= Electrical resistivity
{title2=$\rho_{\rm el}=1/\sigma$}
Electrical resistivity is the inverse of scalar <electrical conductivity> in an isotropic Ohmic medium. In its rest frame, $\mathbf E=\rho_{\rm el}\mathbf j$, so the <electric field> does work per unit volume $\mathbf j\cdot\mathbf E=\rho_{\rm el}|\mathbf j|^2$. In <Gaussian units>, finite conductivity gives <magnetic diffusivity> $\eta=c^2/(4\pi\sigma)$; neglecting this diffusion is a distinct approximation from neglecting electron inertia in <Hall magnetohydrodynamics>.
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