= Dielectric
{wiki}
A <dielectric> is a material in which an <electric field> produces <electric polarization> while transport of <electric charge> is sufficiently small for the intended use. An insulating solid in a <capacitor> is an example; real <dielectrics> can still have <dielectric loss> and nonzero <electrical conductivity>.
The important distinction is between rearranging bound <electric charge> and transporting mobile <charge carriers> through the specimen. Bound <electric charges> can move a little within a <chemical atom> or <molecule>, or an existing <electric dipole> can turn. Their averaged <electric dipole moments> produce <electric polarization> even if there is negligible steady <electric current> between the <electrodes>. A changing <electric polarization> does give a <bound current>; the absence of steady conduction does not mean that the material is electrically unresponsive.<The Feynman Lectures on Physics, Volume II by Richard Feynman and others/Chapter 10: Dielectrics>[II.10–2 and II.10–3: induced atomic dipoles and relaxation of bound surface charge]{ref}
For uniform <electric polarization> $\mathbf P=P\widehat{\mathbf x}$ in a rectangular slab, <polarization density> gives zero bound volume <charge density> in the interior and opposite bound <surface charge densities> $-P,+P$ on the left and right faces. The <electric charge> has been redistributed, not created. More generally, the <divergence theorem> makes the cancellation explicit:
$$
\int_{\mathcal V}(-\nabla\cdot\mathbf P)\,d\mathcal V
+\int_{\partial\mathcal V}\mathbf P\cdot\mathbf n\,dA=0.
$$
Here $\mathbf n$ is the outward <normal vector> on the material boundary. This accounts for both bound volume <charge density> and bound <surface charge density>; omitting either can falsely suggest that an initially neutral <dielectric> acquired net <electric charge>.
An <electrical insulator> between <capacitor> plates supports a useful combination: appreciable <electric polarization>, little leakage <electric current>, and tolerance of the intended <electric field>. This lets a small <parallel-plate capacitor> store separated <electric charge> and recoverable <capacitor energy>. The comparison in <dielectric capacitor response> explains why specifying fixed <electric charge> or fixed <electric potential difference> changes the answer. High <relative permittivity> alone does not establish good insulation or a large usable <energy density>; <dielectric loss> and <dielectric breakdown> matter as well.
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