# Class 12 Physics Chapter 5 Important Questions Magnetism and Matter

1 Q:- Define the term 'magnetic moment' of a magnetic dipole or a bar magnet. Ans:- The magnetic moment of a magnetic dipole is equal to the magnetic

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Magnetism and Matter. The best solution of the problem is to practice as many Physics Class 12 Chapter 5 Important Questions as possible to clear the doubts.

1 Q:- Define the term 'magnetic moment' of a magnetic dipole or a bar magnet.
Ans:- The magnetic moment of a magnetic dipole is equal to the magnetic moment of an equivalent solenoid which produces exactly the same magnetic field as produced by the magnetic dipole.

2 Q:- What happens if a bar magnet is cut into two pieces:

• (a) transverse to its length, and
• (b) along its length?

Ans:- In either case, one gets two magnets and each magnet has a North as well as South Pole. Of course, the magnetic moment of each piece is less than that of the original bar magnet.

3 Q:- Is magnetic dipole moment of a bar magnet a vector quantity? If yes, what is its direction?
Ans:- Yes, dipole moment of a bar magnet is a vector quantity whose direction is from South Pole of magnet towards north pole of magnet.

4 Q:- State Gauss' Theorem in magnetism. What does it signify?
Ans:- According to Gauss's theorem for magnetism, the net magnetic flux through any closed surface is zero.

Mathematicall,

\oint\vec{B}.d\vec{A}=0

Gauss's theorem in magnetism signifies the fact that no isolated magnetic monopoles exist in nature and the the simplest magnetic element is a magnetic dipole or a current loop behaving as a dipole. Thus, there are no sources or sinks of magnetic field B inside a closed surface and next flux is zero.

5 Q:- Does a bar magnet exert a torque on itself due to its own field? Does one element of a current-carrying wire exert a force on another element of the same wire?
Ans:- A bar magnet does not exert a torque on itself due to its own field. In general, one element of a current-carrying wire exerts a force on another element of the same wire. However, for a straight current-carrying wire, force exerted by one element on another element is zero.

6 Q:- What is the basic difference between magnetic and electric field lines?
Ans:- Magnetic field lines form closed paths but electrical field lines do not make closed paths or curves.

7 Q:- What is the order of magnitude of the Earth's magnetic field at a place on its surface?
Ans:- Magnitude of the Earth's magnetic field at a place on its surface of the order of 10-5 Tesla.

8 Q:- Approximately upto what distance from its surface, we can detect the presence of the Earth's magnetic field?
Ans:- Presence of the Earth's magnetic field can be detected even at a distance of about 30000 km from the surface of the Earth.

9 Q:- What is geomagnetic equator?
Ans:- Geomagnetic equator is the locus of all those places on the Earth where the value of angle of dip is zero.

10 Q:- In which direction, should a compass needle align if taken to (a) North, and (b) South Pole of the Earth?
Ans:- At either Pole, the value of angle of dip is 90 degree and a freely suspended needle must remain Vertical in equilibrium state. A compass needle cannot align itself vertically because it is pivoted about an vertical Axis. Consequently, at the poles, a compass needle has no preferred directions and many comes to rest along any direction.

11 Q:- Where on the surface of the Earth, is the angle of dip 90 degree?
Ans:- At the magnetic poles of the Earth, the value of angle of dip is 90 degree.

12 Q:- What is the SI unit of magnetic induction vector (M)?
Ans:- The SI unit of magnetic induction vector is ampere per metre (A/m).

13 Q:- Define magnetic susceptibility of a material.
Ans:- Magnetic susceptibility of a given sample is defined as the ratio of magnetic induction M induced in the sample to the magnetic intensity H in which the sample is place.

Thus,

\vec{M}=\chi\vec{H}

14 Q:- What does negative value of magnetic susceptibility of a material signify?
Ans:- Negative value of magnetic susceptibility of a material signifies that magnetization developed in the material is in a direction opposite to that of external magnetic field. In other words, negative value of susceptibility signifies that the value of magnetic field is slightly reduced due to the presence of the given material.

15 Q:- From the viewpoint of atomic magnetic dipole moment, what is the difference between diamagnetic and paramagnetic materials?
Ans:- An atom of diamagnetic material has no permanent magnetic dipole moment of its own but an atom of paramagnetic material processes a net permanent magnetic dipole moment of its own.

16 Q:- What is Curie's temperature?
Ans:- Curie's temperature is a temperatures for a ferromagnetic materials above which it losses its ferromagnetic characters and begins to behave as a paramagnetic material.

17 Q:- Why should the materials used for making permanent magnets have a high coercivity?
Ans:- A materials used for making permanent magnet should have a high coercivity so that its magnetization is not a erased by stray magnetic fields, temperature fluctuations or a minor mechanical damage.

18 Q:- Write any three characteristics which a ferromagnetic substance should process if it is to be used to make permanent magnet. Give an examples of such material.
Ans:- A ferromagnetic material, which may be used to design a permanent magnet, should have:

• (a). high coercivity,
• (b). high permeability, and
• (c). high retentivity.

Steel, Cobalt and certain alloys like Alnico or the Ticonal are used for designing permanent magnets.

19 Q:- Why do magnetic field lines prefer to pass through ions than through air?
Ans:- Iron is ferromagnetic materials having a very high value of relative permeability (μ_r) and a magnetic susceptibility (\chi). Consequently, magnetic field lines banned by a large extent toward the iron sample.

20 Q:- Can two magnetic field lines intersect each other at some point? Justify your answer.
Ans:- Two magnetic field lines can never intersect each others at any point because if they do so, then there will be two possible directions of the magnetic field at the point of intersection which is not possible.

