= Electrical insulator
{wiki=Insulator_(electricity)}
An <electrical insulator> has sufficiently small transport of <electric charge> for its operating conditions. Insulation is a practical requirement involving <electric potential difference>, <temperature>, geometry and duration, rather than a universal claim of zero <electrical conductivity>. <Electric polarization> can be substantial in an <electrical insulator>; <dielectric breakdown> can end the insulating regime.
For an idealized uniform slab with small constant <electrical conductivity> $\sigma$, the <Ohmic conductor> model predicts leakage <electrical resistance> $R=d/(\sigma A)$ in parallel with <capacitance> $C=\epsilon A/d$. An isolated <capacitor> then has
$$
\frac{dQ}{dt}=-\frac{Q}{RC},\qquad
Q(t)=Q(0)e^{-t/(RC)},\qquad RC=\frac{\epsilon}{\sigma}.
$$
This is a derived model of slow discharge, not an assertion that every leakage mechanism obeys <Ohm's law>. Its time scale matches <charge relaxation>. The fixed-charge comparison in <dielectric capacitor response> needs an observation time short compared with $RC$, but long enough for <electric polarization> to settle. During a maintained voltage, leakage consumes <power> $V^2/R$ as <Joule heating>; delayed <electric polarization> can add <dielectric loss> under changing <electric fields>.<Materials Measurement: Dielectric Materials>[Overview: real and imaginary permittivity, storage and dissipated loss]{ref}
Large <relative permittivity> alone does not rule out either mechanism.<University Physics Volume 2 by Samuel J. Ling and others/8.5 Molecular Model of a Dielectric>[§8.5, paragraph following Table 8.1: high dielectric constant does not ensure a good insulator; mobile ions permit conduction]{ref}
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