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JEE Main & Advanced Physics Magnetism Question Bank Critical Thinking

  • question_answer
    A bar magnet has coercivity \[4\times {{10}^{3}}\,A{{m}^{-1}}.\] It is desired to demagnetise it by inserting it inside a solenoid 12 cm long and having 60 turns. The current that should be sent through the solenoid is

    A)            2 A

    B)            4 A

    C)            6 A

    D)            8 A

    Correct Answer: D

    Solution :

                       The bar magnet coercivity \[4\times {{10}^{3}}A{{m}^{-1}}\]i.e., it requires a magnetic intensity \[H=4\times {{10}^{3}}A{{m}^{-1}}\]to get demagnetised. Let i be the current carried by solenoid having n number of turns per metre length, then by definition H = ni. Here \[H=4\times {{10}^{3}}\]Amp turn metre-1                    \[n=\frac{N}{l}=\frac{60}{0.12}=500\] turn metre?1            \[\Rightarrow i=\frac{H}{n}=\frac{4\times {{10}^{3}}}{500}=8.0\ A\]


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