Question

Compute the capacitance between two concentric shells of inner radius 2m and the outer radius is infinitely large.

(a) 0.111 nF

(b) 0.222 nF

(c) 4.5 nF

(d) 5.4 nF

This question was addressed to me during an online interview.

My question is from Resistances and Capacitances in section Electrostatic Boundary Value Problem of Electromagnetic Theory

Answer 1

The correct choice is (b) 0.222 nF

The explanation is: The concentric shell with infinite outer radius is considered to be an isolated sphere. The capacitance C = 4πε/(1/a – 1/b). If b->∞, then C = 4πεa. Put a = 2m, we get C = 4π x 8.854 x 10^-12 x 2 = 0.222 nF.