1. The magnetic susceptibility does not depend upon the temperature in
Correct Answer: c) Diamagnetic substances
Diamagnetic substances have magnetic susceptibility that is negative and nearly independent of temperature. In contrast, paramagnetic and ferromagnetic substances show temperature-dependent susceptibility.
2. The phenomenon by which a material opposes the formation of magnetic flux in it is called
Correct Answer: b) Reluctance
Reluctance is the opposition offered by a material to the formation of magnetic flux. It is analogous to resistance in electrical circuits.
3. If a ferromagnetic material is inserted in a current carrying solenoid, the magnetic field of solenoid
Correct Answer: a) Largely increases
Ferromagnetic materials have very high permeability (μ≫1), so inserting them into a solenoid greatly increases the magnetic flux density (B = μH), resulting in a much stronger magnetic field.
4. The universal property of all substances is
Correct Answer: a) Diamagnetism
All materials exhibit diamagnetism, which is a weak repulsion from a magnetic field. This is due to the orbital motion of electrons. Other magnetic properties like paramagnetism or ferromagnetism are superimposed on this basic diamagnetic behavior.
5. The magnetic moment of an atom is primarily due to
Correct Answer: d) Both b and c
The magnetic moment of an atom arises from two main sources: the spin of electrons (spin magnetic moment) and the orbital motion of electrons (orbital magnetic moment). Nuclear spin contributes negligibly to the total magnetic moment.
6. Demagnetisation of magnets can be done by
Correct Answer: d) All the above
Demagnetization can occur through various methods: mechanical shock (rough handling) can disrupt magnetic domains, heating above the Curie temperature destroys magnetic order, and applying a reverse field can neutralize magnetization.
7. The energy stored in a magnetic field is given by
Correct Answer: a) \(\frac{1}{2}LI^2\)
The energy stored in a magnetic field is given by \(E = \frac{1}{2}LI^2\) where L is inductance and I is current. For magnetic materials, energy density is \(\frac{1}{2}B \cdot H\) per unit volume.
8. A ferromagnetic material is heated above its curie temperature. Which one is a correct statement
Correct Answer: b) Ferromagnetic domains becomes random
Above the Curie temperature, thermal energy overcomes the exchange interaction that maintains parallel alignment of atomic magnetic moments. The material loses its ferromagnetic properties and becomes paramagnetic with randomly oriented moments.
9. When a magnetic substance is heated, then it
Correct Answer: b) Losses its magnetism
Heating increases thermal agitation which tends to randomize the alignment of magnetic moments. For ferromagnetic materials, heating above the Curie temperature causes complete loss of spontaneous magnetization.
10. The magnetic flux density (B) inside a superconductor is
Correct Answer: d) Zero
Superconductors exhibit perfect diamagnetism (Meissner effect), expelling all magnetic flux from their interior. Thus, B = 0 inside a superconductor in the superconducting state.
11. In the hysteresis cycle, the value of H needed to make the intensity of magnetisation zero is called
Correct Answer: b) Coercive force
Coercivity or coercive force (Hc) is the reverse magnetic field required to reduce the magnetization of a ferromagnetic material to zero after it has been magnetized to saturation.
12. The magnetic moment of atomic neon is
Correct Answer: a) Zero
Neon is a noble gas with completely filled electron shells (1s22s22p6). All electron spins are paired and orbital moments cancel out, resulting in zero net magnetic moment.
13. The magnetic susceptibility of a superconductor is
Correct Answer: b) -1
Superconductors exhibit perfect diamagnetism with χ = -1 due to the Meissner effect, where they completely expel magnetic flux from their interior (B = 0 inside).
14. Temperature above which a ferromagnetic substance becomes paramagnetic is called
Correct Answer: d) Curie temperature
The Curie temperature (Tc) is the critical temperature above which a ferromagnetic material loses its spontaneous magnetization and becomes paramagnetic due to thermal energy overcoming exchange interactions.
15. Which one of the following is a non-magnetic substance
Correct Answer: d) Brass
Brass is an alloy of copper and zinc, both of which are non-magnetic (diamagnetic). Iron, nickel, and cobalt are the three elemental ferromagnetic materials at room temperature.
16. Susceptibility of ferromagnetic substance is
Correct Answer: a) > 1
Ferromagnetic materials have very large positive susceptibility (χ ≫ 1), often in the range of 103 to 105, due to their strong spontaneous magnetization and domain alignment.
17. Magnets cannot be made from which of the following substances
Correct Answer: c) Copper
Copper is diamagnetic and cannot be made into permanent magnets. Iron and nickel are ferromagnetic and can be made into magnets. The correct answer should be only copper, as iron and nickel can be made into magnets.
