A model is proposed for describing the electrodynamics of hard superco
nductors with dimensions comparable to the London depth. The nonlinear
response of the superconductor to a change in the external magnetic f
ield is examined for the case in which there is an irreversible motion
of Abrikosov vortices. Certain aspects of the low-frequency surface i
mpedance of a superconducting plate are explained successfully by this
model.