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A charge Q is distributed over three concentric spherical shells 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 -

1. ) c b a
2. c b a ( Q 3 3 3 0 2 2 2 + + πε + +
3. ) c b a ( 4 Q 0 + + πε
4. 0 12 Q πε abc ca bc ab + + (4) ) c b a ( 4 ) c b a ( Q 2 2 2 0 + + πε + +

Correct answer: 0 12 Q πε abc ca bc ab + + (4) ) c b a ( 4 ) c b a ( Q 2 2 2 0 + + πε + +

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