KVPY Sample Paper KVPY Stream-SX Model Paper-24

  • question_answer
    'A charge Q is distributed over three concentric spherical shell of radii a, b, c (a < b < c) such that their surface charge densities are equal to one another. The total potential at a point at distance r from their common centre, where r < a, would be:

    A) \[\frac{Q}{12\pi {{\varepsilon }_{0}}}\frac{ab+bc+ca}{abc}\]

    B) \[\frac{Q({{a}^{2}}+{{b}^{2}}+{{c}^{2}})}{4\pi {{\varepsilon }_{0}}({{a}^{3}}+{{b}^{3}}+{{c}^{3}})}\]

    C) \[\frac{Q}{4\pi {{\varepsilon }_{0}}(a+b+c)}\]

    D) \[\frac{Q(a+b+c)}{4\pi {{\varepsilon }_{0}}({{a}^{2}}+{{b}^{2}}+{{c}^{2}})}\]

    Correct Answer: D

    Solution :

    \[{{Q}_{1}}+{{Q}_{2}}+{{Q}_{3}}=Q....(1)\]
    \[\frac{{{Q}_{1}}}{4\pi {{a}^{2}}}=\frac{{{Q}_{1}}}{4\pi {{b}^{2}}}=\frac{{{Q}_{3}}}{4\pi {{c}^{2}}}=k....(2)\]
    Subs.\[{{Q}_{1}},{{Q}_{2}},{{Q}_{3}}\]in\[(1)\]
    \[k=\frac{Q}{4\pi ({{a}^{2}}+{{b}^{2}}+{{c}^{2}})}\]
    \[v=\frac{k{{Q}_{1}}}{a}+\frac{k{{Q}_{2}}}{b}+\frac{k{{Q}_{3}}}{c}.\]


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