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Let a total charge 2 Q be distributed in a sphere of radius R, with the charge density given by ρ (r) = kr, where r is the distance from the centre. Two charges A and B, of –Q each, are placed on diametrically opposite points, at equal distance, a from the centre. If A and B do not experience any force, then :

1. a = 8 –1/4 R
2. a = 2 –1/4 R
3. a = 4 / 1 2 R 3
4. a = R/ 3

Correct answer: a = 8 –1/4 R

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