A Quantitative Version of Feynman's Static Field Momentum Example: https://arxiv.org/abs/1911.05893
The Feynman demonstration that electromagnetic field momentum is real, even for static fields, can be made more useful by simplifying its geometry. Instead of Feynman's disk with charged balls on its surface, use a hollow non-conducting sphere with uniform surface charge density. The initial field angular momentum and the final mechanical angular momentum can then be calculated in closed form and shown to be equal. The methods used in the calculation are those available to the average upper-division physics student.
This simplified geometry also provides a test for the current idea that electromagnetic field energy can be considered a form of inertial mass. The mass motion in the simplified Feynman example can be modeled as the static, circular, incompressible flow of a fluid, with distributed nonzero vorticity. But such motion requires a centripetal force or pressure that has yet to be identified.