Loop crossing a magnetic field boundary

A square resistive loop of side $a$, moving normally across a sharp field edge at speed $v$, has induced current magnitude $Bav/R$ and drag magnitude $B^2{a}^{2}v/R$ while partially overlapping the field. Completely enclosed in a uniform static field it has no changing flux and, with neglected self-inductance, no current.

For a complete entry and exit of a small square loop, integrating the drag over two distances $a$ gives impulse magnitude $2B^2{a}^3/R$. This is independent of speed in the ideal sharp-edge, zero-inductance model, though dissipated energy is not.