12th Class Physics Magnetism Question Bank MCQ - Magnetism and Matter

  • question_answer
    A domain in ferromagnetic iron in the form of cube is having \[5\times {{10}^{10}}\] atoms. If the side length of this domain is \[1.5\mu m\] and each atom has a dipole moment of \[8\times {{10}^{-24}}A{{m}^{2}}\], then magnetisation of domain is:

    A) \[118\times {{10}^{5}}A{{m}^{-1}}\]            

    B)             \[118\times {{10}^{4}}A{{m}^{-1}}\]

    C)             \[235\times {{10}^{4}}A{{m}^{-1}}\]

    D)             \[118\times {{10}^{5}}A{{m}^{-1}}\]

    Correct Answer: D

    Solution :

    (d) \[118\times {{10}^{5}}A{{m}^{-1}}\] The volume of the cubic domain is                             \[V={{(15\times {{10}^{-6}}m)}^{3}}\]             \[=3.38\times {{10}^{-18}}{{m}^{3}}=3.38\times {{10}^{-12}}c{{m}^{3}}\] Number of atoms in domain \[(N)=5\times {{10}^{10}}\]atoms         Since each iron atom has a dipole moment (M)           \[=8\times {{10}^{-24}}A{{m}^{2}}\] \[{{M}_{\max }}\]= total number of dipole moment for all atoms                 \[=N\times M=5\times {{10}^{10}}\times 8\times {{10}^{-24}}\]             \[=40\times {{10}^{-14}}=4\times {{10}^{-13}}A{{m}^{2}}\] Now the consequent magnetisation  \[M{{'}_{\max }}=\frac{{{M}_{\max }}}{Domain\,\,volume}\] \[=\frac{4\times {{10}^{-13}}A{{m}^{2}}}{3.38\times {{10}^{-18}}{{m}^{2}}}=118\times {{10}^{5}}A{{m}^{-1}}\]


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