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

  • question_answer
    A compass needle whose magnetic moment is \[60\text{ }A{{m}^{2}}\] pointing geographical north at a certain place where the horizontal component of Earth's magnetic field is \[40\times {{10}^{-6}}Wb{{m}^{-2}}\] experiences a torque of \[1.2\times {{10}^{-3}}\text{ }Nm\]. The declination of the place is:

    A) \[20{}^\circ \]                          

    B)             \[45{}^\circ \]       

    C)             \[~60{}^\circ \]                       

    D)             \[30{}^\circ \]

    Correct Answer: D

    Solution :

    (d) \[30{}^\circ \] In stable equilibrium, a compass needle points along magnetic north and experiences no torque. When it is turned through declination a, it points along geographic north and experiences torque.             \[\tau =MB\,\sin \alpha \] \[\therefore \sin \alpha =\frac{\tau }{MB}=\frac{12\times {{10}^{-3}}}{60\times 40\times {{10}^{-6}}}=\frac{1}{2}\] Or         \[\alpha =30{}^\circ \]


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