37: Magnetic Properties Of Matter : Cbse-xi-physics

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CBSE-XI-Physics

37: Magnetic Properties of Matter

with Solutions - page 2

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Qstn #4
A long, straight wire carries a current i. The magnetising field intensity H is measured at a point P close to the wire. A long, cylindrical iron rod is brought close to the wire, so that the point P is at the centre of the rod. The value of H at P will
(a) increase many times
(b) decrease many times
(c) remain almost constant
(d) become zero
digAnsr: c
Ans : (c) remain almost constant
From the Biot-Savart law, magnetic field `` \left(B\right)`` at a point P close to the wire carrying current i is given by,
`` \stackrel{\to }{B}=\frac{{\mu }_{0}}{4\,\mathrm{\,\pi \,}}\frac{i\stackrel{\to }{dl}\times \stackrel{\to }{r}}{{r}^{3}}``
Magnetising field intensity (H) will be,
`` H=\frac{B}{{\mu }_{0}}=\frac{1}{4\,\mathrm{\,\pi \,}}\frac{i\stackrel{\to }{dl}\times \stackrel{\to }{r}}{{r}^{3}}``
Now, as the cylindrical rod is brought close the wire such that centre of the rod is at P, then distance of point P from the wire(r) will remain same. Hence, magnetic field intensity will remain almost constant. Also even when the rod is carrying any current then B will be zero at the centre of the rod so the value of Magnetising field intensity will remain the same at point P.
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Qstn #5
The magnetic susceptibility is negative for
(a) paramagnetic materials only
(b) diamagnetic materials only
(c) ferromagnetic materials only
(d) paramagnetic and ferromagnetic materials
digAnsr: b
Ans : (b) diamagnetic materials only
Magnetic susceptibility is defined as the ratio of the intensity of magnetisation induced in the material to magnetising foorce applied on it.
Magnetic susceptibility is negative for diamagnetic materials. Magnetic susceptibility is positive for paramagnetic and ferromagnetic materials.
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Qstn #6
The desirable properties for making permanent magnets are
(a) high retentivity and high coercive force
(b) high retentivity and low coercive force
(c) low retentivity and high coercive force
(d) low retentivity and low coercive force
digAnsr: a
Ans : (a) high retentivity and high coercive force
Permanent magnets should have high retentivity so that the magnet is strong and high coercive force, so that magnetisation is not erased by stary magnetic fields, temperature change or due to rough handling etc.
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Qstn #7
Electromagnets are made of soft iron because soft iron has
(a) high retentivity and high coercive force
(b) high retentivity and low coercive force
(c) low retentivity and high coercive force
(d) low retentivity and low coercive force
digAnsr: d
Ans : (d) low retentivity and low coercive force
Electromagnets are made of soft iron because soft iron has
(a) low retentivity - When soft iron is placed inside a solenoid to make an electromagnet and current is passed through the solenoid,magnetism of the solenoid is incresed thousand folds. When the current is switched off, the magnetism is removed instantly because of the low retentivity of soft iron.
(a) Low coercivity - it has low coercivity so that area under the hysteresis cureve for soft iron is very small hysterisis loss in case of soft iron is small.
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#
Section : iii

