  The cage model for polymer reptation is extended to simulate DC
electrophoresis. The drift velocity v of a polymer with length L in an electric
field with strength E shows three different regions: if the strength of field
is small, the drift velocity scales as E/L; for slightly larger strengths, it
scales as E^2, independent of length; for high fields, but still E much smaller
than 1, the drift velocity decreases exponentially to zero. The behaviour of
the first two regions are in agreement with earlier reports on simulations of
the Duke-Rubinstein model and with experimental work on DNA polymers in agarose
gel.
