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CBSE-XI-Physics

37: Magnetic Properties of Matter

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  • #
    Section : i
  • Qstn #1
    When a dielectric is placed in an electric field, it gets polarised. The electric field in a polarised material is less than the applied field. When a paramagnetic substance is kept in a magnetic field, the field in the substance is more than the applied field. Explain the reason of this opposite behaviour.
    Ans : This opposite behaviour is due to the opposite behaviour of magnetic dipole as compared to electric dipole. When the paramagnetic substance is kept in magnetic field, the direction of magnetic field at the centre of magnetic dipole of substance is along the direction of magnetic moment which is opposite to the case of dipole in electric field. Also, when paramagnetic substance is kept in the magnetic field then its magnetic dipole aligns in the direction of field. Thus, magnetic field due to the magnetic dipole adds up to the applied magnetic field.
    Hence, an extra magnetic field produced in the direction of applied field.
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  • Qstn #2
    The property of diamagnetism is said to be present in all materials. Then, why are some materials paramagnetic or ferromagnetic?
    Ans : When a diamagnetic material is placed in magnetic field, dipole moment are induced in its atoms by the applied magnetic field. The direction of magnetic field due to induced dipole moment is opposite to the applied magnetic field therefore the 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. However, some material consists of atoms having some magnetic moment on their own (without applying magnetic field). As a result of it, when they are placed in magnetic field, they aligns their atomic dipole in the direction of applied magnetic field and hence their resultant magnetic field is more then the applied magnetic field and exhibit paramagnetism or ferromagnetism.
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  • Qstn #3
    Do permeability and relative permeability have the same dimensions?
    Ans : Magnetic permeability (`` \mu ``) is the ratio of magnetic flux density (B) to the magnetising field strength (H).
    `` \mu =\frac{B}{H}``
    In CGS ( centimeter-gram-second) dimension of B and H is same. Hence, magnetic permeability is dimensionless. But in SI unit, dimension of ​B and H is not same. Thus, permeability is not dimensionless.
    Relative ​permeability is defined as the ratio of magnetic permeabity of any medium to the permeability of the vaccum. Hence, it is dimensionless. Thus, permeability and relative permeability have the same dimensions in CGS system.
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  • Qstn #4
    A rod, when suspended in a magnetic field, stays in the east-west direction. Can we be sure that the field is in the east-west direction? Can it be in the north-south direction?
    Ans : No, it depends on the nature of rod. As we know that when the diamagnetic substance is suspended in a uniform field they set their longer axis right angles to the direction of magnetic field. So, if the material of the rod will be diamagnetic then it will stay in the east-west direction in perpendicular magnetic field ( i.e. along north-south direction). But if the material of the rod is paramagnetic or ferromagnetic it will stay in east-west direction having magnetic field in east-west direction.
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  • Qstn #5
    Why is it not possible to make permanent magnets from paramagnetic materials?
    Ans : Permanent magnets are made from the material that are easily magnetized and retain the magnetization even reverse magnetizing field is applied (high coercivity). Paramagnetic materials get small magnetization, if they are placed in magnetic field, they lose their magnetization easily if the reverse field is applied. Hence, they are not used to make permanent magnet.
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  • Qstn #6
    Can we have magnetic hysteresis in paramagnetic or diamagnetic substances?
    Ans : No, magnetic hysteresis is the lagging of intensity of magnetization (I) behind magnetising force (H). When diamagnetic and paramagnetic materials are placed in a magnetic field they get weekly magnetised. Also, they lose their magnetization as the magnetic field is removed (low retentively). Therefore, they do not form magnetic hysteresis curve.
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  • Qstn #7
    When a ferromagnetic material goes through a hysteresis loop, its thermal energy is increased. Where does this energy come from?
    Ans : When a ferromagnetic material is taken through the cycle of magnetisation, magnet dipoles of the material orient and reorient with time. This molecular motion within the material results in the production of heat, which increses thermal energy of material.
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  • Qstn #8
    What are the advantages of using soft iron as a core, instead of steel, in the coils of galvanometers?
    Ans : The material used as a core in the moving coil galvanometer undergoes cycle of magnetization for long period. Therefore, low hysterisis loss is the first requirement for such material. In soft ron core, area under the hysteresis curve is small thus loss of energy is less as compared to steel. Further, it is easily magnetized by the magnetizing field, which increase the magnetic field and hence sensitivity of galvanometer.
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  • Qstn #9
    To keep valuable instruments away from the earth’s magnetic field, they are enclosed in iron boxes. Explain.
    Ans : As we know that iron have high permeability, therefore it will provide easy path for the magnetic field lines to pass. As a result of this, all the magnetic field lines of earth's magnetic field will prefer to pass through the wall of the box making magnetic field inside the box zero. Hence, it will keep the valuable instruments away from the earth's magnetic field.
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  • #
    Section : ii
  • Qstn #1
    A paramagnetic material is placed in a magnetic field. Consider the following statements:
    (A) If the magnetic field is increased, the magnetisation is increased.
    (B) If the temperature is increased, the magnetisation is increased.
    (a) A and B are true.
    (b) A is true but B is false.
    (c) B is true but A is false.
    (d) A and B are false.
    digAnsr:   b
    Ans : (b) A is true but B is false.
    if the magnetic field is increased, magnetisation of paramagnetic material placed in magnetic field is also increases. Hence, option
    (a) is correct.
    Magnetization (I) is given by,
    `` \stackrel{\to }{I}=\frac{\stackrel{\to }{M}}{V}``
    As the temperature is increased, magnetic moments of paramagnetic material becomes more randomly aligned due to incresed thermal motion. This leads decrease in the magnetization I. Hence, option
    (b) is incorrect.
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  • Qstn #2
    A paramagnetic material is kept in a magnetic field. The field is increased till the magnetisation becomes constant. If the temperature is now decreased, the magnetisation
    (a) will increase
    (b) will decrease
    (c) will remain constant
    (d) may increase or decrease
    digAnsr:   c
    Ans : (c) will remain constant
    Magnetisation becomes constant i.e all the magnetic moments have got aligned in the direction of the applied field. So now, if the temperature is decreased, thermal vibration of the paramagnetic material will reduce, But as all the magnetic moments are already aligned in the direction of the field so no further alignment can take place due to reduced thermal motion. Thus, there will be no negative effect of decreasing the temperature on the magnetisation. Hence correct option is
    (c).
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  • Qstn #3
    A ferromagnetic material is placed in an external magnetic field. The magnetic domains
    (a) increase in size
    (b) decrease in size
    (c) may increase or decrease in size
    (d) have no relation with the field
    digAnsr:   c
    Ans : (c) may increase or decrease in size
    Atoms of ferromagnetic material in unmagnetised state form domains inside the ferromagnetic material. These domains have large magnetic moments due to the magnetic moment of atoms. In the absence of magnetic field, these domains have magnetic moment in different directions. But when the magnetic field is applied, domains aligned in the direction of the field grow in size and those aligned in the direction opposite to the field reduce in size. Hence, option
    (c) is correct.
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