= Electrostatic interface boundary conditions
{title2=$[D_n]=\sigma_{\rm fixed}$}
Across a charged dielectric interface the tangential <electric field> is continuous and the normal displacement jump is the fixed <surface charge density>. Without a dipole sheet the <electrostatic potential> is continuous. For a charge sheet with equal <permittivity> on both sides, its normal potential derivative has jump $-\sigma_{\rm fixed}/\varepsilon$. Setting the field to zero on one side is an additional physical boundary assumption, as in a conductor or a negligible-exterior-admittance idealization.
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