# Tag Info

$\vec\Phi = K_i \nabla u_i$ is the flux across the interface. For example, if $u$ is the thermal energy density and $K$ the thermal conductivity, then $\vec\Phi$ is the thermal energy flux. Energy conservation then dictates that whatever flows into the interface on one side ($\vec n_1 \cdot K_1 \nabla \vec\Phi_1$) better be equal to what flows out on the ...