A solid sphere of radius $R$ has a charge $Q$ distributed in its volume with a charge density $\rho=\kappa r^a$, where $\kappa$ and $a$ are constants and $r$ is the distance from its centre. If the electric field at $r=\frac{R}{2}$ is $\frac{1}{8}$ times that at $r=R$, find the value of $a$.

  • [IIT 2009]
  • A

    $1$

  • B

    $2$

  • C

    $3$

  • D

    $4$

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  • [KVPY 2011]

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  • [NEET 2020]

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