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Magnetic Materials & Magnets Test - 1

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Magnetic Materials & Magnets Test - 1
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  • Question 1
    1 / -0.25
    Magnetism at the center of a bar magnet is ______?
    Solution

    CONCEPT:

    • Magnetism: It is a class of physical phenomena that are mediated by magnetic fields.
      • Electric currents and the magnetic moments of elementary particles give to a rise a magnetic field, which acts on other currents and magnetic moments.
    • Bar-Magnet: A bar magnet is a rectangular piece of an object, made up of iron, steel or any other ferromagnetic substance or ferromagnetic composite, that shows permanent magnetic properties.
      • It has two poles, a north and a south pole such that when suspended freely, the magnet aligns itself so that the northern pole points towards the magnetic north pole of the earth.
    • Uses of Bar Magnet. Bar magnets are used as stirrers in the laboratory for magnetic experiments.
      • Electronic devices such as telephones, radios, and television sets use magnets.

    CALCULATION:

    • If we consider a bar magnet of length l, then the magnetic moment is M = 2m × I

    Where m is the magnitude of poles.

    • Considering the distance r from the center of the bar magnet, we can write magnetic field intensity as a product of magnetic field strength and positional feature.
    • Therefore, the field intensity at a distance r is given by (r- l2)2.

    The magnetic field of a bar magnet at an axial point is given by

    \(B = \frac{{{\mu _o}}}{{4\pi }}\frac{{2Mr}}{{{{\left( {{r^2} - {l^2}} \right)}^2}}}\)

    At the centre of the magnet, r = 0

    So \(B = \frac{{{\mu _o}}}{{4\pi }}\frac{{2Mr}}{{{{\left( {{r^2} - {l^2}} \right)}^2}}} = \frac{{{\mu _o}}}{{4\pi }}\frac{{2M \times 0}}{{{{\left( {{r^2} - {l^2}} \right)}^2}}} = 0 \) 

    Hence option 3 is correct.

  • Question 2
    1 / -0.25
    For ferromagnetic materials magnetic susceptibility (χ) is ________.
    Solution

    Concept:

    • The measurement of how much a magnetic material will get magnetized in a magnetic field is called susceptibility.
    • It is denoted by χ.
    • Magnetic susceptibility is the ratio of magnetization and the applied magnetizing field intensity. 
    • Magnetic Susceptibility (χ) = Magnetization/magnetizing field intensity
    • The material which exhibits strong magnetism in a magnetic field in the direction of the magnetic field is called a ferromagnetic substance.
    • The magnetic materials which are weekly attracted by a magnet are called a paramagnetic substance.
    • The magnetic material which is repelled by a magnet is called diamagnetic material.

     

    Explanation:

    • As ferromagnetic materials exhibit strong magnetism in a magnetic field. So the susceptibility of ferromagnetic is much greater than 1.
    • Hence option 1 is correct among all

     

    Extra Points:

    These are susceptibility of different materials

    χ ≫ 1 for ferromagnetic substance

    χ > 0 for paramagnetic substance

    χ < 0 for diamagnetic substance
  • Question 3
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    Which of the following will be suitable to demagnetize a magnet?
    Solution

    The correct answer is Hammering of Magnet.

    • The magnet can de be demagnetised by:
      • heating past the Curie point,
      • applying a strong magnetic field,
      • applying alternating current, or
      • hammering the metal. 

    Additional Information

    • With the help of magnetic susceptibility, magnetic materials can be classified as:
    • Paramagnetic material:
      • Magnetic materials that align with the magnetic field are known as paramagnetic materials.
      • Magnetic susceptibility is χ > 0 which means it is always a small positive value for paramagnetic materials.
      • For example Alkaline earth metal, aluminium, oxygen.
    • Diamagnetic material:
      • Magnetic materials that align against the magnetic field are known as diamagnetic materials.
      • Magnetic susceptibility is χ < 0 which means it is always a negative value for diamagnetic material.
      • For example Gold, tin, mercury, water.
    • Ferromagnetic material:
      • Magnetic materials that are highly magnetized in a magnetic field are known as ferromagnetic materials.
      • These are highly attracted by the magnets and move from weaker fields to stronger fields.
      • For example Iron, cobalt, nickel and their alloys.

