Bihar Board - Class 12 Physics - Chapter 5: Magnestism And Matter Short Answer Question

Short Answer Type Questions

1. Where on the surface of Earth is the vertical component of Earth’s magnetic field zero?  
Answer: At the Magnetic equator the vertical component of Earth’s magnetic field is zero.

6. Is the steady electric current the only source of magnetic field? Justify your answer.
Answer: No. Steady current is not the only source of magnetic field. Magnets are also a source of magnetic field. Unsteady current will also be a source of varying magnetic field.


7. Where on the surface of Earth is the vertical com-ponent of Earth’s magnetic field zero? 
Answer: At the Equator the vertical component of the Earth’s magnetic field is zero.


8. Where on the surface of Earth is the horizontal component of Earth’s magnetic field zero? 
Answer: At poles of Earth the horizontal component of Earth’s magnetic field is zero.


9. In what way is the behavior of a diamagnetic material different from that of a paramagnetic, when kept in an external magnetic field? 
Answer:
1. A diamagnetic specimen would move towards the weaker region of the field; while a paramagnetic specimen would move towards the stronger region.
2. A diamagnetic specimen is repelled by a magnet while a paramagnetic specimen moves towards the magnet.
3. The paramagnetic gets aligned along the field and the diamagnetic perpendicular to the field.

10. The susceptibility of a magnetic material is – 2.6 × 10-5. Identify the type of magnetic material and state its two properties.
Answer: Magnetic material is diamagnetic, because susceptibility of a magnetic material is negative.
Properties are :
In a non-uniform magnetic field, it tends to move slowly from stronger to weaker parts of the field.
A freely suspended diamagnetic rod aligns itself perpendicular to the field.
They expel magnetic field lines.
Such substances are repelled by a magnet.


11. The susceptibility of a magnetic material is 2.6 × 10^-5 Identify the type of magnetic material and state its two properties. 
Answer: The material is paramagnetic.
Its two properties are :
They are feebly attracted by magnets.
In a non-uniform magnetic field, they tend to move slowly from weaker to stronger parts of the field.


12. The relative magnetic permeability of a magnetic material is 800. Identify the nature of magnetic material and state its two properties. 
Answer: Substance is ferromagnetic.
Its properties are :
They are strongly attracted by magnets.
In a non-uniform magnetic field, they tend to move quickly from weaker to stronger parts of the field.


13. (a) How does a diamagnetic material behave when it is cooled to very low temperatures?
(b) Why does a paramagnetic sample display greater magnetisation when cooled? Explain. 
Answer:
(a) When diamagnetic material is cooled to very low temperature then it exhibits both perfect conductivity and perfect diamagnetism.
(b) This is because at lower temperature, the tendency to disrupt the alignment of dipoles (due to magnetizing field) decreases on account of reduced random thermal motion.


14.How will you identify whether the magnetic field at a point is due to the earth or due to some current carrying conductor?
Answer: We take a small size magnetic compass needle and move it in a small region. If it always points in the same direction, the field present is due to the earth's magnetism only. However, if the compass needle points in some other direction and directions gradually change on changing the positions of the needle then the magnetic field may be due to some current carrying conductor.


15.  Out of the two magnetic materials, ‘A’ has relative permeability slightly greater than unity while ‘B’ has less than unity. Identify the nature of the materials ‘A’ and ‘B’. Will their susceptibilities be positive or negative? 
Answer: ‘A’ is paramagnetic
‘B’ is diamagnetic
The susceptibility of material ‘A’ is positive while ‘B’ is negative.


16. Can there be a material which does not show any magnetic behavior?
Answer: No, there is no material which does not show any magnetic behavior. Most materials show diamagnetism but few materials exhibit paramagnetism ferromagnetism.

17. Must every magnetic configuration have a North Pole and a South Pole? What about the field due to a toroid?
Answer: It is not at all necessary that every magnetic configuration must have a North Pole and a South Pole. A toroid and even a solenoid of infinite length do not have any magnetic pole formation.

18. A magnetized needle in a uniform magnetic field Experiences a torque but no net force. An iron nail near a bar magnet, however, experiences a force of attraction in addition to torque. Why?
Answer: If a magnetic needle is placed in a uniform magnetic field, then net force acting on the needle is zero but net torque acting on it.
However, if the magnetic field is non uniform, then a needle should experience net translational force as well as the net torque. When an iron nail is placed near a bar magnet, the nail begins to behave as a magnetic needle on account of magnetic induction. However, as the field of a bar magnet is non uniform, hence the nail experiences a force as well as a torque.

19. Magnetic field lines can be entirely confined within the core of a toroid, but not within a straight solenoid. Why? 
Answer: If magnetic field lines were entirely confined within the core of a straight solenoid, the magnetic flux through the cross-section at each end of it would have been nonzero. But the magnetic flux of field B through any closed surface, as per Gauss' law, must always be zero. It means that magnetic field lines of a straight solenoid cannot be entirely confined within its core. Magnetic field lines can be confined within the core of a toroid because it has ends.

20. Magnetic field arises due to charges in motion. Can a system have magnetic moments even though its net charges 0.
Answer: Yes, it is possible that although net charge on a system may be zero but net magnetic dipole moment due to various current loops may not be zero. A para and ferromagnetic material is an example of such a situation. The net charge on an atom of a paramagnetic material is zero but the net magnetic dipole moment of an atom is finite.

21. An unmagnetized piece of iron is attracted towards a bar magnet. What is the real origin of the attractive force experienced by the iron piece?
Answer: Iron is a ferromagnetic material and each atom of it has a finite magnetic dipole moment. These atomic dipoles form domains. In one domain, all the atomic dipole moments are aligned in the same directions but different domains are oriented randomly so it has no magnetization.
When a bar magnet is brought near the iron piece, the domains in iron tend to align themselves along the directions of a field of the bar magnet. As a result, the iron piece acquires some magnetization and is accordingly attracted towards the magnet.

22. Why do paramagnetic materials display greater magnetisation for the same magnetizing field when cooled? How does a diamagnetic material respond to similar temperature changes?
Answer: We know that each atom of a paramagnetic material has a definite magnetic dipole moment. External magnetizing fields tend to align these atomic dipoles in the directions of the field but the ceaseless thermal motion of the atoms opposes this alignment. As temperature is lowered, the effect of the random thermal motion is gradually reduced and consequently, magnetization of the sample increases even for the same value of the magnetizing field.
Property of diamagnetism is, however, independent of the temperature.

23. In the Northern hemisphere, would magnetic field lines due to the Earth's magnetic field point towards or away from the Earth? Why?
Answer: In the Northern hemisphere, the magnetic field lines due to Earth's magnetic field point into the earth. It is no account of the fact that the magnetic South Pole of the Earth's magnet is situated in the Northern hemisphere.

24. What should be the qualities of a ferromagnetic material for preparing an Electromagnet?
Answer: A ferromagnetic material being used to design an Electromagnet should have high permeability, low retentivity and low coercivity. Moreover, if the Electromagnet has to go through an AC cycle of magnetization for a long period, then the hysteresis curve of the paramagnetic materials being used must be narrow so that the magnetic energy dissipated is small.

25. A magnetic needle is placed on a large cork piece floating in a still lake in the Northern hemisphere. Does the needle cork combination drift towards the north of the lake? Why?
Answer: The magnetic needle is placed on piece is the magnetic dipole capable of free motion in the magnetic field of the earth. At a given place, the Earth's magnetic field is uniform and so the needle experiences a net torque but not net force. As a result, the cork needle arrangement will not drift. However, it can rotate about its axis till the magnetic needle aligns itself along the magnetic Meridian.

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