KVPY Sample Paper KVPY Stream-SX Model Paper-18

  • question_answer
    A wire of length \[\ell \] and carrying a current i is placed along X-axis in a magnetic field given by \[\overset{\to }{\mathop{B}}\,={{B}_{0}}(\hat{i}+\hat{j}+\hat{k}).\] magnitude of the force acting on the wire is -

    A) \[{{B}_{0}}\,\,i\,\,\ell \]

    B) \[\sqrt{2}\,\,{{B}_{0}}\,\,i\,\,\ell \]          

    C) \[2\,\,{{B}_{0}}\,\,i\,\,\ell \]

    D) \[{{B}_{0}}\,\,i\,\,\ell \,\,/\sqrt{2}\]

    Correct Answer: B

    Solution :

    Magnetic force on wire \[=i\,(\vec{\ell }\times \vec{B})\]
    Where \[\vec{\ell }=\ell \hat{i}\](as wire is lying along x- axis)
    \[\vec{B}=B.(\hat{i}+\hat{j}+\hat{k})\]
    \[\therefore \,\,\,\,\vec{F}=I\vec{\ell }\times \vec{B}\]
    \[\Rightarrow I\ell \,\,(\hat{i})\times {{B}_{0}}\,\,(\hat{i}+\hat{j}+\hat{k})\]
    \[\vec{F}={{B}_{0}}I\ell (\hat{k}-\hat{j})\]
    \[\Rightarrow \vec{F}=\sqrt{2}\,{{B}_{0}}I\ell \]


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