What is the terminal velocity of a spherical colloidal parti

What is the terminal velocity of a spherical colloidal particle having an electric charge e in an electric field of strength E? How is this used in the Millikan oil-drop experiment?

Solution

For a colloidal solution

force on a charged particle in electrical field = E*e

Drag force on the particle = 3*pi*mu*D * V_e / C_c     (where mu is the viscosity of medium, D is dia of particle , Cc is the drag coefficient)

both the forces equate at terminal velocity therefore

V_e   = C_c * E *e / (3 * pi * mu * D )

In millikan oil drop experiment Millikan determined the charge on a drop. Then he redid the experiment numerous times, each time varying the strength of the x-rays ionizing the air, so that differing numbers of electrons would jump onto the oil molecules each time. He obtained various values for q.
The charge q on a drop was always a multiple of -1.6 x 10 -19 C, the charge on a single electron.