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The region between two concentric spheres of radii 'a' and 'b', respectively (see figure), has volume charge density ρ = r A , where A is a constant and r is the distance from the centre. At the centre of the spheres is a point charge Q. The value of A such that the electric field in the region between the spheres will be constant, is a Q b

1. 2 a 2 Q π
2. ) a b ( 2 Q 2 2 − π
3. ) b a ( Q 2 2 2 − π
4. 2 a Q 2 π

Correct answer: 2 a 2 Q π

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