  The Feynman demonstration that electromagnetic field momentum is real-even
for static fields-can be made more pedagogically useful by simplifying its
geometry. Instead of Feynman's disk with charged balls on its surface, this
article uses the geometry of a hollow non-conducting sphere with uniform
surface charge density. With only methods available in a typical upper-division
electrodynamics course, the initial field angular momentum and the final
mechanical angular momentum can then be calculated in closed form and shown to
be equal.
  This simplified geometry also provides a counterexample for the idea that
electromagnetic field momentum is due to the flow of an electromagnetic
inertial mass, defined as the energy density divided by the square of the speed
of light. The curved flow lines of an inertial field momentum would require a
centripetal force to bend them, but no such force can be identified.
