NCERT Solutions For Class 12 Physics Chapter 5 Magnetism and Matter

Magnetism and Matter is a conceptual and high-weightage chapter in Class 12 Physics that explains the magnetic properties of materials and the behavior of magnets. This chapter covers key topics such as bar magnet as an equivalent solenoid, magnetic field and field lines, Earth’s magnetism, magnetic susceptibility, permeability, and classification of magnetic materials (dia-, para-, and ferromagnetic). These concepts are frequently tested in CBSE board exams and competitive exams like JEE and NEET.

NCERT Solutions for Class 12 Physics Chapter 5 – Magnetism and Matter are prepared strictly according to the latest CBSE syllabus and exam pattern. The solutions are written in simple, step-by-step language with clear derivations, diagrams, and examples, helping students build strong conceptual clarity and score well in board examinations.

NCERT Solutions For Class 12 Physics Chapter 5 Magnetism and Matter

Magnetism and Matter
Medium
Q.
Answer the following questions regarding earth’s magnetism:
(a) A vector needs three quantities for its specification. Name the three independent quantities conventionally used to specify the earth’s magnetic field.
(b) The angle of dip at a location in southern India is about 18o. Would you expect a greater or smaller dip angle in Britain?
(c) If you made a map of magnetic field lines at Melbourne in Australia, would the lines seem to go into the ground or come out of the ground?
(d) In which direction would a compass free to move in the vertical plane point to, if located right on the geomagnetic north or South Pole?
(e) The earth’s field, it is claimed, roughly approximates the field due to a dipole of magnetic moment 8 × 1022 J T−1 located at its centre. Check the order of magnitude of this number in some way.
(f) Geologists claim that besides the main magnetic N-S poles, there are several local poles on the earth’s surface oriented in different directions. How is such a thing possible at all?
Magnetism and Matter
Medium
Q.
A short bar magnet has a magnetic moment of 0.48 J T−1. Give the direction and magnitude of the magnetic field produced by the magnet at a distance of 10 cm from the centre of the magnet on (a) the axis, (b) the equatorial lines (normal bisector) of the magnet.
Magnetism and Matter
Medium
Q.
A compass needle free to turn in a horizontal plane is placed at the centre of circular coil of 30 turns and radius 12 cm. The coil is in a vertical plane making an angle of 45o with the magnetic meridian. When the current in the coil is 0.35 A, the needle points west to east.
(a) Determine the horizontal component of the earth’s magnetic field at the location.
(b) The current in the coil is reversed, and the coil is rotated about its vertical axis by an angle of 90o in the anticlockwise sense looking from above. Predict the direction of the needle. Take the magnetic declination at the places to be zero.
Magnetism and Matter
Medium
Q.
A long straight horizontal cable carries a current of 2.5 A in the direction 10o south of west to 10° north of east. The magnetic meridian of the place happens to be 10o west of the geographic meridian. The earth’s magnetic field at the location is 0.33 G, and the angle of dip is zero. Locate the line of neutral points (ignore the thickness of the cable).
(At neutral points, magnetic field due to a current-carrying cable is equal and opposite to the horizontal component of earth’s magnetic field.)
Magnetism and Matter
Medium
Q.
Answer the following questions:
(a) Explain qualitatively on the basis of domain picture the irreversibility in the magnetisation curve of a ferromagnet.
(b) The hysteresis loop of a soft iron piece has a much smaller area than that of a carbon steel piece. If the material is to go through repeated cycles of magnetisation, which piece will dissipate greater heat energy?
(c) ‘A system displaying a hysteresis loop such as a ferromagnet, is a device for storing memory?’ Explain the meaning of this statement.
(d) What kind of ferromagnetic material is used for coating magnetic tapes in a cassette player, or for building ‘memory stores’ in a modern computer?
(e) A certain region of space is to be shielded from magnetic fields. Suggest a method.
Magnetism and Matter
Medium
Q.
Answer the following questions:
(a) Why does a paramagnetic sample display greater magnetisation (for the same magnetising field) when cooled?
(b) Why is diamagnetism, in contrast, almost independent of temperature?
(c) If a toroid uses bismuth for its core, will the field in the core be (slightly) greater or (slightly) less than when the core is empty?
(d) Is the permeability of a ferromagnetic material independent of the magnetic field? If not, is it more for lower or higher fields?
(e) Magnetic field lines are always nearly normal to the surface of a ferromagnet at every point. (This fact is analogous to the static electric field lines being normal to the surface of a conductor at every point.) Why?
(f) Would the maximum possible magnetisation of a paramagnetic sample be of the same order of magnitude as the magnetization of a ferromagnet?
Magnetism and Matter
Medium
Q.
A short bar magnet of magnetic moment 5.25 × 10−2 J T−1 is placed with its axis perpendicular to the earth’s field direction. At what distance from the centre of the Magnet, the resultant field is inclined at 45o with earth’s field on (a) its normal bisector and (b) its axis. Magnitude of the earth’s field at the place is given to be 0.42 G. Ignore the length of the magnet in comparison to the distances involved.
