Some basic concept: If a mixture of electrical charges molecules are kept in electric filed for field strength E, they shall move toward opposite electrode. This process is called electrophoresis.

Velocity of movement (v) is defined by following equation:

As clear from the equation, mobility depends on various factors like net charge (q) and frictional coefficient (f). Size of biological molecule is proportional (not strictly as we see during next lecture) to the mass. Molecule with equal charge /mass ratio have identical electrophoretic mobility in a given medium.