For 10 points, identify this representation of energy flux density, proportional to the cross product of the electric and magnetic fields.
An instantaneous elastodynamic variety of this entity can be calculated as the product of a particle's velocity and the stress tensor of a given medium. Snell's Law can be used to show that the "bending" of this quantity normal to the surface of a conductor is responsible for heating caused by resistive dissipation. The total rate of this quantity can be given as the sum of stored energy densities and heat loss as shown by the theorem named for this quantity's namesake, which states the time derivative of energy density plus this quantity's divergence is equal to the negative dot product of current density and the electric field. The radiation pressure of an electromagnetic wave is the average intensity of this quantity divided by the speed of light. For 10 points, identify this representation of energy flux density, proportional to the cross product of the electric and magnetic fields.
An instantaneous elastodynamic variety of this entity can be calculated as the product of a particle's velocity and the stress tensor of a given medium.
Poynting vector 