June 14 Information

In the following identity, ${\displaystyle \int \int \int _{V}\phi div(grad(\psi ))+grad(\phi )\cdot grad(\psi )dV=\int \int _{S}\phi {\frac {\partial \psi }{\partial n}}dS}$, what does ${\displaystyle n}$ represent? ? Widener ( talk) 20:40, 14 June 2012 (UTC) reply

It represents a normal -- this is the directional derivative in the direction perpendicular to the surface. Looie496 ( talk) 21:18, 14 June 2012 (UTC) reply