From the electric field between two conductors: the resulting total force

4

Hi,
in order to maximize your chances of obtaining an answer, you should first follow the guidelines, especially concerning posting a Minimal Working Example. Without that it is hard to help you without a code.
In any case, I would expect something like:

n = FacetNormal(mesh)
assemble(Constant(epsilon_0)/2*dot(grad(v), grad(v))*n[2]*ds(1))

assuming that your surface S_t has been tagged as 1 and if grad(v) is the electrostatic field.

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