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Electricity and Magnetism Level 3

A solid sphere has a charge \(Q\) distributed in its volume with a charge density \(d=kr^a\) where \(k\) and \(a\) are constants and \(r\) is distance from center. If electric field at \(r=\frac R2\) is \( \frac 18\) times stronger than that of at \(r=R\), find \(a\).

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