Magnetism and Matter
Medium
Q.
If the bar magnet in exercise 5.13 is turned around by 180o, where will the new null points be located?
Magnetism and Matter
Medium
Q.
A short bar magnet placed in a horizontal plane has its axis aligned along the magnetic north-south direction. Null points are found on the axis of the magnet at 14 cm from the centre of the magnet. The earth’s magnetic field at the place is 0.36 G and the angle of dip is zero. What is the total magnetic field on the normal bisector of the magnet at the same distance as the null−point (i.e., 14 cm) from the centre of the magnet? (At null points, field due to a magnet is equal and opposite to the horizontal component of earth’s magnetic field.)
Magnetism and Matter
Medium
Q.
At a certain location in Africa, a compass points 12o west of the geographic north. The north tip of the magnetic needle of a dip circle placed in the plane of magnetic meridian points 60o above the horizontal. The horizontal component of the earth’s field is measured to be 0.16 G. Specify the direction and magnitude of the earth’s field at the location.
Magnetism and Matter
Medium
Q.
Answer the following questions:
(a) The earth’s magnetic field varies from point to point in space. Does it also change with time? If so, on what time scale does it change appreciably?
(b) The earth’s core is known to contain iron. Yet geologists do not regard this as a source of the earth’s magnetism. Why?
(c) The charged currents in the outer conducting regions of the earth’s core are thought to be responsible for earth’s magnetism. What might be the ‘battery’ (i.e., the source of energy) to sustain these currents?
(d) The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth’s field in such distant past?
(e) The earth’s field departs from its dipole shape substantially at large distances (greater than about 30,000 km). What agencies may be responsible for this distortion?
(f) Interstellar space has an extremely weak magnetic field of the order of 10−12 T. Can such a weak field be of any significant consequence? Explain.
[Note: Exercise 5.2 is meant mainly to arouse your curiosity. Answers to some questions above are tentative or unknown. Brief answers wherever possible are given at the end. For details, you should consult a good text on geomagnetism.]
Magnetism and Matter
Medium
Q.
A magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has its north tip pointing down at 22o with the horizontal. The horizontal component of the earth’s magnetic field at the place is known to be 0.35 G. Determine the magnitude of the earth’s magnetic field at the place.
Magnetism and Matter
Medium
Q.
A circular coil of 16 turns and radius 10 cm carrying a current of 0.75 A rests with its plane normal to an external field of magnitude 5.0 × 10−2 T. The coil is free to turn about an axis in its plane perpendicular to the field direction. When the coil is turned slightly and released, it oscillates about its stable equilibrium with a frequency of 2.0 s−1. What is the moment of inertia of the coil about its axis of rotation?
Magnetism and Matter
Medium
Q.
A closely wound solenoid of 2000 turns and area of cross-section 1.6 × 10−4 m2, carrying a current of 4.0 A, is suspended through its centre allowing it to turn in a horizontal plane.
(a) What is the magnetic moment associated with the solenoid?
(b) What is the force and torque on the solenoid if a uniform horizontal magnetic field of 7.5 × 10−2 T is set up at an angle of 30o with the axis of the solenoid?
Magnetism and Matter
Difficult
Q.
A bar magnet of magnetic moment 1.5 JT−1 lies aligned with the direction of a uniform magnetic field of 0.22 T.
(a) What is the amount of work required by an external torque to turn the magnet so as to align its magnetic moment: (i) normal to the field direction, (ii) opposite to the field direction?
(b) What is the torque on the magnet in cases (i) and (ii)?
Magnetism and Matter
Medium
Q.
If the solenoid in Exercise 5.5 is free to turn about the vertical direction and a uniform horizontal magnetic field of 0.25 T is applied, what is the magnitude of torque on the solenoid when its axis makes an angle of 30° with the direction of applied field?
Magnetism and Matter
Medium
Q.
A closely wound solenoid of 800 turns and area of cross section 2.5 × 10−4 m2 carries a current of 3.0 A. Explain the sense in which the solenoid acts like a bar magnet. What is its associated magnetic moment?
Magnetism and Matter
Medium
Q.
A short bar magnet of magnetic moment m = 0.32 J T−1 is placed in a uniform magnetic field of 0.15 T. If the bar is free to rotate in the plane of the field, which orientation would correspond to its
(a) stable, and
(b) unstable equilibrium?
What is the potential energy of the magnet in each case?
Magnetism and Matter
Medium
Q.
A short bar magnet placed with its axis at 30o with a uniform external magnetic field of 0.25 T experiences a torque of magnitude equal to 4.5 × 10−2 J. What is the magnitude of magnetic moment of the magnet?
Magnetism and Matter
Difficult
Q.
The magnetic moment vectors μs and μl associated with the intrinsic spin angular momentum S→ and orbital angular momentum l→, respectively, of an electron are predicted by quantum theory (andverified experimentally to a high accuracy) to be given by:μs= –e/m S→,μl = –e/2ml→Which of these relations is in accordance with the result expected classically?Outline the derivation of the classical result.

