NCERT Solution Physics Class 12 Chapter 5 Magnetism and Matter
NCERT Solution Physics Class 12 Chapter 5 Magnetism and Matter all questions and answers. Physics Class 12 5th Chapter Magnetism and Matter exercise solution and experts answer. As one of online learning platforms, we (netex.) are excited to offer the NCERT Solution Physics Class 12 Chapter 5. This solution is designed to help students who are looking to brush up on their physics concepts on Chapter 5 Magnetism and Matter.
Chapter 5 Magnetism And Matter
5.1 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 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?
ANSWER-
(a) Following are three quantities used to specify the earth’s magnetic field are magnetic inclination, magnetic declination and horizontal and vertical components of magnetic field.
(b) As we know that Britain is more close to north pole than southern India. And angle of dip represents the position of the point from the North Pole or South Pole hence for Britain angle of dip is more than 18°.
(c) To answer this question one should know the assumption related to earth’s magnetic field. It is assumed that earth is a big bar magnet with magnetic north near geographic south and vice versa. And magnetic lines are always originating from magnetic north and going to magnetic south. So if we consider Melbourne Australia which is located in geographic south region (i.e. magnetic north region) hence the magnetic lines seem to come out of ground.
(d) when compass is located on the geometric north or south pole the magnetic field is in vertical direction and horizontal component is same everywhere hence the compass will point out exactly vertical as angle of dip at pole is 900.
(e) given the magnetic moment
M = 8 × 1022 JT–1
Magnetic field strength is given by
(f) Various minerals which are having magnetic properties may be deposited on various places on earth which leads to the several local poles on the earth’s surface oriented in different directions.
5.2 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.
ANSWER-
(a)The earth’s magnetic field varies from point to point with respect to space along with this it also change with time. But therate of change field with respect to time is not known to the exact value but we can say it change noticeably with few hundred years of time passage.
(b) The source of magnetism is the ferromagnet. But iron present in the core is not ferromagnetic in nature hence geologists do not consider this as a source of the earth’s magnetism.
(c)The charged currents in the outer conducting regions of the earth’s core have source resides in the core which is radioactivity.
(d) rocks present on earth have experienced magnetic changes over the years during their solidification processes hence by analyzing the rocks geologists concluded The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years.
(e) Inearth’s atmosphere there is one layer called ionosphere where different ions are present in motion which produces their own magnetic field and modify the field hence The earth’s field departs from its dipole shape substantially at large distances.
(f) Such a small magnitude of magnetic field (10–12 T) will not affect motion of large charged particles over large distance but it can only affect the motion of some small charged particles.
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5.16 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 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 magnetisation of a ferromagnet?
ANSWER-
(a) Magnetism in the material is due to sum of many small dipole moments. If these dipole moments are randomly oriented then some may cancel dipole moment of other and hence net dipole moment will be less. If these dipole moments are aligned in particular direction this will add to the net dipole moment and increases the magnetism. When paramagnetic sample is cooled this settles down the random motion of the ions and hence aligned more dipole moments in same direction results in greater magnetisation.
(b) In diamagnetic materials all dipole moments cancels each other and hence the magnetic field is zero. Hence it is independent of the temperature hence diamagnetism is independent of temperature.
(c) Field in the core of toroid depends upon the material present in the core in the order of ferromagnetic > paramagnetic> diamagnetic> empty space. Hence when bismuth which is diamagnetic material is placed in the core, the field in the core be greater than when the core is empty.
(d) The permeability of a ferromagnetic material inversely related to magnetic field. The smaller the field the higher the permeability and vice versa.
(e) Magnetic field lines are always normal to the surface having permeability greater than one. We all know that the permeability for ferromagnetic material is greater than one hence field lines are normal to these materials.
(f) The maximum possible magnetisation of a paramagnetic sample be of the same order of magnitude as the magnetisation of a ferromagnet.
5.17 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 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.
ANSWER-
(a) Given figure shows the magnetic hysteresis loop i.e. the curve of magnetic field in ferromagnetic material(B) vs applied field intensity (H). From graph when H =0 we see that B ¹0 Means even though external field is removed then also ferromagnetic material will have magnetisation this is the reversibility of ferromagnetic phenomenon.
(b) We know that area under hysteresis loop represents the energy lost in the form of heat .as carbon steel piece has more area hence heat dissipation will be more in case of carbon steel piece than soft iron piece.
(c) We know that hysteresis loop is the magnetic history of the sample hence It stores the information related to history of cycles performed on magnet hence can be used as storage medium.
(d) Ceramic as a ferromagnetic material is used for coating magnetic tapes in cassette players and for building memory stores in modern computers.
(e) For shielding of any region from magnetic field one can surround that area by soft iron rods or shield. As soft iron has property that magnetic lines are get concentrated inside the soft iron rod that will prevent the area of concern from magnetic lines.
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5.25 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.
ANSWER-
Second equation is in accordance with classical theory as follows.
To find magnetic moment associated with thee angular momentum is given by,
Magnetic moment is given by,