# NCERT solutions for Physics Class 12 chapter 5 - Magnetism and Matter [Latest edition]

## Solutions for Chapter 5: Magnetism and Matter

Below listed, you can find solutions for Chapter 5 of CBSE NCERT for Physics Class 12.

Exercise
Exercise [Pages 200 - 203]

### NCERT solutions for Physics Class 12 Chapter 5 Magnetism and Matter Exercise [Pages 200 - 203]

Exercise | Q 5.1 (a) | Page 200

Answer the following question regarding earth’s magnetism:

A vector needs three quantities for its specification. Name the three independent quantities conventionally used to specify the earth’s magnetic field.

Exercise | Q 5.1 (b) | Page 200

Answer the following question regarding earth’s magnetism:

The angle of dip at a location in southern India is about 18°. Would you expect a greater or smaller dip angle in Britain?

Exercise | Q 5.1 (c) | Page 200

Answer the following question regarding earth’s magnetism:

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?

Exercise | Q 5.1 (d) | Page 200

Answer the following question regarding earth’s magnetism:

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?

Exercise | Q 5.1 (e) | Page 200

Answer the following question regarding earth’s magnetism:

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.

Exercise | Q 5.1 (f) | Page 200

Answer the following question regarding earth’s magnetism:

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?

Exercise | Q 5.2 (a) | Page 200

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?

Exercise | Q 5.2 (b) | Page 200

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

Exercise | Q 5.2 (c) | Page 200

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?

Exercise | Q 5.2 (d) | Page 200

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?

Exercise | Q 5.2 (e) | Page 200

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?

Exercise | Q 5.2 (f) | Page 200

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.

Exercise | Q 5.3 | Page 20

A short bar magnet placed with its axis at 30° 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?

Exercise | Q 5.4 | Page 200

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?

Exercise | Q 5.5 | Page 201

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?

Exercise | Q 5.6 | Page 201

If the solenoid 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?

Exercise | Q 5.7 | Page 201

A bar magnet of magnetic moment 1.5 J T –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)?

Exercise | Q 5.8 | Page 201

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 30° with the axis of the solenoid?

Exercise | Q 5.9 | Page 201

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?

Exercise | Q 5.10 | Page 201

A magnetic needle free to rotate in a vertical plane parallel to the magnetic meridian has its north tip pointing down at 22° 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.

Exercise | Q 5.11 | Page 201

At a certain location in Africa, a compass points 12° west of the geographic north. The north tip of the magnetic needle of a dip circle placed in the plane of magnetic meridian points 60° 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.

Exercise | Q 5.12 | Page 201

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.

Exercise | Q 5.13 | Page 201

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.)

Exercise | Q 5.14 | Page 201

If the bar magnet is turned around by 180°, where will the new null points be located?

Exercise | Q 5.15 | Page 202

A short bar magnet of magnetic moment 5.25 × 10−2 J T1 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 45° 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.

Exercise | Q 5.16 (a) | Page 202

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

Exercise | Q 5.16 (b) | Page 202

Why is diamagnetism, in contrast, almost independent of temperature?

Exercise | Q 5.16 (c) | Page 202

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?

Exercise | Q 5.16 (d) | Page 202

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

Exercise | Q 5.16 (e) | Page 202

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?

Exercise | Q 5.16 (f) | Page 202

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

Exercise | Q 5.17 (a) | Page 202

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

Exercise | Q 5.15 (b) | Page 202

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?

Exercise | Q 5.17 (c) | Page 202

‘A system displaying a hysteresis loop such as a ferromagnet, is a device for storing memory?’ Explain the meaning of this statement.

Exercise | Q 5.17 (d) | Page 202

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

Exercise | Q 5.17 (e) | Page 202

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

Exercise | Q 5.18 | Page 202

A long straight horizontal cable carries a current of 2.5 A in the direction 10° south of west to 10° north of east. The magnetic meridian of the place happens to be 10° 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.)

Exercise | Q 5.19 | Page 202

A telephone cable at a place has four long straight horizontal wires carrying a current of 1.0 A in the same direction east to west. The earth’s magnetic field at the place is 0.39 G, and the angle of dip is 35°. The magnetic declination is nearly zero. What are the resultant magnetic fields at points 4.0 cm below the cable?

Exercise | Q 5.20 | Page 203

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 45° 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 90° in the anticlockwise sense looking from above. Predict the direction of the needle. Take the magnetic declination at the places to be zero.

Exercise | Q 5.21 | Page 203

A magnetic dipole is under the influence of two magnetic fields. The angle between the field directions is 60°, and one of the fields has a magnitude of 1.2 × 10–2 T. If the dipole comes to stable equilibrium at an angle of 15° with this field, what is the magnitude of the other field?

Exercise | Q 5.22 | Page 203

A monoenergetic (18 keV) electron beam initially in the horizontal direction is subjected to a horizontal magnetic field of 0.04 G normal to the initial direction. Estimate the up or down deflection of the beam over a distance of 30 cm (me = 9.11 × 10–31 kg).

Exercise | Q 5.23 | Page 203

A sample of paramagnetic salt contains 2.0 × 1024 atomic dipoles each of dipole moment 1.5 × 10–23 J T–1. The sample is placed under a homogeneous magnetic field of 0.64 T, and cooled to a temperature of 4.2 K. The degree of magnetic saturation achieved is equal to 15%. What is the total dipole moment of the sample for a magnetic field of 0.98 T and a temperature of 2.8 K? (Assume Curie’s law)

Exercise | Q 5.24 | Page 203

A Rowland ring of mean radius 15 cm has 3500 turns of wire wound on a ferromagnetic core of relative permeability 800. What is the magnetic field B in the core for a magnetising current of 1.2 A?

Exercise | Q 5.25 | Page 203

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 (and verified experimentally to a high accuracy) to be given by:

µs = –(e/m) S,

µl = –(e/2m) l

Which of these relations is in accordance with the result expected classically? Outline the derivation of the classical result.

Exercise

## NCERT solutions for Physics Class 12 chapter 5 - Magnetism and Matter

Shaalaa.com has the CBSE Mathematics Physics Class 12 CBSE solutions in a manner that help students grasp basic concepts better and faster. The detailed, step-by-step solutions will help you understand the concepts better and clarify any confusion. NCERT solutions for Mathematics Physics Class 12 CBSE 5 (Magnetism and Matter) include all questions with answers and detailed explanations. This will clear students' doubts about questions and improve their application skills while preparing for board exams.

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Concepts covered in Physics Class 12 chapter 5 Magnetism and Matter are Magnetic Substances, Magnetic Dipole Moment of a Revolving Electron, Current Loop as a Magnetic Dipole and Its Magnetic Dipole Moment, Introduction of Magnetism, Magnetisation and Magnetic Intensity, Magnetism and Gauss’s Law, Magnetic Properties of Materials, The Bar Magnet, Permanent Magnet and Electromagnet, Curie Law of Magnetism, Hysteresis Loop, The Earth’s Magnetism, Torque on a Magnetic Dipole (Bar Magnet) in a Uniform Magnetic Field, Dipole in a Uniform External Field, Magnetic Field Intensity Due to a Magnetic Dipole (Bar Magnet) Perpendicular to Its Axis, Magnetic Field Intensity Due to a Magnetic Dipole (Bar Magnet) Along Its Axis.

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Get the free view of Chapter 5, Magnetism and Matter Physics Class 12 additional questions for Mathematics Physics Class 12 CBSE, and you can use Shaalaa.com to keep it handy for your exam preparation.

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