For 10 points, name this vector quantity symbolized with a boldface S, which represents the flux of electromagnetic energy, and "points" in the direction of propagation.
A generalization of this quantity that applies to liquid media is also named for Nikolay Umov. When computing the divergence of the Maxwell stress tensor, this quantity appears divided by the square of the speed of light and represents the momentum density. The original derivation of this quantity relies on taking the time derivative of the energy density and then using a theorem named for the namesake of this quantity, which causes the negative divergence of this quantity to appear. This quantity is found by taking the cross product of the E-field and the H-field. For 10 points, name this vector quantity symbolized with a boldface S, which represents the flux of electromagnetic energy, and "points" in the direction of propagation.
A generalization of this quantity that applies to liquid media is also named for Nikolay Umov.
Poynting vector [prompt on "intensity"]