     

  • Question 4
    1 / -0.25

    Choose the incorrect statement:

    Solution

    Concept:

    Magnet: It is defined as a material that can produce its own magnetic field. There are two types of magnets,

    • Permanent magnet
    • Temporary magnet

    Permanent magnet: These magnets do not lose their magnetic property once they are magnetized. For examples alnico, samarium cobalt, ferrite.

    Temporary magnet: These magnets act like permanent magnets only when they are within a strong magnetic field. It is made up of soft iron. for example electromagnet.

    Alnico:

    • The alloy is permanent magnets that are primarily made up of a combination of aluminum, nickel, and cobalt but can also include copper, iron, and titanium.
    • It can be easily magnetized in an external magnetic field.
    • Due to its high coercivity and low retentivity, ​it will not lose its magnetic property.
    • It has excellent temperature stability.

    Explanation :

    • The magnets lose their properties on heating, hammering, and dropping from some height.
    • On heatingOn dropping, and On hammering-  the magnetic domains of the magnet which were aligned in the same direction gain energy due to heat and lose their orientation, thus becoming randomized.
    • If this heat energy is given for a long time then the magnet loses all its magnetic properties.
    • The strength of the field keeps the magnets strong and magnetic domains in alignment.
    • The strength of the pole and the distance between them are related by inverse square law. When we keep them in a pair (with unlike pairs on the same side) we are reducing the distance which in turn keeps the magnets strong.

    Hence, the incorrect statement is Nickel and Cobalt do not get attracted by magnets. Additional Information

    • Metals that are attracted to the metals are called ferromagnetic; these magnets firmly stick to these metals.
    • For example, iron, cobalt, steel, nickel, manganese, gadolinium, and lodestone are all ferromagnetic metals
  • Question 5
    1 / -0.25
    The electric current of an electromagnet is switched off then the magnetic property of the electromagnet will-
    Solution

    CONCEPT:

    • An electromagnet is a temporary magnet which should ideally have the property to behave as a magnet when current passes through it and lose magnetism as soon as current is stopped.
    • Soft iron is generally used for making electromagnets because it has high magnetic permeability, i.e. it can easily gain magnetic properties when current is passed around the core and quickly lose when current is stopped.
    • The soft iron inside the coil makes the magnetic field stronger because it becomes a magnet itself when the current is flowing

     

    EXPLANATION:

    • As the electromagnet is a temporary magnetic which is only working till when we give the current to it.
    • Once the electric current is stopped the magnetic property of the electromagnet vanished.
    • But there is a very small time duration, in which the magnetic property of the electromagnet exists after stopping the electric current in it. This is due to electromagnetic induction in the coil of the electromagnet. Hence option 1 is correct.
  • Question 6
    1 / -0.25
     When does a ferromagnetic material become a paramagnetic material?
    Solution

    CONCEPT:

    • Curie temperature: Curie temperature is the temperature at which the magnetic properties of a material change.
      • When the temperature is greater than the Curie temperature, ferromagnetic material becomes paramagnetic material.

     

    • Ferromagnetic material: Ferromagnetic materials have a large, positive susceptibility to an external magnetic field. They exhibit a strong attraction to magnetic fields and can retain their magnetic properties after the external field has been removed. For examples Iron, nickel, and cobalt.
    • Paramagnetic material: Paramagnetic materials have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. For examples magnesium, molybdenum, lithium.

     

    EXPLANATION:

    • Curie temperature is the temperature at which the magnetic properties of a material change. When the temperature is greater than Curie temperature, ferromagnetic material becomes paramagnetic material.

    So option 3 is correct.

  • Question 7
    1 / -0.25

    Which of the following material when placed in an external magnetic field behaves as shown in the figure?

    Solution

    The correct answer is option 2) i.e. Diamagnetic

    CONCEPT:

    • Diamagnetic substance: Diamagnetic substances are those that are capable of weakly getting magnetized when placed in an external magnetic field.
      • The magnetization takes place in a direction that is opposite to the external magnetic field

    • When an external magnetic field is applied, the dipoles are induced in the diamagnetic materials that arrange themselves in such a way that they start opposing the external magnetic field.
    • The atoms of diamagnetic substances have paired electrons.
    • A few examples of diamagnetic substances are Copper, Zinc, Bismuth, Silver, and Gold.


