Such a terminal secure to an insulating support entering more or less into its interior, and I likewise connect the circuit to it inside or, generally, at points where the electric density is small.

But now, if I integrate the charge density over the entire region, then what I will get is actually the total amount of electric charge inside the region.

There is no realistic prospect for widespread electric air travel: the jet engines on aircraft need the high-energy density that only chemical fuels can provide.