Three long concentric conducting cylindrical shells have radii R, 2R and 2√2R. The inner and outer shells are connected to each other. The capacitance per unit length between the middle shell and the connected inner–outer combination is:
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Question

Three long concentric conducting cylindrical shells have radii R, 2R and 2√2R. The inner and outer shells are connected to each other. The capacitance per unit length between the middle shell and the connected inner–outer combination is:

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G1 Physics · Accepted Answer

The middle shell forms two coaxial capacitors to the connected inner and outer shells; these two capacitances are in parallel. Adding them gives 6πε₀/ln 2 per unit length. Hence B.