Question

Assume that the magnetic field is uniform in a cubical region and zero outside. Can you project a charged particle from outside into the field, so that the particle describes a complete circle in the field?

Answer 1

Any charged particle from outside the cubical area can't be sent there in a way that makes it form a full circle inside the field. The particle will leave the cube before going all the way around because the magnetic field stops at the cube's edges.