= Cylindrical screening charge
{title2=$Q(r)=2\pi\int_a^r s\rho(s)\,ds$}
The positive mobile charge per unit axial length accumulated from cylinder radius $a$ to $r$ is $Q(r)=2\pi\int_a^r s\rho(s)\,ds$. For fixed negative line charge $-\lambda$, <Gauss's law> gives signed radial <electric field> $E_r=2[-\lambda+Q(r)]/(\epsilon r)$ in <Gaussian units>. Its absolute value is the field magnitude. An integral over a local infinite-dilution profile measures condensed charge, rather than every <ion> in a finite neutral cell.
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