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NCERT Class 12 Physics Textbook Chapter 5 With Answer PDF Free Download
Chapter 5: Magnetism and Matter
5.1 Introduction
Magnetic phenomena are universal in nature. Vast, distant galaxies, the tiny invisible atoms, men, and beasts all are permeated through and through with a host of magnetic fields from a variety of sources.
The earth’s magnetism predates human evolution. The word magnet is derived from the name of an island in Greece called magnesia where magnetic ore deposits were found, as early as 600 BC.
Shepherds on this island complained that their wooden shoes (which had nails) at times stayed
stuck to the ground.
Their iron-tipped rods were similarly affected. This attractive property of magnets made it difficult for them to move around.
The directional property of magnets was also known since ancient times. A thin long piece of a magnet, when suspended freely, pointed in the north-south direction.
A similar effect was observed when it was placed on a piece of cork which was then allowed to float in still water.
The name lodestone (or loadstone) given to a naturally occurring ore of iron magnetite means leading stone. The technological exploitation of this property is generally credited to the Chinese.
Chinese texts dating 400 BC mention the use of magnetic needles for navigation on ships. Caravans crossing the Gobi desert also employed magnetic needles.
The Bar Magnet
One of the earliest childhood memories of the famous physicist Albert Einstein was that of a magnet gifted to him by a relative.
Einstein was fascinated and played endlessly with it.
He wondered how the magnet could affect objects such as nails or pins placed away from it and not in any way connected to it by a spring or string.
The magnetic field lines
The pattern of iron filings permits us to plot the magnetic field lines. This is shown both for the bar-magnet and the current-carrying solenoid in Fig. 5.3. For comparison refer to Chapter 1, Figure 1.17(d). Electric field lines of an electric dipole are also displayed in Fig. 5.3(c).
The magnetic field lines are a visual and intuitive realization of the magnetic field. Their properties are:
(i) The magnetic field lines of a magnet (or a solenoid) form continuous closed loops. This is unlike the electric dipole where these field lines begin from a positive charge and end on the negative charge or escape to infinity.
(ii) The tangent to the field line at a given point represents the direction of the net magnetic field B at that point.
(iii) The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field B. In Fig. 5.3(a), B is larger around region ii than in region i .
(iv) The magnetic field lines do not intersect, for if they did, the direction of the magnetic field would not be unique at the point of intersection.
One can plot the magnetic field lines in a variety of ways. One way is to place a small magnetic compass needle at various positions and note its orientation. This gives us an idea of the magnetic field direction at various points in space.
The Earth’s Magnetism
Earlier we have referred to the magnetic field of the earth. The strength of the earth its value being of the order of 10 T.
What causes the earth to have a magnetic field is not clear.
Originally the magnetic field was thought of as arising from a giant bar magnet placed approximately along the axis of rotation of the earth and deep in the interior.
However, this simplistic picture is certainly not correct. The magnetic field is now thought to arise due to electrical currents produced by the convective motion of metallic fluids (consisting mostly of molten iron and nickel) in the outer core of the earth.
This is known as the dynamo effect. The magnetic field lines of the earth resemble that of a (hypothetical) magnetic dipole located at the center of the earth.
The axis of the dipole does not coincide with the axis of rotation of the earth but is presently titled by approximately 11.3º with respect to the latter.
In this way of looking at it, the magnetic poles are located where the magnetic field lines due to the dipole entering or leaving the earth.
The location of the north magnetic pole is at a latitude of 79.74º N and a longitude of 71.8º W, a place somewhere in northern Canada. The magnetic south pole is at 79.74º S, 108.22º E in Antarctica
| Author | NCERT |
| Language | English |
| No. of Pages | 31 |
| PDF Size | 701 KB |
| Category | Physics |
| Source/Credits | ncert.nic.in |
NCERT Solutions Class 12 Physics Chapter 5 Magnetism and Matter
Q 5.3) 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\times 10^{-2}\; J4.5×10−2J. What is the magnitude of the magnetic moment of the magnet?
Answer 5.3:
Magnetic field strength, B = 0.25 T
Torque on the bar magnet, T = 4.5\times 10^{-2}\; J4.5×10−2J
The angle between the bar magnet and the external magnetic field, \theta = 30°θ=30°
Torque is related to magnetic moment (M) as:T = MB sin\thetaT=MBsinθ∴ M = \frac{T}{B sin\theta }∴M=BsinθT
= \frac{4.5\times 10^{-2}}{0.25\times sin 30°} = 0.36\; J\; T^{-1}0.25×sin30°4.5×10−2=0.36JT−1
Hence, the magnetic moment of the magnet is 0.36\; J\; T^{-1}0.36JT−1.
Q 5.4) A short bar magnet of magnetic moment m = 0.32 JT–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?
Answer 5.4:
Moment of the bar magnet, M = 0.32\; J\; T^{-1}0.32JT−1
External magnetic field, B = 0.15 T
(a) The bar magnet is aligned along the magnetic field. This system is considered as being in stable equilibrium. Hence, the angle \thetaθ, between the bar magnet and the magnetic field is 0°0°.
Potential energy of the system = -MB cos\theta−MBcosθ
= -0.32\times 0.15\; cos 0°−0.32×0.15cos0°
= -4.8\times 10^{-2}\; J−4.8×10−2J
Q 5.5) 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?
Answer 5.5:
Number of turns in the solenoid, n = 800
Area of cross-section, A = 2.5\times 10^{-4}\;m^{2}2.5×10−4m2
Current in the solenoid, I = 3.0 A
A current-carrying solenoid behaves like a bar magnet because a magnetic field develops along its axis, i.e., along with its length.
The magnetic moment associated with the given current-carrying solenoid is calculated as:
M = n I A
= 800\times 3\times 2.5 \times 10^{-4}800×3×2.5×10−4
= 0.6\; J\; T^{-1}0.6JT−1
Q 5.6) 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 the torque on the solenoid when its axis makes an angle of 30° with the direction of applied field?
Answer 5.6:
Magnetic field strength, B = 0.25 T
Magnetic moment, M = 0.6 T^{-1}T−1
The angle \thetaθ, between the axis of the solenoid and the direction of the applied field, is 30°30°.
Therefore, the torque acting on the solenoid is given as:\tau = MB sin \thetaτ=MBsinθ
= 0.6\times 0.25\; sin 30°0.6×0.25sin30°
= 7.5\times 10^{-2}7.5×10−2 J
NCERT Class 12 Physics Textbook Chapter 5 With Answer PDF Free Download