18. Substances in which the magnetic moment of a single atom is not zero, is known as
Correct Answer: c) Paramagnetism
Paramagnetic materials have atoms with permanent magnetic moments (not zero) that align with an external field but are randomly oriented without one. Ferromagnetism and ferrimagnetism involve collective ordering of these moments.
19. The material of permanent magnet has
Correct Answer: d) High retentivity, high coercivity
Permanent magnets require both high retentivity (to retain magnetization) and high coercivity (to resist demagnetization). Materials like alnico and rare-earth magnets exhibit these properties.
20. The energy product (BH)max represents
Correct Answer: b) Maximum energy density of a permanent magnet
The (BH)max product represents the maximum magnetic energy density a permanent magnet can supply to an external circuit. It's an important figure of merit for permanent magnet materials.
21. An example of a diamagnetic substance is
Correct Answer: b) Copper
Copper is diamagnetic (χ ≈ -10-5), weakly repelled by magnetic fields. Aluminium is paramagnetic, while iron and nickel are ferromagnetic at room temperature.
22. The domain theory of ferromagnetism explains that
Correct Answer: d) All of the above
The domain theory states that ferromagnetic materials are divided into small regions (domains) with uniform magnetization. In unmagnetized state, domains are randomly oriented, causing no net magnetization. When magnetized, domains align to produce net magnetization.
23. Diamagnetic substances are
Correct Answer: c) Feebly repelled by magnets
Diamagnetic materials have weak negative susceptibility (χ ≈ -10-5 to -10-6) and are slightly repelled by magnetic fields. This effect is much weaker than paramagnetic or ferromagnetic attraction.
24. The permanent magnet is made from which one of the following substances
Correct Answer: c) Ferromagnetic
Permanent magnets are made from ferromagnetic materials (like iron, nickel, cobalt, or their alloys) that retain their magnetization after the external field is removed due to high coercivity and retentivity.
25. If a magnetic substance is kept in a magnetic field, then which of the following is thrown out
Correct Answer: c) Diamagnetic
Diamagnetic materials are weakly repelled by magnetic fields (negative susceptibility), so they tend to move toward weaker field regions. Paramagnetic and ferromagnetic materials are attracted to stronger field regions.
26. The Weiss molecular field is proportional to
Correct Answer: b) Magnetization
The Weiss molecular field (Hm) is an internal field in ferromagnetic materials that is proportional to the magnetization (Hm = λM), where λ is the molecular field constant.
27. The use of study of hysteresis curve for a given material is to estimate the
Correct Answer: b) Hysteresis loss
The area of the hysteresis loop represents the energy lost as heat per unit volume during one complete magnetization cycle. This is called hysteresis loss, important in transformer cores and other AC applications.
28. The relative permeability is represented by μr and the susceptibility is denoted by χ for a magnetic substance. Then for a paramagnetic substance
Correct Answer: d) μr > 1, χ > 0
For paramagnetic materials: χ is small and positive (0 < χ < 10-3), and μr = 1 + χ is slightly greater than 1. The relationship between them is μr = 1 + χ.
29. The magnetic moment of an electron due to its orbital motion is given by
Correct Answer: a) \(\frac{e\hbar}{2m}\)
The orbital magnetic moment of an electron is given by μl = -(e/2m)L, where L is the orbital angular momentum. For L = ħ, we get the Bohr magneton μB = eħ/2m.
30. Which of the following is true
Correct Answer: b) Paramagnetism is temperature dependent
Paramagnetic susceptibility follows Curie's law: χ = C/T, where C is the Curie constant and T is absolute temperature. Diamagnetic susceptibility is nearly temperature independent.
31. If the angular momentum of an electron is J then the magnitude of the magnetic moment will be
Correct Answer: b) \(\frac{eJ}{2m}\)
The magnetic moment μ is related to angular momentum J by μ = γJ, where γ = -e/2m is the gyromagnetic ratio for orbital motion. Thus, |μ| = eJ/2m.
32. Magnetic permeability is maximum for
Correct Answer: c) Ferromagnetic substance
Ferromagnetic materials have very high permeability (μ ≫ μ0), often thousands of times greater than paramagnetic materials. Diamagnetic materials have μ slightly less than μ0.
33. Which of the following statements are true about the magnetic susceptibility χm of paramagnetic substance
Correct Answer: a) Value of χm is inversely proportional to the absolute temperature of the sample
Paramagnetic susceptibility follows Curie's law: χm = C/T, where C is the Curie constant. Thus, χm is positive and inversely proportional to absolute temperature.
34. The magnetic susceptibility is negative for
Correct Answer: b) Diamagnetic materials
Diamagnetic materials have negative susceptibility (χ < 0) as they weakly oppose applied magnetic fields. Paramagnetic and ferromagnetic materials have positive susceptibility.
