  We derive asymptotic expansions of the current, the electrostatic capacity,
and the far-field of the electrostatic potential resulting from small
perturbations in the shape of an isolated conductor with
$\mathcal{C}^2$-surface. Our derivation is rigourous by using systematic way,
based on layer potential techniques and the field expansion (FE) method. We
then use these results to study the sensibility analysis of the first
eigenvector of the $L^2$-adjoint of the Neumann-Poincar\'e (NP) operator with
respect to small perturbations in the surface of its domain.
