Ques 21. The north pole of a magnet is moved away from a metallic ring. The induced current in the ring flows
Clockwise✔
Anticlockwise
First anticlockwise and then clockwise
First clockwise and then anticlockwise
When the north pole of the magnet is moved away from the coil the direction of the induced current is such as to make the nearer pole of the coil a south pole which makes clockwise induced current flow in the coil or the ring.
Ques 22. Energy stored in the inductor is given by
(LI)2/√2
L2I/2
1/√LI
LI2/2✔
The energy stored in the magnetic field of an inductor can be expressed as
W = 1/2 L I2
where
W = energy stored (joules, J)
L = inductance (henrys, H)
I = current (amps, A)
Ques 23. A coil with a certain number of turns has a specified time constant. If the number of turns is doubled, its time constant would
Remain unaffected
Become Double
Become four-fold✔
Get halved
The self inductance of a solenoid is given as
L = μN2A/I = L1
where
N is the number of turns of the solenoid
A is the area of each turn of coil
l is the length of the solenoid
and μ is the permeability constant
so, if the number of turns was to be doubled the self-inductance would be
L2 = u (2N)2A/l
or
L2 = 4L1
it would be quadrupled or increased fourfold.
Ques 24. Hysteresis is the phenomenon in the magnetic circuit by which
H lags behind B
B lags behind H✔
B and H are always same
Setting up a constant flux is done
The phenomenon of flux density B lagging behind the magnetizing force H in a magnetic material is known as Magnetic Hysteresis.
Ques 25. The flux through each turn of a 100-turns coil is (t3 – 2t)mWh, where ‘t’ is in seconds. Find the magnitude of the induced emf at t = 2s
1 V✔
0.8 V
0.4 V
0.2 V
Induced EMF E is
E = NdΦ/dt
Ques 26. A circuit has inductance of 2 H. If the circuit current changes at the rate of 10A/sec, then self induced emf is
5V
0.2V
20V✔
10V
The self induced EMF is given as
E = Ldi/dt
= 2 × 10
= 20 V
Ques 27. The B-H curve for _______ will be the straight line passing through the origin.
Air✔
Soft Iron
Hardened Steel
Silicon Steel
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 a constant quantity, therefore, B ∝ H
Ques 28. Magnetic lines of force coming from the magnet.
Intersect at Infinity
Intersect within the magnet
Cannot intersect at all✔
Cancel at pole faces
The two magnetic field lines do not intersect each other because If the lines intersected, then at the point of intersection 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 is not possible.
Ques 29. The main advantage of temporary magnets are that we can
Change the magnetic flux✔
Use any magnetic material
Decreases the hysteresis loss
Magnetize without any source
Magnets have two regions called magnetic poles with magnetism both in and around a magnetic circuit generating 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 30. The magnetic material used in permanent magnets is
Iron
Soft-steel
Nickel
Hardened Steel✔
The permanent magnet is made of hardened steel because soft iron and steel remain magnets only under the power or influence of other magnets 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.