35. The exchange interaction in ferromagnetic materials is
Correct Answer: c) Quantum mechanical in nature
The exchange interaction is a quantum mechanical phenomenon arising from the Pauli exclusion principle and electrostatic Coulomb interaction between electrons, leading to parallel alignment of spins in ferromagnets.
36. Which of the following statements is incorrect about hysteresis
Correct Answer: c) The hysteresis loop area is independent of the thermal energy developed per unit volume of the material
The area of the hysteresis loop represents the energy lost as heat per unit volume per cycle. Thus, it is directly related to thermal energy loss, making statement c incorrect.
37. Curies law can be written as
Correct Answer: a) \(χ = \frac{C}{T}\)
Curie's law states that the magnetic susceptibility χ of a paramagnetic material is inversely proportional to the absolute temperature T: χ = C/T, where C is the material-specific Curie constant.
38. The magnetic moment of a material with 1023 atoms each having a moment of 1μB when fully aligned is
Correct Answer: d) 9.27 × 10-1 A·m2
1 Bohr magneton (μB) = 9.27 × 10-24 A·m2. For 1023 aligned moments: 1023 × 9.27 × 10-24 = 9.27 × 10-1 A·m2.
39. Curie-Weiss law is obeyed by iron at a temperature ......
Correct Answer: b) Above Curie temperature
The Curie-Weiss law (χ = C/(T - Tc)) describes the susceptibility of ferromagnetic materials in their paramagnetic phase (above Tc). Below Tc, spontaneous magnetization occurs.
40. A small rod of bismuth is suspended freely between the poles of a strong electromagnet. It is found to arrange itself at right angles to the magnetic field. This observation establishes that bismuth is
Correct Answer: a) Diamagnetic
Diamagnetic materials tend to move to regions of weakest field. Bismuth is strongly diamagnetic, so the rod orients perpendicular to the field lines where the field gradient is weakest.
41. The magnetic susceptibility is
Correct Answer: a) \(\frac{M}{H}\)
Magnetic susceptibility (χ) is defined as the ratio of magnetization (M) to the applied magnetic field strength (H): χ = M/H. It measures how much a material becomes magnetized in response to an applied field.
42. A diamagnetic material in a magnetic field moves
Correct Answer: c) From stronger to the weaker parts of the field
Diamagnetic materials are repelled by magnetic fields, so they tend to move toward regions of weaker field strength. This is opposite to the behavior of paramagnetic and ferromagnetic materials.
43. Liquid oxygen remains suspended between two pole faces of a magnet because it is
Correct Answer: b) Paramagnetic
Liquid oxygen is paramagnetic and is attracted to the strong magnetic field between the poles. This attraction balances gravity, allowing it to remain suspended.
44. The materials suitable for making electromagnets should have
Correct Answer: b) Low retentivity and low coercivity
Electromagnets require materials that magnetize easily (low coercivity) and don't retain magnetization when current is switched off (low retentivity). Soft iron is commonly used.
45. The magnetic anisotropy energy is responsible for
Correct Answer: b) Preferred directions of magnetization
Magnetic anisotropy energy causes certain crystallographic directions to be easier for magnetization (easy axes) than others. This affects domain orientation and hysteresis properties.
46. For an isotropic medium B, μ, H and M are related as (where \(B = \mu_0(H + M)\) and M have their usual meaning in the context of magnetic material
Correct Answer: a) \(B = \mu_0(H + M)\)
The fundamental relationship is B = μ0(H + M), where B is magnetic flux density, H is magnetic field strength, and M is magnetization. For linear materials, B = μH where μ = μ0(1 + χ).
47. Which of the following is diamagnetism
Correct Answer: d) Bismuth
Bismuth is one of the strongest diamagnetic materials. Aluminium is paramagnetic, nickel is ferromagnetic, and quartz (SiO2) is weakly diamagnetic but bismuth shows much stronger diamagnetism.
48. Curie temperature is the temperature above which
Correct Answer: b) A ferromagnetic material becomes paramagnetic
Above the Curie temperature (Tc), thermal energy disrupts the spontaneous alignment of magnetic moments in ferromagnetic materials, causing them to become paramagnetic.
49. The Bloch wall energy is primarily due to
Correct Answer: d) All of the above
The Bloch wall (domain wall) energy results from competition between exchange energy (favors parallel spins), anisotropy energy (favors easy axis alignment), and magnetostatic energy.
50. The magnetic susceptibility of any paramagnetic material changes with absolute temperature T as
Correct Answer: c) Inversely proportional to T
According to Curie's law, the susceptibility of paramagnetic materials is inversely proportional to absolute temperature: χ = C/T, where C is the Curie constant characteristic of the material.