Qstn #1
Pick the correct options.
(a) All electrons have magnetic moment.
(b) All protons have magnetic moment.
(c) All nuclei have magnetic moment.
(d) All atoms have magnetic moment.
digAnsr: a,b
Ans : (a) All electrons have magnetic moment.
(b) All protons have magnetic moment.
Electrons of an atom moves in circular path around the nucleus and constitute electric current. Since a current loop has magnetic moment, therefore electron also has magnetic moment due to this orbit motion. It also has magnetic moment due to the spinning about its own axis.
Orbital motion of electron around the nucleus give rise to magnetic field around the proton (nucleus). This create a torque and thus magnetic dipole moment on proton.
All nuclei also have magnetic moment but it is several thousand times smaller than the magnetic moment of the electron so it can be ignored in comparison to the magnetic moment of an eletron.
Generally, an atom has no magnetic moment. Because the magnetic moments of electrons of an atom have a tendency to cancel in pairs.
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Qstn #2
The permanent magnetic moment of the atoms of a material is not zero. The material
(a) must be paramagnetic
(b) must be diamagnetic
(c) must be ferromagnetic
(d) may be paramagnetic
digAnsr: d,a
Ans : (d) may be paramagnetic
Diamagnetic material have zero magnetic moment on their own. Hence, option
(a) is incorrect.
Paramagnetic and ferromagnetic materials have non zero permanent magnetic moment. But we are not sure about the material, whether it is Paramagnetic or ferromagnetic. If the material has large value of permanent magnetic moment, then it will be ferromagnetic and if it has small value of permanent magnetic moment, then it will be Paramagnetic.
Hence, option
(d) is correct.
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Qstn #3
The permanent magnetic moment of the atoms of a material is zero. The material
(a) must be paramagnetic
(b) must be diamagnetic
(c) must be ferromagnetic
(d) may be paramagnetic
digAnsr: b
Ans : (b) must be diamagnetic
Paramagnetic and ferromagnetic materials have non zero permanent magnetic moment on their own. Only atoms of diamagnetic materials have zero permanent magnetic moment. Hence,
(b) is correct.
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Qstn #4
Which of the following pairs has quantities of the same dimensions?
(a) Magnetic field B and magnetising field intensity H
(b) Magnetic field B and intensity of magnetisation I
(c) Magnetising field intensity H and intensity of magnetisation I
(d) Longitudinal strain and magnetic susceptibility
digAnsr: c,d,a,b
Ans : (c) Magnetising field intensity H and intensity of magnetisation I
(d) Longitudinal strain and magnetic susceptibility
Dimension of Magnetic field B is given by,
F=Bqv
`` \Rightarrow ```` B=\frac{F}{qv}``
`` \Rightarrow ```` \frac{\left[{\,\mathrm{\,MLT\,}}^{-2}\right]}{\left[\,\mathrm{\,AT\,}\right]\left[{\,\mathrm{\,LT\,}}^{-1}\right]}=\left[{\,\mathrm{\,MT\,}}^{-2}{\,\mathrm{\,A\,}}^{-1}\right]``
Dimension of magnetising field intensity H is given by,
H=`` \left[\frac{Idl}{{r}^{2}}\right]``
`` \Rightarrow ```` \frac{\left[\,\mathrm{\,AL\,}\right]}{\left[{\,\mathrm{\,L\,}}^{2}\right]}=\left[{\,\mathrm{\,AL\,}}^{-1}\right]``
Dimension of intensity of magnetisation I is also `` \left[{\,\mathrm{\,AL\,}}^{-1}\right]`` as both Magnetising field intensity H and intensity of magnetisation I are measured in Am^{`` -1``}.
Hence, option
(a) and option
(b) is incorrect. Option
(c) is correct.
As Longitudinal strain and magnetic susceptibility both are dimensionless quantity. Hence, option
(d) is correct.
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Qstn #5
When a ferromagnetic material goes through a hysteresis loop, the magnetic susceptibility
(a) has a fixed value
(b) may be zero
(c) may be infinity
(d) may be negative
digAnsr: b,c,d
Ans : (b) may be zero
(c) may be infinity
(d) may be negative

We know,
`` \,\mathrm{\,Magnetic\,}\,\mathrm{\,susceptibility\,}=\frac{I}{H}``
At point B, the value of H is zero but I is non zero. Magnetic susceptibility is infinity here. At point C, value of magnetic susceptibility will be negative. Here Magnetic field is applied in opposite direction to reduce the intensity of magnetization to zero. The applied field to reduce the residual magnetization to zero is called coercivity. At point D, I is zero but H is not zero so susceptibility of the material will be zero.
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Qstn #6
Mark out the correct options.
(a) Diamagnetism occurs in all materials.
(b) Diamagnetism results from the partial alignment of permanent magnetic moment.
(c) The magnetising field intensity, H, is always zero in free space.
(d) The magnetic field of induced magnetic moment is opposite the applied field.
digAnsr: a,d,b,c
Ans : (a) Diamagnetism occurs in all materials.
(d) The magnetic field of induced magnetic moment is opposite the applied field.
When a material is placed in magnetic field, dipole moment are induced in the atoms by the applied magnetic field. Since the direction of magnetic field due to induced dipole moment is opposite to the applied magnetic field. Therefore, resultant magnetic field is smaller than the applied magnetic field. This process is called diamagnetism. As this process takes place for all the material, therefore all the material exhibit diamagnetism. Hence, option
(a) and
(d) are correct.
Diamagnetic material do not have permanent magnetic moment on their own. When they are placed in magnetic field, dipole moments are induced by the applied magnetic field. Thus, there is no net alignment of permanent magnetic moment so these mterials do not have any permanenet magnetic momentof their own. Hence, option
(b) is incorrect.
Magnetic field intensity is not zero in free space. Hence, option
(c) is incorrect.
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#
Section : iv

Qstn #1
The magnetic intensity H at the centre of a long solenoid carrying a current of 2.0 A, is found to be 1500 A m^-1. Find the number of turns per centimetre of the solenoid.
Ans : Here,
Current in the solenoid, I = 2 A
Magnetic intensity at the centre of long solenoid, H = 1500 Am^-1
Magnetic field produced by a solenoid`` \left(B\right)`` is given by
B = µ₀ni ...(1)
Here, n = number of turns per unit length
i = electric current through the solenoid
Also, the relation between magnetic field strength`` \left(B\right)`` and magnetic intensity`` \left(H\right)`` is given by
H = `` \frac{B}{{\,\mathrm{\,\mu \,}}_{0}}`` ...(2)
From equations (1) and (2), we get:
H = ni
⇒ 1500 A/m = n × 2
⇒ n = 750 turns/meter
⇒ n = 7.5 turns/cm
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Qstn #2
A rod is inserted as the core in the current-carrying solenoid of the previous problem.
Ans : Given:

#2-a
What is the magnetic intensity H at the centre?
Ans : Intensity of magnetisation, H = 1500 A/m
As the solenoid and the rod are long and we are interested in the magnetic intensity at the centre, the end effects may be neglected.
The sole effect of the rod in the magnetic field of the solenoid is that a magnetisation will be induced in the rod depending on rod magnetic properties.
There is no effect of the rod on the magnetic intensity at the centre.