NCERT Solutions For Class 12 Physics Chapter 5 Magnetism and Matter

Q. 1) Answer the following questions:

(a) The earth’s magnetic field varies from point to point in space. Does it also change with time? If so, on what time scale does it change appreciably?

(b) The earth’s core is known to contain iron. Yet geologists do not regard this as a source of the earth’s magnetism. Why?

(c) The charged currents in the outer conducting regions of the earth’s core are thought to be responsible for earth’s magnetism. What might be the ‘battery’ (i.e., the source of energy) to sustain these currents?

(d) The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth’s field in such distant past?

(e) The earth’s field departs from its dipole shape substantially at large distances (greater than about 30,000 km). What agencies may be responsible for this distortion?

(f) Interstellar space has an extremely weak magnetic field of the order of 10−12 T. Can such a weak field be of any significant consequence? Explain.

[Note: Exercise 5.2 is meant mainly to arouse your curiosity. Answers to some questions above are tentative or unknown. Brief answers wherever possible are given at the end. For details, you should consult a good text on geomagnetism.]

Ans:

(a) The earth’s magnetic field changes with time. Earth takes a few hundred years to change by an appreciable amount. The change in earth’s magnetic field with the time cannot be neglected.

(b) Earth’s core contains iron in the molten form which is not ferromagnetic. Therefore, it cannot be considered as a source of earth’s magnetism.

(c) The radioactivity in the interior of Earth is the source of energy that sustains the currents in the outer conducting regions of the core of Earth. These charged currents are considered to be responsible for earth’s magnetism, but it is not certain.

(d) Earth reversed the direction of its magnetic field several times during its history of 4 to 5 billion years. These magnetic fields got weakly recorded in certain rocks during their solidification. The analysis of this rock magnetism can reveal the geomagnetic history of Earth.

(e) Earth’s magnetic field departs from its dipole shape substantially at large distances (greater than about 30,000 km) due to the presence of the ionosphere. In this region, earth’s field gets modified due to the magnetic field produced by the motion of single ions.

(f) An extremely weak magnetic field can deflect charged particles moving in a circular path. This may be less noticeable for a large radius path. Therefore, over the gigantic interstellar space, the deflection can affect the passage of charged particles.

Q. 2) A closely wound solenoid of 800 turns and area of cross section 2.5 × 10−4 m2 carries a current of 3.0 A. Explain the sense in which the solenoid acts like a bar magnet. What is its associated magnetic moment?

