= Charge relaxation
{title2=$\tau=\epsilon/\sigma$}
In a homogeneous <Ohmic conductor>, the <charge continuity equation> and <Gauss's law> imply exponential decay of the bulk <charge density>, $\rho(t)=\rho(0)e^{-\sigma t/\epsilon}$. For an isolated finite body, bulk charge moves to its boundary; the accumulating <surface charge density> preserves total <electric charge>. The formula uses a constant permittivity $\epsilon$ and the corresponding free-charge convention, or $\epsilon_0$ for total charge in the vacuum form of <Gauss's law>.
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