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Right choice is (d) 1000

The explanation: The reluctance is given by S = L/μ A, where L is the length, A is the area and μ is the PERMEABILITY. On substituting L = 2π x 10^-4, A = 0.5 and μ = 4π x 10^-7, we GET S = 10^3/(2×0.5) = 1000 units.

The correct OPTION is (c) 0.5

Best explanation: The reluctance is defined as the ratio of the mmf and the flux. It is GIVEN by S = mmf/φ. On SUBSTITUTING mmf = 3.5 and φ = 7, we get S = 3.5/7 = 0.5 units.

Right OPTION is (c) RELUCTANCE

To elaborate: The reluctance of a magnetic material is the ABILITY of the material to oppose the magnetic flux. It is the ratio of the magnetic motive force mmf to the flux.

Right answer is (c) LENZ law

To explain I would say: The induced EMF in a material under the INFLUENCE of a MAGNETIC field will oppose the flux that PRODUCES it. This is indicated by a negative sign in the emf equation. This phenomenon is called Lenz law.

Right choice is (B) 300

The explanation is: The self inductance of a coil is given by L = Nφ/I, where N = 50, φ = 3 and I = 0.5. Thus L = 50 X 3/0.5 = 300 UNITS.

Correct option is (a) 21

Easy explanation: For tightly coupled coils, the coefficient of coupling is UNITY. Then the MUTUAL INDUCTANCE will be M = √(L1 x L2)= √(49 x 9) = 21 units.

Correct option is (a) True

To explain: The EXPRESSION is given by L = Ndφ/di. It can be seen that L is proportional to the RATIO of flux to CURRENT. Thus the statement is true.

Right choice is (d) K<0.5

The EXPLANATION is: k is the coefficient of coupling. It lies between 0 and 1. For loosely COUPLED COIL, the coefficient of coupling will be very less. THUS the condition K<0.5 is true.

The correct option is (B) 7

The BEST I can explain: The equivalent inductance of TWO coils in series with opposing FLUX is L = L1 + L2 – 2M, where L1 and L2 are the self inductances and M is the mutual inductance. Thus L = 7 + 2 – 2(1) = 7H.

Right CHOICE is (a) L = N dφ/di

The best EXPLANATION: We know that e = -N dφ/dt and also e = -L di/dt. On EQUATING both we GET, L = Ndφ/di is the expression for inductance.

Right option is (d) 13

To EXPLAIN: The EQUIVALENT INDUCTANCE of two coils in series is given by L = L1 + L2 + 2M, where L1 and L2 are the self inductances and M is the mutual inductance. THUS L = 2 + 5 + 2(3) = 13H.

The correct option is (c) 1:√3

Explanation: From the MAGNETIC boundary conditions, the ratio of PERMEABILITY μ1/μ2 = tan θ1/tan θ2 and θ1 = 45, θ2 = 60. THUS we GET μ1/μ2 = 1/√3. The ratio will be 1:√3.

Correct option is (d) -500

To elaborate: The emf is the product of the turns of the coil and the FLUX RATE. Thus e = -N dφ/dt, where the negative SIGN indicates that the emf induced is OPPOSING the flux. Thus e = -100 X 5 = -500 units.

Right option is (b) CURRENT

The explanation is: The LINE INTEGRAL of the magnetic FIELD intensity is given by ∫H.dl. This is same as the current COMPONENT. From this relation, the Ampere law can be deduced.

Correct OPTION is (a) Same

Easy explanation: The normal COMPONENT and tangential components of the MAGNETIC flux DENSITY will be same. This holds good for any medium.

The correct CHOICE is (c) 2.4

The BEST I can explain: Unlike the ELECTRIC fields, the magnetic flux DENSITY has normal component same in both the MEDIUMS. This gives Bn1 = Bn2.

The correct option is (a) True

The best explanation: SINCE the TANGENTIAL component of the magnetic field intensity will be continuous and B = μH, in air, the tangential component of the FLUX density will ALSO be continuous.

Correct choice is (a) True

For explanation I WOULD say: For two medium of separation, the tangential component of the MAGNETIC field INTENSITY will be CONTINUOUS. This is analogous to the fact that the tangential component of the electric field intensity is continuous at the boundary.

