# NCERT Solutions for class 12th Physics Chapter 5 Magnetism and Matter

#### Exercise

Question 1. 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 18°. 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 togo 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 x 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?

Sol. (a) The three quantities are

1. magnetic declination
2. angle of dip and
3. horizontal component of earths magnetic field.

(b) Since Britian is closer to the magnetic north pole hence angle of dip would be greater.
(c) Since Australia is closer to magnetic south pole hence magnetic field lines would come out of the ground.
(d) At the geomagnetic poles the direction of magnetic field would be in the vertical direction hence a magnetic needle moving in a horizontal plane would point in any direction.
(e) Using formula for field at an equatorial point of a magnetic dipole

Note: Refer Chapter at a Glance (4)

Question 2. 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.]

Sol.

• (a) Yes, due to the motion of its plates and core the earth’s magnetic field does shift. Noticeable changes occur over a large period of time (few hundred years) and variations are there even in small durations of time (few years) which cannot be ignored.
• (b) The core contains molten iron (magnetic domains are destroyed in molten form) which is not ferromagnetic hence it cannot contribute to magnetism of earth.
• (c) One possibility is the radioactivity in the intenior of the earth.
• (d) By study rock magnetism i.e., the solidification of rocks leads to trapping of materials in their magnetised states. Analysing these we can get clues to the history of geomagnetism.
• (e) At large distances external factors such as solar winds, ionospheric particles etc. influences the earths magnetic field, hence it gets distorted.
• (f) From cyclotron relation r = mv/e.B we can say that small field can affect the path of moving charged particle whose radius is large eg. cosmic rays.

Question 3. 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 x 10-2 J. What is the magnitude of magnetic moment of the magnet?

Question 4. A short bar magnet of magnetic moment m = 032 .rr-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?

Question 5. A closely wound solenoid of 800 turns and an area of cross section 2.5 x 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?

Question 6. 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?

Question 7. A bar magnet of magnetic moment 1.5 J11 lies aligned with the direction of a uniform magnetic field of 0.22T. 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? Also find the torque on the magnet in two cases (i) and (ii).

Question 8. A closely wound solenoid of 2000 turns and area of cross-section 1.6 x 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 x 10-2T is set up at an angle of 30° with the axis of the solenoid?

Question 9. 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 x 1o– 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?

Note: Refer Chapter at a Glance (10)

Question 10. 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.

Question 11. 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.

Question 12. 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.

Question 13. 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).

Question 14. If the bar magnet in exercise 5.13 is turned around by 180°, where will the new null points be located?

Question 15. A short bar magnet of magnetic moment 5.25 x 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 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.

Note: Refer Chapter at a Glance (4)

Question 16. 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 used 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? Ifnot, 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 magnetisation of a ferromagnetic ?

Sol. (a) In paramagnetics, the tendency to disrupt the alignment of molecular dipoles with the external magnetising field arising from random thermal motion is reduced at lower temperatures.
(b) In diamagnetics, the molecular dipole moments always align in direction opposite to that of external magnetising field, inspite of the internal motion of atoms.
(c) As bismuth is diamagnetic, so the field in the toroid with bismuth core will be slightly less than when the core is empty.
(d) No, the permeability of a ferromagnetic material is not independent of the magnetic field. It is more at higher fields.
(e) As the magnetic permeabilityµ of a ferromagnetic is much larger than unity i.e. µ >>1, so magnetic field lines are always nearly normal to the surface of a ferromagnetic at every point.
(f) Yes, but for the maximum possible magnetisation of paramagnetic sample impractically very high magnetising fields are required.

Question 17. 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 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 modern computer?
(e) A certain region of space is to be shielded from magnetic fields. Suggest method.

Sol. (a) In a specimen of a ferromagnetic, the atomic dipoles are grouped together in domains. All the dipoles of a domain are aligned in the same direction and have net magnetic moment. In an unmagnetised substance these domains are randomly distributed so that the resultant magnetisation is zero. When the substance is placed in an external magnetic field, these domains align themselves in the direction of the field. Some energy is spent in the process of alignment when the external field is removed, these domains do nor come back into their random positions completely. The substance retains some magnetisation. The energy spent in the process of magnetisation is not fully recovered. The balance of energy is lost as heat. This is the basic cause for irreversibility of the magnetisation curve of a ferromagnetic substance.
(b) Carbon-Steel piece, because the heat produced in complete cycle of mangetisation is directly proportional to the area under the hysteresis loop.
(c) Magnetisation of a ferromagnet is not a single valued function of the magnetising field. Its value for a particular field depends both on the magnetising field and on the history of its magnetisation i.e. how many cycles of magnetisation it has gone through etc. So, the value of magnetisation is a record or memory of its cycles if magnetisation. If information bits can be made to correspond to these cycles, the system displaying such a hysteresis loop can act as a device for storing information.
(d) Ferrites or ceramics which is specially treated barium iron oxides.
(e) By surrounding the region with soft iron rings, as magnetic field lines will be drawn into the rings and the enclosed space becomes free of magnetic field.

Question 18. 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 magnetic field.)

Question 19. A telephone cable at a place has four long straight horizontal wire 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 point 4.0 cm below the cable?

Question 20. 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.

Question 21. 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 x 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?

Question 22. 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 of30 cm (me= 9.11 x 10-19 C). [Note: Data in this exercise are so chosen that the answer will give you an idea of the effect of earth’s magnetic field on the motion of the electron beam from the electron gun to the screen in a TV set]

Note: If the velocity of charged particle is perpendicular to the magnetic field, it performs circular motion.

Question 23. A sample of paramagnetic salt contains 2.0 x 1024 atomic dipoles each of dipole moment 1.5 x 10-23 JT-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)

Question 24. 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?

Question 25. The magnetic moment vectors µs and µ1 associated with the intrinsic spin angular momentum s and orbital angular momentum l respectively, of an electron are predicted by quantum theory (and rarified experimentally to a high accuracy) to be given by

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