    EXPLANATION:

    • Diamagnetic materials have the property of opposing the external magnetic field and do not allow it to pass through them.
    • The given figure shows a diamagnetic specimen placed in an external magnetic field.

    Additional Information

    • Paramagnetic substances: Paramagnetic substances are those substances that get weakly magnetized with the influence of an external magnetic field.
    • Ferromagnetic materialsFerromagnetic substances are those substances that get strongly magnetized when placed in an external magnetic field.
  • Question 8
    1 / -0.25
    Curie – Weiss law is:
    Solution

    Explanation:

    Curie - Weiss law:

    According to Curie - Weiss law, the susceptibility of a ferromagnetic material decreases above a certain temperature called curie temperature and it becomes paramagnetic in nature.

    The susceptibility is given by

    \(\Rightarrow \chi_m =\frac{C}{T-\theta}\)

    Where,

    C = Curie constant,

    TC or θ = Curie temperature, 

    T = Temperature

  • Question 9
    1 / -0.25
    For paramagnetic materials magnetic susceptibility is related with temperature as:
    Solution

    CONCEPT:

    • The measurement of how much a magnetic material will be magnetized in a given magnetic field is called as magnetic susceptibility.
    • It is denoted by χ.
    • Magnetic susceptibility is the ratio of magnetization and the applied magnetizing field intensity.

    Magnetic Susceptibility (χ) = Magnetization/magnetizing field intensity

    Diamagnetic SubstanceParamagnetic substancesFerromagnetic substances
    Diamagnetic substances are those which develop feeble magnetization in the opposite direction of the magnetizing field.Paramagnetic substances are those which develop feeble magnetization in the direction of the magnetizing field.Ferromagnetic substances are those which develop strong magnetization in the direction of the magnetizing field.
    Such substances are feebly repelled by magnets and tend to move from stronger to weaker parts of a magnetic field.Such substances are feebly attracted by magnets and tend to move from weaker to stronger parts of a magnetic field.They are strongly attracted by a magnet and tend to move from weaker to the stronger part of a magnetic field.
    Magnetic susceptibility is small and negative i.e. -1 ≤  χ ≤ 0.Magnetic susceptibility is small and positive i.e.  χ > 0Magnetic susceptibility is very large and positive i.e. χ > 1000
    Examples: Bismuth, copper, lead, zinc, etc.Example: Manganese, aluminum, chromium, platinum, etc.Example: Iron, cobalt, nickel, gadolinium, and alloys like alnico

    EXPLANATION:

    • As we know, for paramagnetic materials, magnetic susceptibility is inversely proportional to the temperature i.e.,

    \(\Rightarrow \chi \propto\frac{1}{T}\propto T^{-1}\)

  • Question 10
    1 / -0.25
    In a permanent magnet at room temperature
    Solution

    Concept:

    • Permanent magnets are magnets that retain their magnetism over long periods of time. Permanent magnets are made of hard ferromagnetic materials.
    • In a paramagnetic material, every individual atom (or ions or molecules) possesses a dipole moment.
    • And this magnetic moment of every atom interacts with one another in such a way that they spontaneously align themselves in a common direction over a macroscopic volume which is termed as the domain.
    • The magnetic strength of a magnetic material is inversely proportional to temperature, since as temperature increases because of thermal agitation orientation of atoms becomes random when leading to a decrease in its magnetic field, and after certain temperature magnetic material loses its magnetic property permanently.
    • Also, Temporary magnets are those which act like a permanent magnet when they are within a strong magnetic field but lose their magnetism when the magnetic field disappears. E.g. Electromagnet.

     

    Explanation:

    • Hence for a permanent magnet at room temperature, all domains can’t be perfectly aligned because of thermal agitation,
    • This means although it will be completely magnetized domains of a permanent magnet at room temperature will be partially aligned
    • Therefore option 3 is correct 
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