Two-wire capacitance at large separation
= Two-wire capacitance at large separation
{title2=$C'=2\pi\epsilon/\ln[d^2/(a_1a_2)]$}
Replace two well-separated cylindrical conductors by opposite axial <line charges>. Their surface-to-surface potential difference is $\lambda[\ln(d/a_1)+\ln(d/a_2)]/(2\pi\epsilon)$, giving the displayed <capacitance per unit length>. The approximation needs $d\gg a_1,a_2$ and a homogeneous surrounding <permittivity>.