Ans:
Number of turns in the solenoid, n = 800
Area of cross-section of the solenoid, A = 2.5 × 10−4 m2
Current in the solenoid, I = 3.0 A

A magnetic field develops along the axis of the current-carrying solenoid. Therefore, it behaves like a bar magnet.

Magnetic moment associated with the given current-carrying solenoid is given as,

M = n I A
= 800 × 3 × 2.5 × 10−4
= 0.6 J T−1

Q. 3) Answer the following questions:

(a) Why does a paramagnetic sample display greater magnetisation (for the same magnetising field) when cooled?

(b) Why is diamagnetism, in contrast, almost independent of temperature?

(c) If a toroid uses bismuth for its core, will the field in the core be (slightly) greater or (slightly) less than when the core is empty?

(d) Is the permeability of a ferromagnetic material independent of the magnetic field? If not, is it more for lower or higher fields?

(e) Magnetic field lines are always nearly normal to the surface of a ferromagnet at every point. Why?

(f) Would the maximum possible magnetisation of a paramagnetic sample be of the same order of magnitude as the magnetisation of a ferromagnet?

Ans:

(a) Due to the random thermal motion of molecules, the alignments of dipoles get disrupted at high temperatures. On cooling, this disruption is reduced due to the reduced random thermal motion. That is why, a paramagnetic sample displays greater magnetisation when cooled.

(b) In a diamagnetic substance, the induced dipole moment is always opposite to the magnetising field. Therefore, the random motion of the atoms does not affect the diamagnetism of a material.

(c) As bismuth is a diamagnetic substance, therefore, a toroid with a bismuth core has a magnetic field slightly greater than a toroid whose core is empty.

(d) The permeability of a ferromagnetic material is not independent of the applied magnetic field. From the hysteresis curve it is clear that, μ is greater for a lower field.

(e) The permeability of a ferromagnetic material is always greater than one. That is why the magnetic field lines are always nearly normal to the surface of such materials.

(f) Yes, maximum possible magnetisation of a paramagnetic sample can be of the same order of magnitude as the magnetisation of a ferromagnet. However, this requires very high magnetising fields for saturation, which are hard to achieve.

Q. 4) Answer the following questions:

(a) Explain qualitatively on the basis of domain picture the irreversibility in the magnetisation curve of a ferromagnet.

(b) The hysteresis loop of a soft iron piece has a much smaller area than that of a carbon steel piece. If the material is to go through repeated cycles of magnetisation, which piece will dissipate greater heat energy?

(c) ‘A system displaying a hysteresis loop such as a ferromagnet, is a device for storing memory?’ Explain.

(d) What kind of ferromagnetic material is used for coating magnetic tapes in a cassette player, or for building ‘memory stores’ in a modern computer?

(e) A certain region of space is to be shielded from magnetic fields. Suggest a method.

Ans:

The hysteresis curve (B–H curve) of a ferromagnetic material is shown in the given figure.

(a) Magnetisation persists even when the external field is removed, indicating irreversibility.

(b) Carbon steel piece will dissipate greater heat energy due to larger hysteresis loop area.

(c) Magnetisation depends on the history of magnetisation; hence the system can store memory.

(d) Ferrites such as FeFe2O4, CoFe2O4, MnFe2O4 are used.

(e) Shielding can be achieved by surrounding the region with a soft iron ring.

Note: Q&A containing MathML or Latex or Katex code cannot be rendered in pdf document.

FAQs: Class 12 Physics Chapter 5 – Magnetism and Matter

Q1. Is Magnetism and Matter important for exams?
Yes, it is a high-weightage chapter for CBSE, JEE, and NEET.

Q2. Which topics are most important in this chapter?
Bar magnet as solenoid, Earth’s magnetism, and magnetic materials.

Q3. Are numericals asked from this chapter?
Yes, problems based on magnetic field and magnetisation are common.

Q4. Are derivations important here?
Yes, derivations related to magnetic field of a bar magnet are frequently asked.

Q5. How do NCERT Solutions help?
They provide NCERT-aligned, exam-ready explanations with diagrams and solved questions.