The invention pertains to a method and apparatus to separate and quantify particles using time-variable force fields. The force fields can be for dielectrophoresis (positive or negative), electrophoresis, or electrohydrodynamic. In one aspect, the fields are translated and/or modified in space at a speed substantially comparable to the speed of translation of the fastest particles in the sample. The translation and/or modification of the force fields can also occur with varying speed. In another aspect, the field is translated and/or modified in a first direction at high speed. In another aspect, the quantity or size of the particles is determined by an indirect measurement of the speed of movement after varying the force field by means of a relationship between the speed of movement and the volume of the particles. This invention also pertains to an apparatus to produce appropriate field configurations necessary for the selective movement of particles.