, air never saturates. So the B-H curve in the air should be a straight line
Air is non-magnetic material and for non-magnetic material the B =µoH and since µo = 4π x 10-7 is constant quantity, therefore, B ∝ H
Ques 93. Magnetic lines of force coming from the magnet. (SSC-2014)
Intersect at Infinity
Intersect within the magnet
Cannot intersect at all
Cancel at pole faces
Answer.3. Cannot intersect at all
Explanation:-
The two magnetic field lines do not intersect each other because If the lines intersected, then at the point of intersections there would be two directions of magnetic field force which means at the point of intersecting the compass needle is showing two different directions which are not possible
Ques 94. The main advantages of temporary magnets are that we can (SSC-2014)
Change the magnetic flux
Use any magnetic material
Decreases the hysteresis loss
Magnetize without any source
Answer.1. Change the magnetic flux
Explanation:-
Magnets have two regions called magnetic poles with the magnetism both in and around a magnetic circuit that generates an invisible line of flux around it. These lines of flux are referred to as the “magnetic field” of the magnet.
Soft magnets are temporary – they only produce magnetic fields while an external magnetic field is present and lose nearly all their magnetism once the external field is removed. Hence the magnetic flux can be changed in temporary magnets.
Ques 95. The magnetic material used in permanent magnets is (SSC-2014)
Iron
Soft-steel
Nickel
Hardened Steel
Answer.4. Hardened Steel
Explanation:-
The permanent magnet is made of hardened steel because soft iron and steel remain magnets only under the power or influence of other magnet and lose their magnetism as soon as the magnet is removed.
Compared to iron, steel cannot be easily demagnetized or magnetized. It can only be magnetized under the influence of a strong magnetic field. But steel has the ability to retain its magnetism once it is magnetized. Therefore steel is best suited to be used in permanent magnets.
Ques 96. The emf induced in a coil is given by f = -N dΦ/dt, where ‘e’ is the induced emf, N is the number of turns and dΦ’ is the instantaneous flux linkage with the coil in time ‘dt’. The negative sign of the above expression is due to (SSC- 2013)
Hans Christian Oersted
Andre-Marie Ampere
Michael Faraday
Emil Lenz
Answer.4. Emil Lenz
Explanation:
According to Lenz’s law, the induced EMF sets up current is such a direction so as to oppose the very cause of producing it. Mathematically this expression is expressed in the negative sign
So, In Faraday’s law, the negative sign shows that the polarity of induced emf is such that it opposes any change in the magnetic flux of coil.
Ques 97. The mutual inductance between two coils having self-inductances 3 Henry and 12 Henry and coupling coefficient 0.85 is (SSC- 2013)
12.75 Henry
5.1 Henry
0.425 Henry
1.7 Henry
Answer.2. 5.1 Henry
Explanation:
Coefficient of coupling or magnetic coupling coefficient is (K) is given as
Where L1 and L2 are self-inductance of coil 1 and 2 respectively
Ques 98. The unit of reluctance of magnetic circuit is (SSC- 2013)
AT/m
Weber/m
AT/Weber
Weber/AT
Answer.3. AT/Weber
Explanation:
Reluctance is defined as
ℜ = F/Φ
F = magnetomotive force (MMF) in ampere-turns
Φ = the magnetic flux in webers.
Therefore the unit of Reluctance is ampere-turns per Weber ( AT/Weber) (a unit that is equivalent to turns per Henry)
Ques 99. The total flux at the end of a long permanent bar magnet is 100×10 -6 Wb.The end of is magnet is withdrawn through a 1000 turn coil in 1/20 seconds. The induced emf in the coil is (SSC- 2013)
20.0 V
2.0 V
0.2 V
0.02 V
Answer.2. 2.0 V
Explanation:
Ques 100. A solenoid of inductance 250 mH and resistance 10 Ω is connected to a battery. The time taken for the magnetic energy 1/4th of its maximum value is (SSC- 2013)
