In this animation we show the charging of two Van de
Graff generators sitting side by side to equal and opposite
charges Q. As we charge them, there is a Poynting flux
outward from the region where we are creating electromagnetic
energy. This region is where the charge is being moved
against the electric field on the conveyer belts of
the two generators (inside the cylindrical shafts of
the generators). This is where the creation rate per
unit volume for electromagnetic energy (-J dot E) is
positive. Energy is created in those regions and flows
out to fill the space around the generators. That flow
can be seen in the motion of the electric field lines,
which is always in the direction of the Poynting flux.
After charging the generators, we turn off the conveyer
belts. We then ionize the air between the two spheres
so that they discharge along the line joining them.
We see the spark discharge and the inflow of energy
from the electrostatic field powers that spark. Again
we see the energy drain out of the electrostatic field
as indicated by field line motion, which is in the direction
of the Poynting flux.