21 Q:- The angle of dip at a place in Kerala is 18 degree. Will its value be more or less at a place in Kashmir? Give reason.
Ans:- Angle of dip increases with increase in latitude of a place. As in comparison to a place in Kerala, Kashmir situated at a higher latitude in Northern hemisphere, hence the value of the dip at Kashmir is more than that at a place in Kerala.

22 Q:- A freely suspended magnet always points along north-south direction. Why?
Ans:- We know that our Earth behaves as if large-size bar magnet is placed deep inside it with its North magnetic pole near the Geographic South Pole and its south magnetic pole near the Geographic north pole. A freely suspended magnet always points along north-south directions due to the force of attractions experience by it due to the opposite poles of the earth's magnet.

23 Q:- Can a magnetic field be mapped out? If yes, how?
Ans:- Yes, we can map out a magnetic field. For this we use a magnetic compass needle. The direction pointed by the north pole of a compass needle is at any place gives the directions of magnetic field at the place.

24 Q:- What do you mean by a neutral point in the context of a magnetic field?
Ans:- A neutral point is a point where the net magnetic field under the combined effect of two magnetic dipoles is zero. Alternatively, we can have a neutral point near a magnet where field of magnet is just nullified by the Earth's magnetic field and the net field is zero. At neutral point, a compass needle will not have any preferred direction.

25 Q:- What do you mean by The 'dynamo effect' in the context of the Earth's magnetic field?
Ans:- In the outer core of the Earth, there is a connective motions of metallic, mostly of iron and nickel, fluid irons. Due to these convective motion of Ions, an electric current is established there. Magnetic field of the Earth is due to these electrical current loops. The production of electric currents due to connective motion of liquid metal ions is called 'dynamo effect' and is responsible for causing the Earth's magnetic field.

26 Q:- The relative permeability of magnetic is 0.9983. Name the type of magnetic material it represents.
Ans:- since for a given magnetic material, the relative permeability is less than 1, the material is a diamagnetic material.

27 Q:- The susceptibility of a magnetic material is 1.9 x 10-5. Name the type of magnetic material it represents.
Ans:- The susceptibility of the given material is extremely small but positive. Hence, the material is a paramagnetic material.

28 Q:- To which of the two polar dielectric or nonpolar dielectric, does a paramagnetic material correspond?
Ans:- A paramagnetic material corresponds to a polar dielectric. As a molecule of a polar the electric processes a finite electric dipole moment, similarly a molecule of paramagnetic material processes a finite magnetic dipole moment.

29 Q:- An iron magnet is heated to 1500 K and then cooled in space devoid of any magnetic field. Will it retain its magnetism? Give reason.
Ans:- Curie's temperatures for iron is 1043 K. If an iron magnet is heated at 1500 K, it will lose its ferromagnetic character. If the sample is cooled again in the regions where there is no external magnet field, it will not be magnetized again. Hence, the iron magnet will now become a simple piece of iron without any magnetism of its own.

30 Q:- What do you mean by retentivity and coercivity of a magnetic materials? What are their units?
Ans:- Retentivity of given magnetic specimen is a measure of magnetic field are retained by the specimen when external magnetizing intensity is removed. The SI unit of retentivity is tesla (T).

Coercivity of a magnetic specimen is a measure of magnetic intensity required in opposite direction so as to destroy the magnetic field retained by the specimen. The SI unit of coercivity is ampere per meter (A/m).

31 Q:- Can there be a materials which does not show any magnetic behaviour?
Ans:- No, there is no materials with does not show any magnetic behaviour. Most materials show diamagnetism but few material exhibit paramagnetism ferromagnetism.

32 Q:- Must every magnetic configuration have a North Pole and a South Pole? What about the field due to a toroid?
Ans:- It is not at all necessary that every magnetic configurations 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.

33 Q:- 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 attractions in addition torque. Why?
Ans:- 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 magnetic field is non uniform, then a needle should experience net translational force as well as the net torque. When a 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.

34 Q:- Magnetic field lines can be entirely confined within the core of a ttoroid, ut not within a straight solenoid. Why?
Ans:- 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.

35 Q:- Magnetic field arises due to charges in motion. Can a systems have magnetic moments even though its net charges 0.
Ans:- 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 net magnetic dipole moment of an atom is finite.

36 Q:- An unmagnetized piece of iron is a attracted towards a bar magnet. What is the real origin of the attractive force experienced by the iron piece?
Ans:- 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.

37 Q:- Why does paramagnetic materials display greater magnetisation for the same magnetising field when cooled? How does a diamagnetic material respond to similar temperature changes?
Ans:- We know that each atom of a paramagnetic material has a definite magnetic dipole moment. External magnetising field tends to align these atomic dipoles in the directions of field but the ceaseless thermal motion of the atoms oppose this alignment. As temperature is lowered, effect of the random thermal motion is gradually reduced and consequently, magnetization of the sample increases even for the same value of magnetising field.

Property of diamagnetism is, however, independent of the temperature.

38 Q:- In the Northern hemisphere, would magnetic field lines due to the Earth's magnetic field point towards or away from the Earth? Why?
Ans:- 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 magnetic South Pole of the Earth's magnet is situated in the Northern hemisphere.

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

40 Q:- A magnetic needle is placed on a large size cork piece floating in a still lake in the Northern hemisphere. Does the needle cork combination drift towards the north of the lake? Why?
Ans:- The magnetic needle is placed on piece is the magnetic dipole capable for free motion in the magnetic field of the earth. At a given place, the Earth's magnetic field is uniform one 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 align itself along the magnetic Meridian.

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