Electrophoresis is a sedimentation technique used in DNA analysis and in the determination of protein molecular weight.  The sample to be analyzed is placed along one end of a gel with carefully controlled diffusion properties.  The DNA and protein fragments are prepared so that in the environment of the gel they have a net charge.  The gel is placed into a constant electric field.  The net force acting on a sample molecules is the difference between the electric force and the drag force.  In the gel the drag force is proportional to the speed of the fragment. 

F_net = ma = qE - bv,

where q is the charge on the molecule and b is its drag coefficient.  The molecules quickly reach terminal velocity.  The drag force then exactly cancels the electric force and the acceleration a is zero.  We have for the terminal speed

v_t = qE/b.

The speed of the molecules is measured by determining the distance traveled in a given time interval.  Given that the fragments have been properly prepared (denatured, unfolded, and charged),  the speed of the molecules can be compared to that of accurately calibrated standards, and the molecular weight can be determined.  As is often the case, the basic physics theory is simple, but the application involves integrating knowledge from many different fields.

Try it:  GEL ELECTROPHORESIS VIRTUAL LAB