Magnetic Properties of Matter
Questions for Short Answer
1. When a dielectric is placed in an electric field, it gets polarized. The electric field in a polarized material is less than the applied field. When a paramagnetic substance is kept in a magnetic field, the field in the substance is more than the applied field. Explain the reason for this opposite behavior.
ANSWER: In the case of a dielectric placed in an electric field, the dielectric gets polarized. Due to the polarization, there is a field inside it that is opposite to the applied field. Thus the net electric field in the polarised material is less than the applied field.
When a paramagnetic substance is kept in a magnetic field, the randomly oriented magnetic moments of the atoms get aligned along the magnetic field. In this case, unlike the polarized dielectric in the electric field, the magnetic moments of the aligned magnetic fields of the substance are in the same direction as the applied field. Thus the magnetized substance produces an extra magnetic field in the direction of the applied field. The resultant magnetic field in the material is greater than the applied field.
2. The property of diamagnetism is said to be present in all materials. Then, why are some materials paramagnetic or ferromagnetic?
ANSWER: When a substance is placed in an applied magnetic field, dipole moments are induced in the atoms. From Lenz's law, the magnetic field due to the induced magnetic moments opposes the original field. The resultant field is smaller than the applied field. This property is called diamagnetism and it is present in all materials. But some materials have a permanent atomic magnetic moment that gets aligned according to the applied field and produces an extra magnetic moment along the field. This paramagnetism or ferromagnetism property is much stronger than diamagnetism. Such types of materials are paramagnetic or ferromagnetic.
3. Do permeability and relative permeability have the same dimensions?
ANSWER: No. The unit of permeability is T-m/A. Hence its dimensions are,
[ยต] =[MT⁻²I⁻¹][L][I⁻¹]
=[MLT⁻²I⁻²]
The relative permeability is a factor by which the magnetic field B is increased when a material is brought into the magnetic field. It is also the comparison factor by which the permeability of a material is greater than the permeability of a vacuum. Thus it is unitless and dimensionless.
4. A rod when suspended in a magnetic field stays in the east-west direction. Can we be sure that the field is in the east-west direction? Can it be in a north-south direction?
ANSWER: We cannot be sure that the field is in the east-west direction because it depends upon the substance of the rod. If the substance of the rod is paramagnetic or ferromagnetic then the field will be in the east-west direction but if it is diamagnetic then the field will be in the north-south direction.
5. Why cannot we make permanent magnets from paramagnetic materials?
ANSWER: Paramagnetic materials show a small magnetization in the direction of the field. When the applied field is removed, that small magnetization too disappears. So the retentivity is nil, thus permanent magnets cannot be made from paramagnetic materials.
6. Can we have magnetic hysteresis in paramagnetic or diamagnetic substances?
ANSWER: No, magnetic hysteresis is only shown by ferromagnetic substances.
7. When a ferromagnetic material goes through a hysteresis loop, its thermal energy is increased. Where does this thermal energy come from?
ANSWER: The aligning process of the atomic magnets does not occur simultaneously with the magnetizing field but lags behind in ferromagnetic materials. Even when the applied magnetic field is withdrawn, some magnetism is retained by it. To make it zero, an opposite magnetic field is applied which is called coercive force. So we have to spend energy to make the residual magnetization to zero. This energy is converted to thermal energy in a similar manner to that of the friction between two surfaces.
8. What are the advantages of using soft iron as a core, instead of steel, in the coils of galvanometers?
ANSWER: Soft iron has a small area of the hysteresis loop because the retentivity and the coercive force are smaller than steel. So it is easily magnetized and only a small magnetization is retained when the field is removed. Also, the loss of energy is small during periodic variations in the magnetizing fields. The area of the hysteresis loop of steel is larger, so retentivity and coercive force are higher. Energy loss is also higher in periodic variations of the magnetic fields in the case of steel. Hence soft iron is more suitable as a core in the coils of the galvanometer.
9. To keep valuable instruments away from the earth's magnetic field, they are enclosed in iron boxes. Explain.
ANSWER: Iron being ferromagnetic, has a relative permeability of the order of thousands. So the earth's magnetic field lines near the iron box pass through the material of the box and do not go inside the box. Thus the valuable instruments inside the iron box remain unaffected from the earth's field. 
Diagram for Q-9
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Links to the Chapters
Links to the Chapters
CHAPTER- 36- Permanent Magnets
CHAPTER- 35- Magnetic Field due to a Current
CHAPTER- 34- Magnetic Field
CHAPTER- 33- Thermal and Chemical Effects of Electric Current
CHAPTER- 36- Permanent Magnets
CHAPTER- 35- Magnetic Field due to a Current
CHAPTER- 34- Magnetic Field
CHAPTER- 33- Thermal and Chemical Effects of Electric Current
CHAPTER- 32- Electric Current in ConductorsCHAPTER- 31- CapacitorsCHAPTER- 30- Gauss's Law
CHAPTER- 29- Electric Field and Potential
CHAPTER- 28- Heat Transfer
OBJECTIVE -I
CHAPTER- 26-Laws of Thermodynamics
CHAPTER- 25-CALORIMETRY
Questions for Short Answer
OBJECTIVE-I
OBJECTIVE-II
EXERCISES - Q-11 to Q-18
CHAPTER- 24-Kinetic Theory of Gases
CHAPTER- 23 - Heat and Temperature
CHAPTER- 21 - Speed of Light
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 32- Electric Current in Conductors
CHAPTER- 31- Capacitors
CHAPTER- 30- Gauss's Law
CHAPTER- 29- Electric Field and Potential
CHAPTER- 28- Heat Transfer
CHAPTER- 26-Laws of Thermodynamics
CHAPTER- 25-CALORIMETRY
Questions for Short Answer
OBJECTIVE-I
OBJECTIVE-II
CHAPTER- 24-Kinetic Theory of Gases
CHAPTER- 23 - Heat and Temperature
CHAPTER- 21 - Speed of Light
CHAPTER- 20 - Dispersion and Spectra
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
CHAPTER- 15 - Wave Motion and Waves on a String
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 8 - Work and Energy
Click here for → Question for Short Answers
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → Exercises (1-10)
Click here for → Question for Short Answers
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Click here for → OBJECTIVE-II
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CHAPTER- 7 - Circular Motion
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CHAPTER- 6 - Friction
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CHAPTER- 6 - Friction
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Click here for → Friction - OBJECTIVE-II
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Click here for → Friction - OBJECTIVE-II
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Click here for → EXERCISES (21-31)
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CHAPTER- 5 - Newton's Laws of Motion
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Click here for→ Newton's laws of motion - Objective - I
Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
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Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)
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CHAPTER- 4 - The Forces
The Forces-
"Questions for short Answers"
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Click here for "The Forces" - OBJECTIVE-II
Click here for "The Forces" - Exercises
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CHAPTER- 3 - Kinematics - Rest and Motion
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Click here for EXERCISES (Question number 1 to 10)
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Click here for EXERCISES (Question number 21 to 30)
Click here for EXERCISES (Question number 31 to 40)
Click here for EXERCISES (Question number 41 to 52)
CHAPTER- 2 - "Physics and Mathematics"
CHAPTER- 2 - "Physics and Mathematics"