The correct choice is (C) J = Curl(M)

Easiest explanation: The curl of the MAGNETIZATION gives the magnetic field intensity theoretically. From MAXWELL equation, we can correlate that with the CURRENT density (Ampere law)

Correct choice is (c) Paramagnetic

Easiest explanation: Paramagnetic materials are CHARACTERIZED by a small and POSITIVE SUSCEPTIBILITY. The susceptibility and the temperature are DIRECTLY RELATED.

Right choice is (a) 12.5

Easy EXPLANATION: The MAGNETIZATION is the product of the susceptibility and the FIELD intensity given by M = χmH. Put χm = 50 and H = 0.25, then M = 50 x 0.25 = 12.5 UNITS.

Right ANSWER is (c) T = BIA sin θ

Easy explanation: The torque is given by the product of the flux density, magnetic MOMENT IA and the sine ANGLE of the conductor held by the FIELD. This gives T = BIA sin θ.

Correct choice is (c) Diamagnetic

Explanation: In the diamagnetic materials, the susceptibility is very small and NEGATIVE. Thus the susceptibility will be independent of the temperature. The atoms of SOLIDS having CLOSED shells and metals LIKE gold have this property.

The correct answer is (b) 36

Easy explanation: The Lorentz is GIVEN by the product of the current, DIFFERENTIAL length and the magnetic flux density. Put B = 12, I = 1.5 and L = 2, thus we get F = BIL = 12 x 1.5 x 2 = 36 units.

The correct CHOICE is (b) 20

The best EXPLANATION: The SUSCEPTIBILITY in magnetic materials is GIVEN by χm = C/(T-θ), where C is the curie constant, T is the critical temperature and θ is the paramagnetic curie temperature. PUT C = 0.2 and T-θ = 0.01, thus we get susceptibility as 0.2/0.01 = 20.

Correct option is (a) Antiferromagnetic

The best I can explain: In antiferromagnetic materials, the SUSCEPTIBILITY will decrease with increase in TEMPERATURE. They have RELATIVELY SMALL susceptibility at all temperatures.

Right choice is (C) Villari effect

For explanation I would SAY: When a STRAIN is applied, the change in magnetic FIELD is observed. This is the converse of the magnetostriction phenomenon and is called Villari effect.

Right answer is (c) Ferromagnetic

The best I can explain: In MATERIALS LIKE IRON, the magnetic properties DEPEND on the direction in which they are MEASURED. This is magnetic anisotropy. The material iron is a ferromagnetic material type.

Right choice is (b) Magnetostriction

Best explanation: The PIEZOELECTRIC effect is the mechanical strain caused on a MATERIAL like quartz when subjected to an electric FIELD. The same is observed in a ferromagnetic material CALLED magnetostriction.

Right CHOICE is (d) 19.4

To explain I would say: From Curie law, the internal field of a magnetic material is GIVEN by H = Ho + χ M, where χ is the MOLECULAR field constant. Put χ = 0.1, M = 74 and Ho = 12, we get H = 12 + (0.1)74 = 19.4 units.

The CORRECT OPTION is (b) 13.33

The explanation: The magnetization is the ratio of the MAGNETIC moment to the volume. Thus M = m/v, where m = 16 and v = 1.2. We get M = 16/1.2 = 13.33 units.

The correct choice is (d) eh/4πm

The best I can explain: In atomic physics, the Bohr magneton (symbol μB) is a PHYSICAL CONSTANT and the natural unit for expressing the magnetic moment of an ELECTRON caused by either its orbital or spin angular momentum. It is GIVEN by eh/4πm, where H is the Planck’s constant, e is the charge of the electron and m is the mass of the electron.

Right choice is (d) –e/2m

To explain I would say: The ORBITAL dipole moment is GIVEN by M = 0.5 x eVangx r^2 and the orbital angular moment is given by Ma = m x Vangx r^2. Their ratio M/Ma is given by –e/2m, the negative SIGN indicates the CHARGE of electron.

Correct ANSWER is (a) 64

To explain: The orbital dipole moment is given by M = 0.5 X eVangx R^2, where e = 1.6 x 10^-19 is the CHARGE of the electron, Vang = 2 and r = 0.2. On substituting, we get M = 0.5 x 1.6 x 10^-19x 2 x 0.2^2= 64 x 10^-22 units.