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3 A charge Q is distributed over two concentric conducting thin spherical shells radii r and R (R > r). If the surface charge densities on the two shells are equal, the electric potential at the common centre is :

1. ) r R ( 2 Q ) r 2 R ( 4 1 2 2 0   
2. Q ) r R
3. r R ( 4 1 2 2 0   
4. Q ) r R ( ) r R ( 4 1 2 2 0    (4) Q ) r R ( ) r R 2 ( 4 1 2 2 0   

Correct answer: r R ( 4 1 2 2 0   

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