= Bound charge
{title2=$\rho_{\mathrm b}=-\nabla\cdot\mathbf P$}
{title2=$\sigma_{\mathrm b}=\mathbf P\cdot\mathbf n$}
The effective <electric charge> associated with <electric polarization> is bound charge. For <polarization density> $\mathbf P$, its volume <charge density> is $\rho_{\mathrm b}=-\nabla\cdot\mathbf P$. At a boundary with unpolarized surroundings, its <surface charge density> is $\sigma_{\mathrm b}=\mathbf P\cdot\mathbf n$, where $\mathbf n$ is the outward <unit normal vector>.
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